acoustics

Article Handclap for Acoustic Measurements: Optimal Application and Limitations

Nikolaos M. Papadakis 1,2,* and Georgios E. Stavroulakis 1

1 Institute of Computational Mechanics and Optimization (Co.Mec.O), School of Production Engineering and Management, Technical University of Crete, 73100 Chania, Greece; [email protected] 2 Department of Music Technology and Acoustics, Hellenic Mediterranean University, 74100 Rethymno, Greece * Correspondence: [email protected]



Received: 25 March 2020; Accepted: 7 April 2020; Published: 26 April 2020


Abstract: Handclap is a convenient and useful acoustic source. This study aimed to explore its optimal application and limitations for acoustic measurements as well for other possible utilizations. For this purpose, the following steps were performed: investigation of the optimal hand configuration for acoustic measurements and measurements at different microphone source distances and at different spaces and positions. All measurements were performed with a handclap and a dodecahedron speaker for comparison. The results indicate that the optimal hand configuration (among 11) is with the hands cupped and held at an angle due to the superior low frequency spectrum. This configuration produced usable acoustic parameter measurements in the low frequency range in common room background levels unlike other configurations. The reverberation time was measured across different spaces and positions with a deviation less than three and just a noticeable difference of the signal-to-noise ratio within or near the ISO 3382-1 limits for each corresponding octave band. Other acoustic parameters (i.e., early decay time, clarity) were measured with greater deviations for reasons discussed in the text. Finally, practical steps for measurements with a handclap as an acoustic source are suggested.

Keywords: acoustic measurements; acoustic parameters; impulse response; sound source; reverberation time; ISO 3382; noise compensation method; dodecahedron; just noticeable difference

1. Introduction A source that is concentrated at a point and produces an omnidirectional sound field is called a simple source or a monopole source [1]. The conceptually simplest sound source with finite extension is the spherical source, often referred to as “pulsating” or “breathing sphere” [2]. A sound source with omnidirectional characteristics, a reminder of a monopole, is required in many acoustic measurements. A list and description of these measurements can be found in Reference [3]. The most common practical implementation of a pulsating sphere producing an omnidirectional field used for acoustic measurements is a dodecahedron speaker. The omnidirectional directivity is approached by placing 12 electrodynamic loudspeakers (direct radiator type) in a regular 12 face polyhedron. However, other polyhedron loudspeakers can also be usedd which in some cases can be viewed as equally omnidirectional [4]. Commercially available dodecahedron speakers are manufactured and tested in order to meet the ISO 3382-1 [5] sound source requirements which are the most referred for acoustic measurements. Similar but less stringent requirements (for maximum deviation of directivity) are described in ISO 16283-1 [6] and ISO 10140-5 [7]. The requirements of ISO 3382-1 are:

The sound source shall be as close to omnidirectional as possible. A maximum deviation of • directivity of source in decibels for excitation with octave bands of pink noise and measured in free field is expected (a table of maximum deviation per frequency is featured in ISO 3382-1);

Acoustics 2020, 2, 224–245; doi:10.3390/acoustics2020015 www.mdpi.com/journal/acoustics Acoustics 2020, 2 225

The sound source shall produce a sound pressure level sufficient to provide decay curves with • the required minimum dynamic range, without contamination by background noise. In the case of measurements of impulse responses using pseudo-random sequences (e.g., maximum-length sequence [8]), the required sound pressure level might be quite low because a strong improvement of the signal-to-noise ratio by means of synchronous averaging is possible. In the case of measurements which do not use a synchronous averaging (or other) technique to augment the decay range, a source level will be required that gives at least 45 dB above the background level in the corresponding frequency band (also an alternative source level above the background level is provided in ISO 3382-1 which is discussed in the Methods, Section2).

Dodecahedron speakers, since they cannot emit sufficient acoustic power if an impulsive signal is applied directly, utilize alternative excitation signals. The most common ones are maximum-length sequence (MLS) [8] and exponential sine sweep (ESS) [9], which are described in Annex A and B of ISO 18233 [10], respectively. Optimum signal-to-noise ratios can be found in Stan et al. [11], while comparison of the two methods can be found in Reference [12]. The appropriate choice of excitation signal for acoustic measurements can be dependent on the background noise [13]. Despite the widespread use of dodecahedron speakers, there exist certain drawbacks associated with them. Namely,we can mention deviation from omnidirectionality and low frequency performance [3]. There are also drawbacks due to the fact of practical reasons. Especially, their high cost makes them probably the most expensive equipment in an acoustic measurement setup. Their heavy weight combined with their large volume makes transportation difficult, e.g., transfer to airports. There are also cases where their use is required in places where there is no electricity supply. An external generator or an appropriate dodecahedron speaker with an internal generator can be used which further increases the cost. Beside dodecahedron speakers, there are alternative sound sources that can be used for acoustic measurements such as balloons [14], gunshots [15], firecrackers [16], inverse horn designs [17], wooden clappers [18], rotation of a directional speakers [19], ultrasound piezoelectric transducers [20], and ring radiators [21]. A detailed review of all the source alternatives to a dodecahedron speaker can be found in Reference [3]. Handclap is one of the sound sources which is useful, convenient, and easily available. It is a common practice for acousticians to use clapping to excite a space in order to assess its acoustic characteristics. It can be used to detect unwanted acoustic phenomena such as echoes and flutter-echoes [22]. In a questionnaire [23] designed to collect data from industrial, research, and academic communities, handclap was ranked sixth among sound sources/test signals (excitation system) that the participants have used in the past for the measurement of acoustic characteristics of rooms or indoor spaces (the first five results were in preference order: pink noise, balloon, sine sweep, starter pistol, and MLS signal). Sound generation and the underlying physics of a handclap were examined in a study by Fletcher et al. [24]. In particular, the effect of the hand profile was investigated which may be either nearly complementary between the two hands, giving a nominally flat impact, or else domed so that there is a significant enclosed volume. It was shown that a shock wave is generated for configurations of the hands that produce a loud sharp sound. The addition of a Helmholtz-type resonance is involved in the case of domed impacts. A study by Hargather et al. [25] also noted the presence of shock waves in some handclap sounds and examined them by Schlieren photography. In a study by Repp [26], eight different clapping modes according to hand configuration were identified. The author states that additional variations may derive from such factors as hand curvature, stiffness, fleshiness of the palms, tightness of the fingers, precision, and striking force. Repp’s taxonomy of handclaps was utilized for classification in a study to infer hand configuration [27]. A relevant study by Peltola et al. [28] presents an analysis for synthesizing hand clapping sounds. Sound generation of a handclap creates distinctive acoustic characteristics. Collected relevant publications presenting topics, such as omnidirectionality and adequate sound power, required from ISO 3382-1 as well as repeatability, frequency response, and accuracy in measurement of acoustic parameters can be found in Reference [3]. Acoustics 2020, 2 226

Omnidirectionality of the handclap was studied by Griesinger [29] revealing directional characteristics. Measurements in different directions showed differences more than 15 dB in certain frequency ranges, well above the maximum deviation allowed by requirements of ISO 3382-1 [5]. Omnidirectionality ensures uniform space excitation necessary for correct acoustic measurements since the directivity of the source influences the measurement of impulse responses and acoustic parameters of spaces [30,31]. Concerning sound pressure levels, a study by Rizzi et al. [32] measured the levels of acoustic sources and found the handclap to be one with the lowest. In another study by Seetharaman and Tarzia [33], the handclap averaged 26.4 dB above the background level (in a concert hall with typical background noise levels) with a standard deviation of 4.4 dB across measurements. As stated before in ISO 3382-1 requirements, the source level must be at least 45 dB above the background level in the corresponding frequency band in order to obtain satisfactory results in acoustic measurements. Relating to frequency response, the handclap can be generated in different ways which results in different frequency responses. In frequency responses of the eight different clapping modes that were presented in a study by Repp [26], it can be seen in all of them that a roll off is evident below 500 Hz. The same was reported in a similar study [32] and is in accordance with Griensiger [29] which states “handclaps suffer from poor low frequency content”. In the study by Fletcher [24], measurements of the sound of nominally flat and cupped natural handclaps were made in an anechoic environment. The author states that “a flat clap produces broad-band sound that typically extends to about 10 kHz while the spectrum of a domed clap usually has a subsidiary maximum somewhere below 1 kHz and then declines with frequency more rapidly than does the flat clap”. Concerning repeatability, the handclap may have one of the lowest among acoustic sources. The reason being that the slightest variation of hand configuration may alter the generated impulse and spectral characteristics [26]. Also, there is a considerable variability in spectral shapes of handclaps across individuals. In the same study [26], the amplitude standard deviations ranged from 0.7 to 5.2 dB across subjects. However, it was also found that it is possible to improve the repeatability of a handclap if it is produced by the same individual utilizing the exact same hand configuration every time. Source repeatability ensures that the same sound field is produced from the sound source for each measurement and, hence, similar impulse response and acoustic parameters are measured. Regarding measurements of acoustic parameters, few studies were found in the literature where the handclap was used as an acoustic source [32–35]. Unfortunately, a limitation for all the aforementioned studies was that a smartphone was used as a sound recorder. Reverberation time (T) measurements in a study by Seetharamn and Tarzia [33] showed that results for frequencies below 250 Hz were unreliable, while for the high frequency range results, they were consistent with small deviations. However, the measurements have to be considered with caution, since they were compared with measurements made with a balloon source. Another study by Huang [34] measured acoustic parameters, T and Centre Time (Ts), in the Bayreuth Festspielhaus. Acoustic parameters were compared with results that were found in the literature, and deviations were found that were attributed to reasons such as low sound-to-noise ratios (SNRs) and the performance of micro-electromechanical systems (MEMS) microphones found in smartphones. Another study by Brown and Evans [35] compared sound pressure levels and T measured with a smartphone and a sound level meter, with various sources such as handclap, slamming of a thick book, and pink noise (interrupted pink noise for T measurements). However, the study was not conducted to evaluate the effectiveness of the clap as an acoustic source but of the smartphone as a measuring device. No detailed results were provided, but differences were still observed between the handclap and the source utilizing interrupted pink noise measured with a sound level meter. Measurements with a handclap were also performed by Halmrast [36], utilizing an “in-ear” microphone, mainly to access how an individual perceives the reverberation of its own clapping. Finally, handclap can also be used for other applications such as clap based electrical appliance control [37,38], speech dereverberation [39–41], acoustic environment identification using unsupervised learning [42], and for sonic interactions with a computer [43,44]. Handclap can also be used to form a Acoustics 2020, 2 227 language as a common means of communication between humans and robots [45], to estimate the performance of acoustic absorbers [46], and as a source for a localization technique using a mobile phone [47]. The novelty of this study, in relation to earlier publications, can be considered to consist of the following points:

To the best of our knowledge, this is the first study that suggests the optimal hand configuration • (among 11) for acoustic measurements. Previous studies have either identified the hand configurations (e.g., [26]) or performed measurements without considering the optimal hand configuration [32–35]; Another contribution of this study is that it suggests the expected measurement accuracy utilizing • a handclap as a sound source for some of the most common acoustic parameters (i.e., reverberation time, early decay time, clarity); No known empirical research has focused on comparing measurements performed with a handclap • as a sound source and measurements performed with a dodecahedron speaker following the recommendations of ISO 3382-1; Three new hand configurations were suggested and measured; • Practical steps for measurements with a handclap as an acoustic source are suggested. • This paper has been organized in the following way: Section2 is concerned with the methodology employed for this study, while Section3 presents the findings of the research. Section4 concerns the discussion and analyzes the data, addresses the research questions, identifies areas for further research, and proposes practical steps for measurements with a handclap as an acoustic source. Finally, the conclusion is provided in Section5 and provides a brief summary and contextualizes the research.

2. Methods The methods section is divided into the subsections “Measurements with a Handclap”, “Measurements with a Dodecahedron Speaker”, and “Acoustic Parameters-Compensation Method”. The first and main subsection describes all the necessary measurements performed in order to evaluate the handclap as an acoustic source. “Measurements with a Dodecahedron Speaker” is a short subsection which describes typical measurements according to ISO 3382-1 that were performed for comparison reasons. The final subsection includes a description of the acoustic parameters and a discussion about the compensation method that was used.

2.1. Measurements with A Handclap as A Sound Source The measurements that were performed to investigate optimal application and limitations of the handclap as an acoustic source for the estimation of acoustic parameters are the following:

Measurements to investigate optimal hand configuration; • Measurements at different source–receiver distances; • Measurements in different spaces. • 2.1.1. Measurements to Investigate Optimal Hand Configuration One of the major aims of this research was to investigate the optimal hand configuration of the handclap for the best possible excitation of a space. In previous studies by Repp [26] and Fletcher et al. [24], different hand configurations were examined. Especially in the study by Repp [26], the different sounds of a handclap were categorized in eight clapping modes according to hand configuration. A description and a figure for each configuration can be found in Reference [26]. For this study, eleven hand configurations (Figure1) were evaluated (P1, P2, P3, A1, A2, A3, A1 +, A1 , O1, O2, O3). The nomenclature used by Repp was kept (letters P, A, and O correspond to the − words parallel, aligned, and opposite). Modes Pl–P3 kept the hands parallel and flat but changed Acoustics 2020, 2 228 Acoustics 2020, 2 FOR PEER REVIEW 5 theirthe vertical words parallel, alignment aligned, from and palm-to-palm opposite). Modes (P1) Pl to–P3 fingers-to-palm kept the hands parallel (P3) with and P2flat halfwaybut changed between their vertical alignment from palm-to-palm (P1) to fingers-to-palm (P3) with P2 halfway between these extremes (i.e., with the right hand lowered by approximately 4 cm). Modes A1–A3 varied the these extremes (i.e., with the right hand lowered by approximately 4 cm). Modes A1–A3 varied the alignment in a similar way but with the hands held at an angle (modes P1 and A1 differ in that the alignment in a similar way but with the hands held at an angle (modes P1 and A1 differ in that the fingersfingers of theof the two two hands hands strike strike each each otherother in in P1 P1 but but not not in in A1. A1. Modes Modes P3 and P3 andA3 are A3 more are moresimilar similar to to eacheach other). other). Repp Repp in in his his study study proposed proposed additional variations variations of ofthe the A1 A1configuration. configuration.

P1 P2 P3

A1 A2 A3

A1+ A1−

O1 O2 O3

FigureFigure 1. Di1. ffDifferenterent hand hand configurations configurations that that were were used used during during the themeasurements measurements (P: parallel (P: parallel hand hand configurations,configurations,A: A aligned: aligned hand hand configurations, configurations, OO: :opposite opposite hand hand configurations). configurations).

HeHe stated, stated, “since “since the the hands hands automaticallyautomatically tended tended to tobe bemore more relaxed relaxed (slightly (slightly cupped) cupped) in the inA the A modesmodes than than in thein the P modes,P modes, two two versionsversions of A1 A1 were were added, added, with with the the hands hands either either very verycupped cupped (A1+) (A1+) or flator flat (A1-)”. (A1-)”. Three Three more more hand handconfigurations configurations were were suggested suggested by bythe the authors authors of this of research. this research. Modes Modes O1–O3 kept the hands opposite but changed their vertical alignment from palm-to-palm (O1) to fingers to palm (O3) with O2 halfway between these extremes. In order to evaluate the hand configurations, twenty-four subjects (18 male, 6 female, project identification code: Protocol number 11/16.04.2020; 24 April 2020, Committee on Ethics and Deontology Acoustics 2020, 2 229 of Research (C.E.D.R), Technical University of Crete), students from the Hellenic Mediterranean University Department of Music Technology and Acoustics (Rethymno, Greece) with a mean age of 21 (SD: 2.5) participated in two sets of acoustic measurements (January 2020). Measurements were performed in a similar fashion as the study by Repp [26]. An important difference was that the subjects performed the handclaps upright to simulate how the actual handclaps would be performed in normal acoustic measurement (in the study by Repp the subjects were seated). For the measurements, subjects performed the handclaps, one at a time, with their hands approximately 60 cm from a sound level meter, 01dB-Steel SdB02 (01 dB-Stell, Limonest, France), similar to the study by Repp [26]. For the first set of measurements, subjects were asked to perform a handclap as they would in measurement conditions. The purpose of these measurements was to observe the natural hand configuration of the subjects. For the second set of measurements, subjects were asked to perform handclaps for each hand configuration. For the measurements, instructions for the hand configurations were given. The subjects were requested to create a maximum sound pressure level in a controlled manner for each hand configuration. For the measurements, sound pressure levels (SPLs), octave band SPLs, and frequency responses were recorded.

2.1.2. Measurements at Different Source–Receiver Distances The purpose of these measurements was initially to observe and evaluate the effect of source–receiver distance which in turn was expected to affect the signal-to-noise ratios in acoustic measurements. As presented in the introduction, ISO 3382-1 sound source recommendations state that “the sound source shall produce a sound pressure level sufficient to provide decay curves with the required minimum dynamic range, without contamination by background noise”. It is reasonable to assume that at shorter source–receiver distances, the signal-to-noise ratio will be higher. Measurements at different source–receiver distances were performed in an amphitheater at the Hellenic Mediterranean University, Department of Music Technology and Acoustics with a volume of 1088 m3 (Figure2A). Handclaps were performed by a trained individual in order to achieve the best repeatability possible [26]. As stated in the introduction concerning repeatability, the handclap may have one of the lowest among acoustic sources. However, it is possible to improve the repeatability if it is produced by the same individual utilizing the exact same hand configuration every time [26]. The height of the microphone above the floor was 1.2 m according to recommendations of ISO 3382-1, corresponding to the ear height of average listeners in typical chairs. The individual who performed the handclaps was facing the microphone at every measurement. The height of the handclap was 1.3 m, the natural height of the position of the handclap of the individual who performed at the measurement sessions. For this set of measurements, different hand configurations proposed from the previous subsection were evaluated (more on this in Section 2.1.1, Section 3.1, and Section 4.1). It should also be noted that there are recommendation about the minimum source–receiver distance in ISO 3382-1 [5] and especially in ISO 3382-2 [48] that were followed for this research. It is stated that [5] “no microphone-position shall be too close to any source-position in order to avoid too strong influence from the direct sound”. The minimum distance, dmin, in meters, can be calculated from Equation (1) [48]: r V dmin = 2 (1) cTˆ V is the volume, in cubic meters; c is the speed of sound, in meters per second; Tˆ is an estimate of the expected T, in seconds. Acoustics 2020, 2 FOR PEER REVIEW 7

is an estimate of the expected T, in seconds.

2.1.3. Measurements at Different Spaces In this part of the research, acoustic measurements were performed at different spaces. The purpose of these measurements was to evaluate the accuracy of the handclap as an acoustic source for measurements at different spaces and with different background noise levels. Measurements were performed in three spaces of the Hellenic Mediterranean University, Department of Music Technology and Acoustics with volumes of 1088, 176 and 146 m3 (Figure 2B, 2C, 2D, respectively). The first space was the one that was used for measurements at different source– receiver distances in a previous part of this research. The hand configuration that was used for this set of measurements was the one that was identified as optimum in different sections of the study Acoustics(Sections2020 3.1, 2. and 4.1.). For each case, impulse response measurements were performed as specified230 in the source and receiver positions depicted in Figure 2B–2D.

8.20 m

12.60 m 12.60 m

R9 R8 (C)

R7 7.28 m R1 R2 R6 R3 R5 R4 S1 S2

R3 y (0,0,0)m S3 R2 x 19.60 m 19.60 R1 m 19.60 6.21 m R1 R3 R2 S S1 S3 y R1 y (0,0,0)m (0,0,0)m S2 R2 (D)

x x 8.05m R3 S1 S2 S3 (A) (B) y m (0,0,0) x Figure 2. (A) presents source (S: red circles) and receiver (R: blue circles) positions for measurements at diFigurefferent 2. source–receiver (A) presents source distances. (S: red ( Bcircles)–D) present and receiver source (R: and blue receiver circles) positions positions for for measurements measurements in theat different large, medium source– andreceiver small distances. space respectively (B–D) present (exact source source and and receiver receiver positions positions for are measurements provided in Tablesin the A1large, and medium A2 in Appendix and smallA ).space respectively (exact source and receiver positions are provided in Tables A1 and A2 in Appendix A). 2.1.3. Measurements at Different Spaces 2.2. MeasurementsIn this part of thewith research, A Dodecahedron acoustic measurementsSpeaker were performed at different spaces. The purpose of theseMeasurements measurements performed was to also evaluate with athe dodecahedron accuracy of speaker the handclap for comparison as an acoustic reasons source according for measurementsto ISO 3382-1 [ at5].di Thefferent source spaces positions and with of the diff dodecahedronerent background speaker noise were levels. the same as the handclap sourceMeasurements positions that were were performedused in previous in three part spaces of the ofresearch the Hellenic (measurements Mediterranean at different University, source– 3 Departmentreceiver positions, of Music measure Technologyments andat different Acoustics spaces with) volumesdepicted ofin 1088,Figure 176 2. and 146 m (Figure2B–D, respectively).A dodecahedron The first loudspeaker, space was the 01 one dB that-Stell, was Type used DO12 for measurements (01 dB-Stell, Limonest, at different France) source–receiver was used. distancesThe ESS signal in a previous [9] was utilized part of for this impulse research. response The hand measurements. configuration The that particular was used excitation for this signal set of measurementswas preferred was because the one of that the was low identified background as optimum noise in[12 di,ff13erent]. The sections sampling of the frequency study (Sections for the3.1 andmeasurements 4.1). For each was case, 44.1 impulse kHz. An response appropriate measurements sequence length were performedand time constant as specified for the in theESS source signal andwas receiver chosen positions according depicted to the in expected Figure2B–D. T. Three iterations were performed for each of the measurement positions. Averaging was used for better a signal-to-noise ratio and to reduce the 2.2. Measurements with A Dodecahedron Speaker temperature fluctuation effect. Temperature fluctuation effects were small and averaged out. The heightMeasurements of the dodecahedron performed in also each with measurement a dodecahedron was 1.3 speaker m, similar for comparison to the height reasons of the according handclap to ISOposition 3382-1 in [ 5the]. Themeasurements source positions (instead of the of dodecahedronthe 1.5 m height speaker that is were proposed the same by asISO the 3382 handclap-1). All source of the positionsmeasurement that were positions used inof previous the omnidirectional part of the research microphone (measurements (Earthworks at different M30, source–receiver Type 4190, positions, measurements at different spaces) depicted in Figure2. A dodecahedron loudspeaker, 01 dB-Stell, Type DO12 (01 dB-Stell, Limonest, France) was used. The ESS signal [9] was utilized for impulse response measurements. The particular excitation signal was preferred because of the low background noise [12,13]. The sampling frequency for the measurements was 44.1 kHz. An appropriate sequence length and time constant for the ESS signal was chosen according to the expected T. Three iterations were performed for each of the measurement positions. Averaging was used for better a signal-to-noise ratio and to reduce the temperature fluctuation effect. Temperature fluctuation effects were small and averaged out. The height of the dodecahedron in each measurement was 1.3 m, similar to the height of the handclap position in the measurements (instead of the 1.5 m height that is proposed by ISO 3382-1). All of the measurement positions of the omnidirectional microphone (Earthworks M30, Type 4190, Earthworks, Milford, CT, USA) with a flat frequency response was used. The same microphone was used also for all the measurements with a handclap as a sound source. Acoustics 2020, 2 231

2.3. Acoustic Parameters-Compensation Method

Reverberation time (T), early decay time (EDT), and clarity (C80) were the acoustic parameters that were compared with a handclap and a dodecahedron as acoustic sources. These parameters were selected, as they are representative of the acoustics of space T which is related to the physical properties of the space; while as stated in ISO 3382-1, EDT is a quantity related to perceived reverberance, and C80 is one of the quantities related to perceived clarity of sound. T is defined as “the duration required for the space-averaged sound energy density in an enclosure to decrease by 60 dB after the source emission has stopped” [5]. It is evaluated from the slope of the logarithmic impulse response–decay curves which is determined from the slope of the best-fit linear regression line [5]. Decay curves are a graphical representation of the decay of the sound pressure level in a room as a function of time after the sound source has stopped. Two methods of measuring T and decay curves are described in the ISO 3382-1: the interrupted noise method and the integrated impulse response method. The latter will be applied for this research for the measurement of T (as well as for EDT and C80). The decay curve for each octave band will be calculated by a backward integration of the squared impulse response. Measurements of T can also be performed for a smaller dynamic range than 60 dB. As it is stated in ISO 3382-1 [5]: “T can be evaluated based on a smaller dynamic range than 60 dB and extrapolated to a decay time of 60 dB. It is then labeled accordingly. Thus, if T is derived from the time at which the decay curve first reaches 5 dB and 25 dB below the initial level, it is labeled T20. If decay values of 5 dB to 35 dB below the initial level are used, it is labeled T30.” It also stated that [5] “if only T20 is to be measured, it is sufficient to create a level at least 35 dB above the background level in each frequency band. If T30 is to be measured, it is necessary to create a level at least 45 dB above the background level in each frequency band”. Also, other forms of T with even smaller dynamic ranges can be found in the literature (e.g., T15 [34]). However, it has to be noted that the larger the available section of the decay curve used to extrapolate Tx to T60 (x < 60), the more Tx approaches T60 [49]. In the context of this work, due to the low sound pressure levels generated by handclaps, especially in the low frequency range, T20 will be measured and compared. Early decay time is defined as the time interval required for the sound energy level to decay 10 dB after the excitation has stopped [42], and it is evaluated from the slope of the impulse response–decay curves (as the conventional T). The slope of the decay curve is determined from the slope of the best-fit linear regression line of the initial 10 dB (between 0 dB and 10 dB) of the decay. To enable direct − comparison with the T, the result is multiplied by a factor of 6. For an ideal exponential decay in a diffuse field, the expected value of the EDT equals T. EDT correlates better with the human perception of reverberation of a room than the T [50], hence it gives a better subjective evaluation. Clarity (C80), or “early to late index”, is defined as the logarithmic ratio of the impulse response’s energy before time te and its energy after te [51] (Equation (2)). The value te = 80 ms is better suited for music, while te = 50 ms is used to express the clarity of speech.

R t e p2(t)dt 0 Cte = 10 log R (2) ∞ p2(t)dt te

Cte is the early-to-late index (C80 is usually “clarity”); te is the early time limit of either 50 ms or 80 ms; p2(t) is the instantaneous sound pressure of the impulse response measured at the measurement point. High values of C80 indicate a large amount of the early energy which corresponds to a subjective sensation of clarity. On the contrary, a low clarity values indicate an unclear, excessively reverberant sound. Subjectively, acceptable values for C80 is –3dB or higher [52]. Background stationary noise is inevitable in every measurement, as it affects the decay range of the impulse response–decay curves. The effect of the background noise on the evaluation of room acoustic parameters has been studied by Hak et al. [49]. Five methods which compensate for the effect Acoustics 2020, 2 232 of background stationary noise were evaluated in a study by Guski and Vorlander [53]. It was found that the performance of each method differs significantly. For this study, the “subtraction of noise” method proposed by Chu [54] were utilized, as it was found to be among the best methods [53]. For this method, the noise level is estimated and subtracted from the impulse response before backward integration of the squared impulse response. The effectiveness of the method (as well as other methods) in various measurements of different acoustic parameters can be found in References [53] and [55].

3. Results The results are presented in accordance with the subsections in the methodology. Those are:

Measurements to Investigate Optimum Hand Configuration • Measurements at Different Source–Receiver Distances • Measurements at Different Spaces • 3.1. Measurements to Investigate Optimum Hand Configuration The results for the measurements for the eleven hand configurations are presented in Table1. Arithmetic mean of the SPL for the 24 subjects for each hand configuration along with the standard deviation (SD), the maximum SPL, and the percentage of the natural configuration are shown. In Figure3 (left), a comparison of the SPL and the percentage of the natural configuration (right) for the measured hand configurations are presented. As presented in Table1, the A2 hand configuration had the maximum mean SPL observed among the subjects. However, it can be seen that the A1+ hand configuration (Figures3 and4) had the maximum SPL observed in the low frequency range (125 Hz and 250 Hz octave band).

3.2. Measurements at Different Source–Receiver Distances Measurements were performed to observe the effect of different source–receiver distances in the evaluation of acoustic parameters. It is reasonable to assume that at shorter source–receiver distances, the signal-to-noise ratio will be higher. The purpose of these measurements was to estimate a recommended source–receiver distance for which acoustic measurements can be performed with the best possible accuracy. Measurements were performed in an amphitheater of the Hellenic Mediterranean University, Department of Music Technology and Acoustics at source–receiver distances of 4 to 12 m with the distance increasing by 1 m in each measurement (Figure2A, Table A1). The minimum recommended source–receiver distance (dmin) for the amphitheater according to Equation (2) is approximately 3.56 m. Measurements were also performed at shorter distances than dmin. Inconsistencies in the results were observed, especially for the measurements with a dodecahedron speaker, so no comparison for those results are presented. A possible explanation is probably due to the fact that the measurements were performed in the near field of the source [56]. The acoustic field of a source is divided into the near field and the far field. The near field can also be divided in the hydrodynamic near field which extends approximately 1/4 of an acoustic wavelength from the source and the geometric near field which extends several source dimensions from the source. The subject is beyond the scope of this research, but the reader can refer to Reference [56] for a detailed explanation. Also, in the near field, an ordinary loudspeaker cannot be considered to be a point source [57]. All the measurements for different source–receiver distances were performed with hand configurations A2 and A1+ that were found to be the optimum for acoustic measurements in the previous section (more on this on Section 4.1). A comparison of the acoustic parameters T20, EDT, C80 measured with a handclap and a dodecahedron speaker is presented in the following subsections. Acoustics 2020, 2 233

TableAcoustics 1. 2020Mean, 2 FOR Sound PEER Pressure REVIEW Level (SPL), standard deviation, and maximum SPL for the measured10 hand configurations. research, but the reader can refer to Reference [56] for a detailed explanation. Also, in the near field, Hand Configuration P1 P2 P3 A1 A2 A3 A1+ A1 O1 O2 O3 an ordinary loudspeaker cannot be considered to be a point source [57]. − SPLAll (dB) the measurements 84.2 for 83.2 different 83.0 source 83.0–receiver 85.2 distances 84.1 83.8 were performed 84.1 76.0 with 84.3 hand 79.5 Standardconfigurations Deviation (dB) A2 and 5.61 A1+ that 7.48 were 6.43 found 5.90to be the 6.28 optimum 7.05 for 4.47acoustic 5.55 measurements 5.69 7.39in the 4.49 Maximum SPL (dB) 95.4 97.0 95.8 97.0 96.3 95.5 89.8 91.8 86.8 95.3 86.1 Naturalprevious Configuration section (%) (more4.54 on this 18.2on Section 0.00 4.1.). 9.00A comparison 63.6 of 0.00 the acoustic 4.54 parameters 0.00 0.00 T20, EDT, 0.00 C80 0.00 measured with a handclap and a dodecahedron speaker is presented in the following subsections.

P1 A1

P2 A2

P3 A3 SPL SPL (dB)

O1 A1+

O2 A1-

O3 Frequency (Hz)

Figure 3. Octave frequency bands for the eleven hand configurations. Subfigures present the octave AcousticsFigure 2020, 3. 2 FOROctave PEER frequency REVIEW bands for the eleven hand configurations. Subfigures present the octave 11 frequencyfrequency band forband each for each hand hand configuration configuratio denotedn denoted by by the the abbreviation abbreviation for each each configuration. configuration.

90 P1 P2 P3 A1 A2 A3 A1+ A1- O1 O2 O3

75

60

45

30 31.5 63 125 250 500 1000 2000 4000 8000 16000 FigureFigure 4. Comparison 4. Comparison of of the the octave octave frequencyfrequency bands bands for for the the eleven eleven hand hand configurations configurations.. Figure 4. Comparison of the octave frequency bands for the eleven hand configurations.

3.2.1.3.2.1.T20 forT20 for Diff Derentifferent Source–Receiver Source–ReceiverDistances Distances 3.2.1. T20 for Different Source–Receiver Distances Measurement of T has been the focus of earlier studies [32–35] as presented in the introduction. Since the handclapMeasurement as an acousticof T has sourcebeen the will focus be used of earlier mainly studies as a survey [32–35 method,] as presented satisfactory in the measurement introduction. of

Since the handclap as an acoustic source will be used mainly as a survey method, satisfactory measurement of T is important. In Figure 5, T20 results for measurements performed with a dodecahedron speaker and with two different hand configurations at nine different source–receiver distances are presented.

S-R1: 4 m S-R2: 5 m S-R3: 6 m

) (s 20 20 T

S-R4: 7 m S-R5: 8 m S-R6: 9 m 2

1.5

1

0.5

0 125 250 500 1000 2000 4000 8000 S-R7: 10 m S-R8: 11 m S-R9: 12 m Frequency (Hz)

Figure 5. T20 measurements at different source–receiver (S–R) distances. Error bars represent measured standard deviation.

3.2.2. EDT for Different Source–Receiver Distances Similar to the previous subsection, EDT results for measurements performed with a dodecahedron speaker and with two different hand configurations at nine different source–receiver distances are presented in Figure 6.

Acoustics 2020, 2 FOR PEER REVIEW 11

AcousticsAcoustics2020, 20202 , 2 FOR PEER REVIEW 11 234 Measurement of T has been the focus of earlier studies [32–35] as presented in the introduction. SinceMeasurement the handclap of as T has an acousticbeen the sourcefocus of will earlier be studies used mainly [32–35 as] as a presented survey method, in the introduction. satisfactory measurement of T is important. In Figure 5, T20 results for measurements performed with a T is important.Since the handclap In Figure as5, anT acousticresults for source measurements will be used performed mainly as with a survey a dodecahedron method, satisfactory speaker and dodecahedron speaker and20 with two different hand configurations at nine different source–receiver with twomeasurement different hand of T configurationsis important. In at Figure nine different 5, T20 results source–receiver for measurements distances performed are presented. with a distancesdodecahedron are presented. speaker and with two different hand configurations at nine different source–receiver distances are presented.

S-R1: 4 m S-R2: 5 m S-R3: 6 m

S-R1: 4 m S-R2: 5 m S-R3: 6 m

)

(s

20 20

)

T

(s

20 20 T

S-R4: 7 m S-R5: 8 m S-R6: 9 m

S-R4: 7 m S-R5: 8 m S-R6: 9 m

S-R7: 10 m S-R8: 11 m S-R9: 12 m

S-R7: 10 m FrequencyS-R8: 11 m(Hz) S-R9: 12 m Frequency (Hz) Figure 5. T20 measurements at different source–receiver (S–R) distances. Error bars represent Figure 5. T measurements at different source–receiver (S–R) distances. Error bars represent measured measured20 standard deviation. standardFigure deviation. 5. T20 measurements at different source–receiver (S–R) distances. Error bars represent measured standard deviation. 3.2.2. EDT for Different Source–Receiver Distances 3.2.2. EDT for Different Source–Receiver Distances 3.2.2.Similar EDT for toD ifferent the previous Source– R subsection,eceiver Distances EDT results for measurements performed with a Similar to the previous subsection, EDT results for measurements performed with a dodecahedron dodecahedronSimilar to speaker the previous and with subsection,two different EDT hand results configurations for measurements at nine different performed source– receiver with a speaker and with two different hand configurations at nine different source–receiver distances are distancesdodecahedron are presented speaker andin Figure with two6. different hand configurations at nine different source–receiver

presenteddistances in Figure are presented6. in Figure 6.

)

(s

) (s

S-R1: 4 m S-R2: 5 m S-R3: 6 m EDT

S-R1: 4 m S-R2: 5 m S-R3: 6 m

EDT

S-R4: 7 m S-R5: 8 m S-R6: 9 m

S-R4: 7 m S-R5: 8 m S-R6: 9 m

Figure 6. Cont.

Acoustics 2020, 2 235 Acoustics 2020, 2 FOR PEER REVIEW 12

S-R7: 10 m S-R8: 11 m S-R9: 12 m Frequency (Hz)

FigureFigure 6. Early 6. Early decay decay time time measurements measurements at differentdifferent source source–receiver–receiver (S-R) (S-R) distances. distances. Error Error bars bars representrepresent measured measured standard standard deviation. deviation.

3.2.3. C80 for Different Source–Receiver Distances 3.2.3. C80 for Different Source–Receiver Distances

Finally, C80 results for measurements performed with a dodecahedron speaker and with two Finally, C80 results for measurements performed with a dodecahedron speaker and with two different hand configurations at nine different source–receiver distances are presented in Figure 7. differentAcoustics hand 2020 configurations, 2 FOR PEER REVIEW at nine different source–receiver distances are presented in Figure127 .

3.3. Measurements at Different Spaces Measurements were performed at three different spaces and for three different source–receiver positions (Figure2B–D, Table A2) to evaluate the accuracy of the handclap as an acoustic source for measurements at different spaces and also to observe the effect of different levels of background noise.

Background noiseS-R1R7 levels: 410 m m for the three spaces alongS-R2R8: 511with m m handclap configurationsS-R3R9: 6 noise12 m m levels are presented in Figure8. Frequency (Hz)

All the measurements for this section were performed with hand configuration A1+ that was found

) Figure 6. Early decay time measurements at different source–receiver (S-R) distances. Error bars dB

to be( the optimum for acoustic measurements in the previous section. Appropriate source–microphone

represent measured standard deviation. 80

distancesC were selected according to the results from the previous section of the research (more on the selection3.2.3. of C80 the for hand Different configurations Source–Receiver and D optimumistances distances in Section 4.1).

A comparison of the acoustic parameters T , EDT, C , measured with a handclap and a Finally, SC-R480 :results 7 m for measurements performedS-R520: 8 mwith a dodecahedron80 speakerS-R6 and: 9 m with two dodecahedrondifferent hand speaker configurations for the three at nine spaces, different is presented source–receiver in the distances following are subsections. presented in Figure 7.

S-R7: 10 m S-R8: 11 m S-R9: 12 m Frequency (Hz) S-R1: 4 m S-R2: 5 m S-R3: 6 m Figure 7. C80 measurements at different source–receiver (S–R) distances. Error bars represent

measured standard deviation.

) dB

( 3.3. Measurements at Different Spaces

80 C Measurements were performed at three different spaces and for three different source–receiver positions (Figure 2B–2D, Table A2) to evaluate the accuracy of the handclap as an acoustic source for

measurementsS- R4at : different7 m spaces and also to observeS-R5: 8 mthe effect of different levelsS -ofR6 bac: 9 mkground noise. Background noise levels for the three spaces along with handclap configurations noise levels are presented in Figure 8. All the measurements for this section were performed with hand configuration A1+ that was found to be the optimum for acoustic measurements in the previous section. Appropriate source– microphone distances were selected according to the results from the previous section of the research (more on the selection of the hand configurations and optimum distances in Section 4.1.). S-R7: 10 m S-R8: 11 m S-R9: 12 m A comparison of the acoustic parameters T20, EDT, C80, measured with a handclap and a dodecahedron speaker for the three spaces, is Frequencypresented (Hz)in the following subsections.

Figure 7. C80 measurements at different source–receiver (S–R) distances. Error bars represent Figure 7. C80 measurements at different source–receiver (S–R) distances. Error bars represent measured measured standard deviation. standard deviation. 3.3. Measurements at Different Spaces Measurements were performed at three different spaces and for three different source–receiver positions (Figure 2B–2D, Table A2) to evaluate the accuracy of the handclap as an acoustic source for measurements at different spaces and also to observe the effect of different levels of background noise. Background noise levels for the three spaces along with handclap configurations noise levels are presented in Figure 8. All the measurements for this section were performed with hand configuration A1+ that was found to be the optimum for acoustic measurements in the previous section. Appropriate source– microphone distances were selected according to the results from the previous section of the research (more on the selection of the hand configurations and optimum distances in Section 4.1.). A comparison of the acoustic parameters T20, EDT, C80, measured with a handclap and a dodecahedron speaker for the three spaces, is presented in the following subsections.

Acoustics 2020, 2 236 Acoustics 2020, 2 FOR PEER REVIEW 13

Acoustics 2020, 2 FOR PEER REVIEW 13

Acoustics 2020, 2 FOR PEER REVIEW 13 100 A1+ 80 Large space

Medium space Small space

60

SPL SPL (dB)

40 SPL (dB) SPL 20

SPL SPL (dB) 0

31.5 63 125 250 500 1000 2000 4000 8000 16000 Frequency (Hz) Frequency (Hz)

FigureFigure 8.8. SPLSPL forfor A1A1++ hand configurationconfiguration andand backgroundbackground noisenoise forfor thethe threethree spaces.spaces. Figure 8. SPL for A1+ hand configurationFrequency and background (Hz) noise for the three spaces. 3.3.1. T for DifferentFigure Spaces8. SPL for A1+ hand configuration and background noise for the three spaces. 3.3.1. T2020 for Different Spaces 3.3.1. T20 for Different Spaces In Figure9, T results for measurements performed with a dodecahedron speaker and hand In3.3.1. FigureIn T 20Figure for9, DT20ifferent20 9 ,results T20 results Spaces for for measurements measurements performed with with a dodecahedron a dodecahedron speaker speaker and hand and hand configuration A1+ for three different spaces are presented. configurationconfigurationIn Figure A1+ for A1+9, Tthree for20 results three different differentfor measurements spaces spaces areare performedpresentpresented.ed. with a dodecahedron speaker and hand configuration A1+ for three different spaces are presented. 2 2

1.5 1.5

1 1

0.5 0.5 arge space L

0 0

125 250 500 1000 2000 4000 8000 125 250 500 1000 2000 4000 8000 space Large space Large 2 2 2

1.5 1.5 1.5

) (s

1 1 1

)

20 20

) (s

T 20 20 (s 0.5 0.5 0.5

edium edium space

T 20 20 M T 0 0 0

125 250 500 1000 2000 4000 8000 125 250 500 1000 2000 4000 8000 125 250 500 1000 2000 4000 8000

Medium space

2 2 2 Medium Medium space

1.5 1.5 1.5

1 1 1

0.5 0.5 0.5 mall space S

0 0 0 Small space 125 250 500 1000 2000 4000 8000 125 250 500 1000 2000 4000 8000 125 250 500 1000 2000 4000 8000

S1-R1 S2-R2 S3-R3

S1-R1 S2-R2 S3-R3 Small space Frequency (Hz) Frequency (Hz) Figure 9. T20 measurements at three positions for three different spaces (i.e., large, medium, small). Figure 9.FigureS1T-20R1 9.measurements T20 measurements at threeat three positions positions for S2 three- threeR2 different different spaces spaces (i.e., (i.e.,large, large, medium, medium, small)S3-R3. small). Error bars represent measured standard deviation. Error barsError represent bars represent measured measured standard standard deviation. deviation.Frequency (Hz)

3.3.2.Figure EDT3.3.2. for 9.EDT T Di20 f fformeasurementserent Different Spaces Spaces at three positions for three different spaces (i.e., large, medium, small). Error barsSimilar represent to the measured previous standard subsection, deviation. EDT results for measurements performed with a SimilarSimilar to the previous to the previous subsection, subsection, EDT results EDT for results measurements for measurements performed performed with a dodecahedron with a dododecahedrondecahedron speaker and with hand configuration A1+ for three different spaces are presented in speaker and with hand configuration A1+ for three different spaces are presented in Figure 10. 3.3.2. EDTFigure for 10 D.. ifferent Spaces

Similar2 to the previous subsection, EDT results for measurements performed with a

)

(s

1.5 dodecahedron speaker and with hand configuration A1+ for three different spaces are presented in ) pace (s

Figure 1 10. EDT

EDT 0.5 arge s

L

Large space Large

) 0 125 250 500 1000 2000 4000 8000

(s

Figure 10. Cont.

EDT Large space Large

Acoustics 2020, 2 237

Acoustics 2020, 2 FOR PEER REVIEW 14

Acoustics 2020, 2 FOR PEER REVIEW 14

Medium Medium space

Medium Medium space

Small space

Small space S1-R1 S2-R2 S3-R3

Frequency (Hz) S1-R1 S2-R2 S3-R3 Frequency (Hz) FigureFigure 10. EDT 10. EDT measurements measurements at at three three positionspositions for for three three different different spaces spaces (i.e., large, (i.e., medium, large, medium, small). small). Error bars represent measured standard deviation. Error barsFigure represent 10. EDT measuredmeasurements standard at three positions deviation. for three different spaces (i.e., large, medium, small). Error bars represent measured standard deviation. 3.3.3. C80 for Different Spaces 3.3.3. C80 for Different Spaces 3.3.3.Finally, C80 for CD80ifferent results S forpaces measurements performed with a dodecahedron speaker and with hand Finally,configuration C80 results A1+ for for three measurements different spaces performed are presented with in Figure a dodecahedron 11. speaker and with hand Finally, C80 results for measurements performed with a dodecahedron speaker and with hand

configurationconfiguration A1+ forA1+ three for three diff differenterent spaces spaces are are presentedpresented in in Figure Figure 11. 11.

Large space Large

Large space Large

)

(dB

)

80

C

(dB

80

Medium Medium space C

Medium Medium space

Small space

Small space S1-R1 S2-R2 S3-R3

Frequency (Hz) S1-R1 S2-R2 S3-R3

Figure 11. C80 measurements at three positionsFrequency for three (Hz) different spaces (i.e., large, medium, small). Error bars represent measured standard deviation. Figure 11. C80 measurements at three positions for three different spaces (i.e., large, medium, small). Figure 11. C80 measurements at three positions for three different spaces (i.e., large, medium, small). Error bars represent measured standard deviation. Error4. Discussion bars represent measured standard deviation. 4. Discussion 4. DiscussionThe discussion is presented in accordance with the results and methods in previous sections. TopicsThe of thediscussion discussion is presented are: in accordance with the results and methods in previous sections. • TheTopics discussion ofSuggested the discussion is presented Hand are: Configuration in accordance for Acoustic with Measurements the results and methods in previous sections. • Topics of the• discussion MeasurementSuggestedare: Hand of T Configuration with a Handclap for Acoustic Measurements • • MeasurementMeasurement of of EDT T with with a Handclap a Handclap • 80 Suggested• MeasurementMeasurement Hand Configuration of of C EDTwith with fora Handclap a AcousticHandclap Measurements • • Suggested Steps for Measurements with a Handclap as an Acoustic Source Measurement• Measurement of T with of a C Handclap80 with a Handclap • Measurement• Suggested of EDT Steps with for a Measurements Handclap with a Handclap as an Acoustic Source • Measurement of C80 with a Handclap • Suggested Steps for Measurements with a Handclap as an Acoustic Source • Acoustics 2020, 2 238

4.1. Suggested Hand Configuration for Acoustic Measurements Since the main purpose of this study was to utilize the handclap in the best way possible, the first step was to suggest the optimal hand configuration among the eleven presented (Figure1). The signal-to-noise ratio is of great importance in acoustic measurements; hence, the first candidate was A2 which was found to have the highest SPL (Table1). Hand configuration A1 + was the second candidate with the highest SPL levels in low frequency octave bands (Figures3 and4, mainly 125 and 250 Hz). As it was stated before, the addition of a Helmholtz-type resonance, which is involved in the case of domed impacts, such as hand configuration A1+ [24], is the reason for this characteristic. The two hand configurations (i.e., A2 and A1+) were both tested for comparison reasons in Section 3.2. Measurements for T20, EDT, and C80 (Figures5–7, respectively) revealed that hand configuration A1+ provided better results with smaller deviation and SD than A2, compared to measurements with a dodecahedron speaker (especially in the low frequency range). We believe that the superiority of hand configuration A1+ for acoustic measurements will apply to other spaces besides those applied in this study. The octave band background noise levels and the distribution of those levels in the spectrum measured in this study (Figure8) are typical of those found indoors in various environments (e.g., library, office, church [58]). It can be seen (Figure8) that the smallest signal-to-noise ratio was among the low frequency octave bands. The same will possibly apply to other spaces, since the octave band noise distribution will be similar [58]. The same issue was identified in a study by Seetharman and Tarzia [33], where measurements with a handclap did not produce reliable results in the low frequency range. T results were presented only up to 250 Hz in their study, since as the authors stated, “at low frequencies the clap lacked sufficient energy to compute a reasonable reverberation time”. The same can be seen in the study by Huang [34], where the results for T are presented only from 250 Hz octave band and above. Also, the results for the 250 Hz octave band had the largest deviation compared to values found in the literature. Therefore, it seems reasonable to assume that the hand configuration that would provide the best possible signal-to-noise levels for most frequency bands in typical spaces would be preferable for acoustic measurements. This study suggests that for measurements with typical background noise levels, hand configuration A1+ is the one that will yield the best results.

4.2. Measurement of T with A Handclap Since the handclap in practice will be utilized mainly for surveying acoustic measurements, the best possible estimation of T is one of the main purposes of this research. As stated earlier in the methods (Section 2.3), due to the low sound pressure levels generated by handclaps, especially in the low frequency range, T20 was the proposed form of T to be estimated which is also presented in ISO 3382-1 [5]. The following discussion applies to the measurement of T20 for the A1+ hand configuration which was suggested for acoustic measurements in the previous subsection. Also, in the following paragraphs, the subjects of suggested measurement of source–receiver distance and expected accuracy are discussed. Measurements at an increasing source–receiver distance (Figure5) revealed that the mean absolute error of T20 also increases, especially for the 125 Hz octave band, where the signal-to-noise level is small. In Figure 12, the mean absolute error among octave bands of T20 among measurements with a dodecahedron speaker and a handclap as a sound source for different source–receiver distances is presented. The difference is expected to be greater in larger source–receiver distances (for measurements in larger spaces). However, this remains to be verified in future work. Therefore, for estimation of T20, source–receiver distances near the minimum distance according to Equation (1) are suggested. The influence of the signal-to-noise ratio for measurements with a handclap was investigated by analyzing the T20 measurements at different spaces. From the obtained impulse responses, T20 was calculated for the octave bands from 125 Hz through 8 kHz resulting in 63 calculated values for the three spaces and the three positions for each space. Figure 13 shows the measured T20 parameter value errors in steps of the just noticeable difference (JND) as a function of the signal-to-noise ratio Acoustics 2020, 2 FOR PEER REVIEW 16

Measurements at an increasing source–receiver distance (Figure 5) revealed that the mean absolute error of T20 also increases, especially for the 125 Hz octave band, where the signal-to-noise level is small. In Figure 12, the mean absolute error among octave bands of T20 among measurements with a dodecahedron speaker and a handclap as a sound source for different source–receiver distances is presented. The difference is expected to be greater in larger source–receiver distances (for measurements in larger spaces). However, this remains to be verified in future work. Therefore, for Acoustics 2020, 2 FOR PEER REVIEW 16 estimation of T20, source–receiver distances near the minimum distance according to Equation (1) are suggested. Measurements at an increasing source–receiver distance (Figure 5) revealed that the mean 0.08

absolute error of T20 also increases, especially for the 125 Hz octave band, where the signal-to-noise level is small. In Figure 120.06, the mean absolute error among octave bands of T20 among measurements Acoustics 2020, 2 239 with a dodecahedron speaker and a handclap as a sound source for different source–receiver 0.04 distances is presented. TheM.A.E. (s) difference is expected to be greater in larger source–receiver distances (for

formeasurements different octave in larger bands spaces).0.02 (Figure However8). The value, this of remains JND (5%) to be was verified selected in according future work. to ISO Therefore, 3382-1 [ for5]. 4 5 6 7 8 9 10 11 12 estimation of T20, source–receiver distances near the minimum distance according to Equation (1) are

Results indicate that for SNRs larger than 35 dB, all the values of JND are smaller than three. Source–receiver (S–R) distances (m) suggested. 0.08

Figure 12. Mean absolute error among octave bands of T20 among measurements with a dodecahedron speaker and a handclap as a sound source for different source–receiver distances. 0.06

The influence of the signal0.04 -to-noise ratio for measurements with a handclap was investigated by M.A.E. (s) analyzing the T20 measurements at different spaces. From the obtained impulse responses, T20 was calculated for the octave b0.02ands from 125 Hz through 8 kHz resulting in 63 calculated values for the 4 5 6 7 8 9 10 11 12 three spaces and the three positions forSource each–receiver space. Figure (S–R) distances 13 shows (m) the measured T20 parameter value errors in steps of the just noticeable difference (JND) as a function of the signal-to-noise ratio for Figure 12. Mean absolute error among octave bands of T20 among measurements with a dodecahedron differentFigure octave 12. Mean bands absolute (Figure error 8). among The value octave of bands JND of (5%)T20 among was selected measurements according with a to dodecahedron ISO 3382-1 [5]. Resultsspeakerspeaker indicate andand athata handclaphandclap for SNR asass aa larger soundsound sourcethansource 35 forfor dB, didifferentff allerent the source–receiver sourcevalues– receiverof JND distances. distancesare smaller. than three. It has to be noted that above 35 dB, all the values of JND are smaller than two, except three values ItThe has influence to be noted of the that signal above-to 35-noise dB, all ratio the for values measurements of JND are smallerwith a handclap than two, was except investigated three values by which correspond to measurements for the 8 KHz octave band (large space: S3–R3, medium space: whichanalyzing correspond the T20 measurements to measurements at different for the 8 spaces. KHz octave From band the obtained (large space: impulse S3–R3, responses medium, T space:20 was S1–R1, S3–R3). In general, it can be seen in measurements of T20 (Figures 5, 9) that for the 8 KHz octave S1–R1,calculated S3–R3). for the In general,octave b itands can from be seen 125 in Hz measurements through 8 kHz of Tresulting(Figures in 563 and calculated9) that for values the 8 for KHz the band, all the results from the measurements with a handclap 20(for both hand configurations) had a octavethree spaces band, and allthe the resultsthree pos fromitions the for measurements each space. Figure with 13 a handclap shows the (for measured both hand T20 paramete configurations)r value lower value compared to measurements with a dodecahedron speaker. This result seems to be haderrors a lower in steps value of comparedthe just noticeable to measurements difference with (JND) a dodecahedron as a function speaker. of the signal This result-to-noise seems ratio to for be systemic and will be addressed in a future publication. A possible explanation could be due to the systemicdifferent andoctave will bands be addressed (Figure 8 in). aThe future value publication. of JND (5%) A was possible selected explanation according could to ISO be due3382 to-1 the[5]. source directivity. San Martin and Arena [30] studied the uncertainties caused by source directivity sourceResults directivity. indicate that San for Martin SNRs and larger Arena than [30 35] studieddB, all the the values uncertainties of JND causedare smaller by source than three directivity. in in room-acoustic investigations and found that small differences especially in the high frequency room-acousticIt has to be investigations noted that above and 35 found dB, all that the small values di ffoerencesf JND are especially smaller than in the two high, except frequency three values range range cause variations in the expected measurements. A deviation greater than half the JND of the causewhich variations correspond in theto measurements expected measurements. for the 8 KHz A deviation octave band greater (large than sp halface: the S3 JND–R3, of medium the parameter space: parameter was found at 80% of receivers for the 8 KHz octave band. wasS1–R1, found S3– atR3). 80% In general of receivers, it can for be the seen 8 KHz in measurements octave band. of T20 (Figures 5, 9) that for the 8 KHz octave band, all the results from the measurements with a handclap (for both hand configurations) had a lower value compared to 6 measurements with a dodecahedron speaker. This result seems to be 5 JND

systemic and will be addressed in a future publication. A possible explanation could be due to the 4 source directivity. San Martin and Arena [30] studied the uncertainties caused by source directivity 3 in room-acoustic investigations2 and found that small differences especially in the high frequency range cause variations in the1 expected measurements. A deviation greater than half the JND of the parameter was found at 80%0 of receivers for the 8 KHz octave band. 15 20 25 30 35 40 45 50 55 60 65 Error in steps Errorsteps of in

6 Signal to noise ratio (dB) 5 JND

Figure 13. T2020 parameterparameter value4 value errors errors measured measured with with a ahandclap handclap (compared (compared to to measurements measurements with with a dodecahedron)a dodecahedron) in insteps steps of3 J ofust Just Noticeable Noticeable Difference Difference (JND) (JND) as a asfunction a function of the of signal the signal-to-noise-to-noise ratio forratio different for diff erentoctave octave bands.2 bands. The dashed The dashed line represents line represents a threshold a threshold below below which which all JND all values JND values were weremeasured. measured. 1 0 15 20 25 30 35 40 45 50 55 60 65 Error in steps Errorsteps of in In conclusion, for T measurements using the handclaphandclap asas anan acousticacoustic source,source, TT2200 measurements

are recommended due to the low signal level of the handclap, especially in the low frequency range are recommended due to the low signal levelSignal of to the noise handclap ratio (dB), especially in the low frequency range compared with background noise levels in usual environments [58]. Also, measurements of T near 20 dmin (EquationFigure 13. T (1))20 parameter are suggested. value errors Results measured of this with research a handclap indicate (compared that for to SNRsmeasurements larger than with 35a dB, the valuesdodecahedron) of JND are in smallersteps of J thanust Noticeable three. Difference (JND) as a function of the signal-to-noise ratio for different octave bands. The dashed line represents a threshold below which all JND values were 4.3. Measurementmeasured. of EDT with a Handclap

Similar to T20, the influence of the signal-to-noise ratio for the calculation of EDT with a handclap In conclusion, for T measurements using the handclap as an acoustic source, T20 measurements was investigated by analyzing measurements at the different spaces. From the obtained impulse are recommended due to the low signal level of the handclap, especially in the low frequency range responses, EDT was calculated for the octave bands from 125 Hz through 8 kHz, resulting in 63 calculated values for the three spaces and the three positions. Figure 14 shows the measured EDT parameter value errors in steps of JND as a function of the signal-to-noise ratio for different octave Acoustics 2020, 2 FOR PEER REVIEW 17

compared with background noise levels in usual environments [58]. Also, measurements of T20 near dmin (Equation (1)) are suggested. Results of this research indicate that for SNRs larger than 35 dB, the values of JND are smaller than three.

4.3. Measurement of EDT with a Handclap

Similar to T20, the influence of the signal-to-noise ratio for the calculation of EDT with a handclap was investigated by analyzing measurements at the different spaces. From the obtained impulse Acousticsresponses2020, ,EDT2 was calculated for the octave bands from 125 Hz through 8 kHz, resulting in 240 63 calculated values for the three spaces and the three positions. Figure 14 shows the measured EDT parameter value errors in steps of JND as a function of the signal-to-noise ratio for different octave bandsbands (Figure(Figure8 8).). The The value value of of JNDJND (5%)(5%) waswas selectedselected according according to to ISO ISO 3382-1 3382-1 [ [55].]. The results indicateindicate thatthat forfor SNRsSNRs largerlarger thanthan 3535 dB,dB, allall thethe valuesvalues ofof JNDJND areare smallersmaller thanthan five.five.

Error in steps Errorsteps of JND in

Signal-to-noise ratio (dB)

FigureFigure 14.14. EDT parameter value errors measured with aa handclaphandclap (compared(compared toto measurementsmeasurements withwith aa dodecahedron)dodecahedron) in steps of JND as a functionfunction of the signal-to-noisesignal-to-noise ratio for different different octave bands bands.. TheThe dasheddashed lineline representsrepresents aa thresholdthreshold belowbelow whichwhich allall JNDJND valuesvalues werewere measured.measured.

EvaluatingEvaluating the results, it cancan bebe seenseen thatthat thethe errorerror inin measurementsmeasurements ofof EDTEDT (evaluated(evaluated byby thethe JNDs)JNDs) is is greater greater than than the measurements the measurement of T20s .o Af possibleT20. A possible explanation explanation could be due could to the be directionality due to the ofdirectionality the handclap of and the thehandclap effect that and it the has effect in the that measurements it has in the ofmeasurements acoustic parameters. of acoustic As statedparameters in the. introduction,As stated in omnidirectionalitythe introduction, omnidirectionality of the handclap was of studied the handclap by Griesinger was studied [29] revealing by Griesinger directional [29] characteristics.revealing direct Theional eff characteristics.ect that the directionality The effect of that the thesource directionality has on the measurements of the source ofhas EDT on canthe bemeasurements found in some of EDT studies can [ 59be, 60found]. The in influence some studies of the [59 directionality,60]. The influence of the of source the directionality was performed of the in asource study was by Wang performed and Vingead in a study [59 ]by revealing Wang and that Vingead reverberation [59] revealing time (T30 that) changed reverberation very little time across (T30) sourcechanged types very and little orientations, across source while both types EDT and and orientations, C80 changed while significantly. both EDT The and same C80 conclusion changed wassignificantly. found in anotherThe same study conclusion [60], where was the found parameters in another dealing study with [60 early], wh sound,ere the such parameters as EDT anddealing C80, werewith moreearly sensitivesound, such than as the EDT parameters and C80, were dealing more with sensitive late sound than suchthe parameters as T20. These dealing studies with do late not contradictsound such the as resultsT20. These reported studies in thedo not previous contradict subsection the results concerning reported the in e fftheect previous of directionality subsection on theconcern measurementing the effect of T offor directionality the 8 KHz octave on the band measurement [30], since of they T for present the 8 resultsKHz octave only upband to the[30] 4, since KHz octavethey present band. results only up to the 4 KHz octave band.

4.4. Measurement of C80 with a Handclap 4.4. Measurement of C80 with a Handclap Acoustics 2020, 2 FOR PEER REVIEW 18 Similar to the previous subsection for the measurements of T20 and EDT, the influence of Similar to the previous subsection for the measurements of T20 and EDT, the influence of the the signal-to-noise ratio for the calculation of C80 with a handclap was investigated by analyzing signalmentioned-to-noise studies ratio [59 for,60 ] thereveal calculation that the effect of C of80 withdirectionality a handclap was wasvery profoundinvestigated in the by calculation analyzing measurements in different spaces. C80 was calculated for the octave bands from 125 Hz through 8 kHz, measurementsof C80. in different spaces. C80 was calculated for the octave bands from 125 Hz through 8 resulting in 63 values for the three spaces and the three positions. Figure 15 presents the measured C80 kHz, Another resulting factor in 63 affecting values formeasurement the three spacesof C80 is an thed scattering the three positions.effect of the Figure person 15 who presents performs the parameter value errors in steps of JND as a function of the signal-to-noise ratio for different octave measuredthe handc lap.C80 Inparameter a study byvalue Guski errors and inVorländer steps of [JND61], a as special a function measurement of the signal scenario-to-noise was designedratio for bands (Figure8). The value of JND (1 dB) was selected according to ISO 3382-1 [ 5]. The results indicate differentto investigate octave the bands effect (Figure of human 8). The-size value scattering of JND (1 objects. dB) was Measurements selected according of C80 to show ISO ed3382 a - large1 [5]. that for SNRs values larger than 35 dB, all the values of JND are smaller than 4.5. Theinfluence results of indicate the scattering that for object SNR,s whivaluesle the larger effect than for 35T wasdB, allnegligible. the values of JND are smaller than 4.5.

Evaluating the results, it can be seen that the error in measurements of C80 was greater than the 8 error of T20 but similar to that of EDT. Again, a possible explanation could be due to the directionality of the handclap and the effect6 that it has in the measurements of acoustic parameters. Previous

4

2

Error in steps Errorsteps of JND in 0 15 20 25 30 35 40 45 50 55 60 65 Signal to noise ratio (dB)

Figure 15. C80 parameter value errors measured with a handclap (compared to measurements Figure 15. C80 parameter value errors measured with a handclap (compared to measurements with witha a dodecahedron) dodecahedron) in stepsin steps of JND of JND as a functionas a function of the signal-to-noiseof the signal-to ratio-noise for diratfferentio for octave different bands. octave bands.The The dashed dashed line representsline represents a threshold a threshold below which below all which JND values all JND were values measured. were measured.

4.5. Suggested Steps for Measurements with A Handclap as an Acoustic Source In order to present a practical methodology that can be followed for acoustic measurements with a handclap as a sound source, the following steps are recommended by the authors: 1. Utilization of hand configuration A1+ for acoustic measurements (Figure 1). This study has shown that this hand configuration will provide the best possible signal-to-noise levels for most frequency bands in typical spaces and will yield the best results (more on this on Section 4.1.); 2. Training of hand configuration A1+ to optimize results is recommended. Training of measurements with a handclap has shown to result in better repeatability of the measurements [26];

3. Source–receiver distance in measurements should be near minimum distance, dmin, (Equation (2)) suggested by the ISO 3382-2 [48]. At shorter source–receiver distance, the signal-to-noise ratio is higher, therefore better measurements can be achieved (more on this on Section 4.2.); 4. If available, background noise levels should be measured in order to evaluate the accuracy of the results according to the signal-to-noise ratio. The results indicating probable measurement accuracy according to the signal-to-background noise level provided in this paper (more on this on Sections 4.2., 4.3., 4.4.); 5. Measurements for the same source–receiver positions should be performed at least four times and averaged; 6. A compensation method for the background noise should be applied in the processing of measurements; Background noise affects the decay range of the impulse response decay curves and the evaluation of the acoustic parameters (more on this on Section 2.3.). Available software for acoustics measurements that provides compensation methods can be found in Reference [62]. 7. If possible, measurements with a mobile phone should be avoided. Measurements of T with a handclap and different cellphones as sound recorders were found to provide different results [34], while in another study, differences of T were within +/−0.2 s compared to measurements with a sound level meter [35]. Measurements with a cellphone have been studied mainly for the measurements of SPL [63]. Differences were found across operating

Acoustics 2020, 2 241

Evaluating the results, it can be seen that the error in measurements of C80 was greater than the error of T20 but similar to that of EDT. Again, a possible explanation could be due to the directionality of the handclap and the effect that it has in the measurements of acoustic parameters. Previous mentioned studies [59,60] reveal that the effect of directionality was very profound in the calculation of C80. Another factor affecting measurement of C80 is the scattering effect of the person who performs the handclap. In a study by Guski and Vorländer [61], a special measurement scenario was designed to investigate the effect of human-size scattering objects. Measurements of C80 showed a large influence of the scattering object, while the effect for T was negligible.

4.5. Suggested Steps for Measurements with A Handclap as An Acoustic Source In order to present a practical methodology that can be followed for acoustic measurements with a handclap as a sound source, the following steps are recommended by the authors:

1. Utilization of hand configuration A1+ for acoustic measurements (Figure1). This study has shown that this hand configuration will provide the best possible signal-to-noise levels for most frequency bands in typical spaces and will yield the best results (more on this on Section 4.1); 2. Training of hand configuration A1+ to optimize results is recommended. Training of measurements with a handclap has shown to result in better repeatability of the measurements [26];

3. Source–receiver distance in measurements should be near minimum distance, dmin, (Equation (2)) suggested by the ISO 3382-2 [48]. At shorter source–receiver distance, the signal-to-noise ratio is higher, therefore better measurements can be achieved (more on this on Section 4.2); 4. If available, background noise levels should be measured in order to evaluate the accuracy of the results according to the signal-to-noise ratio. The results indicating probable measurement accuracy according to the signal-to-background noise level provided in this paper (more on this on Section 4.2, Section 4.3, Section 4.4); 5. Measurements for the same source–receiver positions should be performed at least four times and averaged; 6. A compensation method for the background noise should be applied in the processing of measurements; Background noise affects the decay range of the impulse response decay curves and the evaluation of the acoustic parameters (more on this on Section 2.3). Available software for acoustics measurements that provides compensation methods can be found in Reference [62]. 7. If possible, measurements with a mobile phone should be avoided. Measurements of T with a handclap and different cellphones as sound recorders were found to provide different results [34], while in another study,differences of T were within +/ 0.2 s compared to measurements with a sound − level meter [35]. Measurements with a cellphone have been studied mainly for the measurements of SPL [63]. Differences were found across operating systems (iOS and Android), across different versions of the operating system as well as applications (apps) for those systems. Difference in measurements can be attributed to factors such as small dynamic range, internal filters applied in the low frequency range as well as input gain control applied by (some of) the operating systems. In conclusion, measurements with a mobile phone depend on many factors and the expected accuracy is not easy to predict. However external microphones can be used for better results [64].

5. Conclusions This study provided one of the first comprehensive assessments of the handclap as a source for acoustic measurements. The findings of this study indicate the optimal hand configuration (A1+), a suggested source–receiver distance for acoustic measurements (near dmin), and also the expected deviation of T20, EDT, and C80 compared to measurements with a dodecahedron speaker in steps of JND as a function of the signal-to-noise ratio. Also, practical steps for measurements with a handclap as an acoustic source are suggested. Acoustics 2020, 2 242

We believe that our findings could be useful for a better utilization of a handclap as a source for acoustic measurements as well as for a better understanding of its inherent limitations. Our research may also be useful for other scientific fields that exploit the sound of a handclap for a multitude of reasons. With regard to the research methods, some limitations need to be acknowledged. The study was applied in a limited number of spaces. Therefore, in terms of directions for further work, the methodology applied could be investigated in a larger number of rooms in order to test the generalizability of the findings. We hope that our research will serve as a basis for future studies on further utilization of the handclap as a useful and convenient acoustic source.

Author Contributions: N.M.P. analyzed the data and wrote the manuscript; G.E.S. provided suggestions and guidance for the work and reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: We gratefully acknowledge the help provided by Konstantinos Lafazanis during various stages of this research. Thanks are also due to the students that participated in the measurement sessions from the Hellenic Mediterranean University, Department of Music Technology and Acoustics. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript:

A# Aligned (hand configuration) C80 Clarity EDT Early Decay Time ESS Exponential Sine Sweep ISO International Organization for Standardization JND Just Noticeable Difference MEMS Micro-Electromechanical Systems MLS Maximum Length Sequence O# Opposite (hand configuration) P# Parallel (hand configuration) SD Standard Deviation SNR Sound-to-Noise Ratio SPLs Sound Pressure Levels T Reverberation Time T20 Reverberation Time (based on a 20 dB dynamic range) T30 Reverberation Time (based on a 30 dB dynamic range) Ts Centre Time

Appendix A

Table A1. Source and receiver positions for measurements at different source–receiver distances corresponding to Figure2A.

Source Position Receiver Positions R1(4.58, 8.62, 1.53) R2(4.58, 9.62, 1.66) R3(4.58, 10.61, 1.80) R4(4.58, 11.60, 1.94) R5(4.58, 12.59, 2.09) S(4.58, 4.63, 1.30) R6(4.58, 13.58, 2.24) R7(4.58, 14.57, 2.38) R8(4.58, 15.56, 2.53) R9(4.58, 16.55, 2.68) Acoustics 2020, 2 243

Table A2. Source and receiver positions for measurements at different spaces (i.e., large, medium, small) corresponding to Figure2B–D, respectively.

Large Space Medium Space Small Space S1(3.36, 3.64, 1.30) R1(3.57, 7.71, 1.42) S1(2.55, 2.20, 1.30) R1(2.03, 3.78, 1.20) S1(1.40, 3.15, 1.30) R1(2.16, 5.76, 1.20) S2(5.92, 1.82, 2.20) R2(5.78, 5.77, 1.20) S2(4.70, 1.55, 1.30) R2(3.85, 3.98, 1.20) S2(3.15, 2.53, 1.30) R2(3.57, 4.98, 1.20) S3(8.80, 3.30, 1.30) R3(9.24, 7.27, 1.36) S3(5.62, 1.17, 1.30) R3(6.57, 3.02, 1.20) S3(4.20, 1.40, 1.30) R3(4.31, 3.35, 1.20)

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