Psychoacoustics and Its Benefit for the Soundscape

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ACTA ACUSTICAUNITEDWITH ACUSTICA Vol. 92 (2006) 1 – 1

Psychoacoustics and its Beneﬁt for the Soundscape Approach

Klaus Genuit, André Fiebig HEAD acoustics GmbH, Ebertstr. 30a, 52134 Herzogenrath, Germany. [klaus.genuit][andre.ﬁebig]@head- acoustics.de

Summary The increase of complaints about environmental noise shows the unchanged necessity of researching this subject. By only relying on sound pressure levels averaged over long time periods and by suppressing all aspects of quality, the specific acoustic properties of environmental noise situations cannot be identified. Because annoyance caused by environmental noise has a broader linkage with various acoustical properties such as frequency spectrum, duration, impulsive, tonal and low-frequency components, etc. than only with SPL [1]. In many cases these acoustical properties affect the quality of life. The human cognitive signal processing pays attention to further factors than only to the averaged intensity of the acoustical stimulus. Therefore, it appears inevitable to use further hearing-related parameters to improve the description and evaluation of environmental noise. A first step regarding the adequate description of environmental noise would be the extended application of existing measurement tools, as for example level meter with variable integration time and third octave analyzer, which offer valuable clues to disturbing patterns. Moreover, the use of psychoacoustics will allow the improved capturing of soundscape qualities.

PACS no. 43.50.Qp, 43.50.Sr, 43.50.Rq

1. Introduction disturbances and unpleasantness of environmental noise, a negative feeling evoked by sound. However, annoyance is The meaning of soundscape is constantly transformed and sensitive to subjectivity, thus the social and cultural back- modified. The inflationary use of this term refers to the grounds have an important influence on the subjective at- large diversity of interpretations with respect to the content titudes of people to noise [4] and they must be consid- of the soundscape idea – making it impossible to present a ered besides physical parameters. Therefore, the complex- general acknowledged definition. Often, the institutional ity of noise annoyance and the description of soundscapes, and disciplinary backgrounds of the researchers have a which means more than only the determination of annoy- sustainable effect on their soundscape concept. Schafer, a ance, cannot be simply described with a single parame- “soundscape pioneer”, describes the vital question in this ter, because many factors contribute to it. Hence, individ- context: ”What is the relationship between man and the ual, contextual or physical variables, causing a deviation sounds of his environment [. . . ]? [. . . ] Only a total appreci- from the “law of averages” implied by dose-response rela- ation of the acoustic environment can give us the resources tionships, must be determined with respect to an improved for improving the orchestration of the world soundscape” understanding of annoyance as caused by environmental [2]. A soundscape is not an objectively existing reality noise [5]. (you cannot overflow the soundscape and take a picture), Psychoacoustics will make a meaningful contribution to but it is a culturally-affected environment constituted by it and will allow the detection of specific soundscape fea- human perception [3]. tures. Today, environmental noise is calculated and depicted in noise-maps with the A-weighted sound pressure level 2. Psychoacoustics in soundscape research (SPL). Those noise maps neglecting the specific charac- ter of a soundscape are often the basis for discussions and 2.1. Psychoacoustics and soundscapes interpretations regarding noise annoyance. In this context, it is necessary to reflect also the meaning of annoyance. Psychoacoustics covers one important field of the differ- Annoyance will be used here as an overall evaluation of ent dimensions involved in the environmental noise evaluation process. It describes sound perception mechanisms in terms of several parameters, such as loudness, sharp- Received 1 January 2005, ness, roughness, and fluctuation strength as well as further accepted 1 January 2005. hearing-related parameters.

Soundscape is used as the complex superposition of 2.2. The application of psychoacoustics in the con- natural, human and technical (emitting from machinery text of environmental noise or electroacoustic reproduction devices) noises and their perception. Soundscapes consist of a number of spatially Different calculation methods and models, which, accord- distributed sound sources, which give the soundscapes ing to the rule of distance for SPL reduction, and by tak- their distinctive features. The emitted noise of each source ing into account propagation properties, reflections against could be measured and analyzed in terms of several pa- objects, damping factors, are an attempt in predicting how rameters. However, the annoyance due to given individ- A-weighted SPL evolves as a function of distance, how- ual sound sources cannot be transferred to the overall an- ever, neglecting time and frequency information. In real- noyance of an entire, complex soundscape containing the ity though, the calculated LAeq and the expected corre- noise of different sound sources, because of e.g. masking sponding noise annoyance, often differ widely from the effects [6]. evaluations made by the concerned residents [11]. This explained with the fact that the human hearing, in contrast to Surveys have documented that spatiality plays an im- a sound level meter, is not an absolute measuring instru- portant role for physiological reactions and annoyance. In ment. It shows a differentiated sound perception due to its the context of industrial noises, a study has investigated non-linearity and adaptivity as well as its signal process- the influence of the direction of sound incidence (uni- ing characteristics, paying attention to further factors than directional, multi-directional) on physiological reactions only to the averaged intensity of the sound event [12]. and loudness evaluation. It could be observed that the re- It could be established that the time structure of occur- action to the multi-directional situation was higher than rence of sound events in the context of traffic noise has the reaction to the uni-directional situation, although both a major influence on the noise annoyance. A high traf- noise situations produced the same SPL at the position of fic load noise can be evaluated as more pleasant than low the listener [7]. Therefore, the spatial distribution of sound traffic load noise because of the perceivable single vehicle sources as well as the direction and speed of any move- pass-by’s penetrating the quietness, which “startled” per- ment of these sources can be relevant for the perception manently the exposed persons. For example, traffic noise and evaluation of environmental noise. was measured and analyzed for the understanding of exte- In fact, the determination of noise annoyance caused by rior vehicle noise perception and resulting effects on man complex sound situations arising from the superposition in the European research project “Sound Quality of Exte- of the emitted sounds by a number of sources is very com- rior Vehicle Noise” (SVEN). There, traffic noise recorded plicated and the (binaural) signal processing involved in in L-shaped and U-shaped streets was analyzed and eval- human hearing has to be considered. The use of aurally- uated. The noise for the “L-shaped building” street was accurate measurements provides the opportunity to con- judged by the majority to be a bit or much louder and more sider binaural signal processing phenomena. dangerous, unpleasant and annoying compared with the U- The fact, that annoyance resulting from the listener’s shaped street. With respect to the general judgements the surrounding soundscape is also depending on the personal “L-shaped building” traffic noise was again judged less attitude of the listener, further increases the intricacy of the fa vourable. Therefore, the street shape – U-shaped, L- soundscape approach, which has to integrate important as- shaped – affects also the noise situation (because of dif- pects, such as the physical situation, experience and inter- ferent reflection patterns and time structure) and the noise pretation of the environment into one broad concept [8]. annoyance respectively [13]. Even visual information of the location affects noise eval- Another ongoing European research project is “Quiet uation [9]. Abe et al. concluded that the influence of visual City Transport” (QCity), which underlines the endeavour and verbal information on the auditory evaluation of envi- to further explore the connection between urban noise and ronmental sounds is considerable [10]. In order to capture its perception and evaluation [14]. It was observed that the mentioned difficulties with respect to human sensation many hot-spots regarding complaints cannot be explained and evaluation of environmental noise, the transformation with data from noise maps alone. The LAeq does not suf- of a sound event (the physical situation) into the perceived ficiently describe the environmental noise and its percep- sound event has to be considered. This transformation is tion. influenced by different aspects: first of all, the physical as- Figure 1 illustrates the behaviour of different parame- pect; secondly, the psychoacoustic aspect: the human hear- ters in dependency on distance in free field. It schemat- ing processes sound depending on the time structure and ically depicts on the basis of tonal and broadband noise frequency distribution. And thirdly, the psychological as- the behaviour of different parameters in dependency on pect, including context, the kind of information, the in- the distance. It makes clear the different behaviour of psy- dividual expectation and attitude to the sound, is finally choacoustic parameters compared with the decrease of the leading to the evaluation of the sound event. SPL. Therefore, a multi-dimensional approach, considering Figure 2 shows the calculation of these parameters of these different aspects adequately, is necessary to meet the a vehicle’s pass-by noise recorded in different distances requirements of the soundscape approach, the “[. . . ] total from the moving source. In contrast to the principle of the appreciation of the acoustic environment” [2]. SPL distance dependency, the psychoacoustic parameters

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Figure 1. Relative change of psychoacoustic parameters compared with A-weighted SPL as a function of distance (distance vs. percent) for tonal noise and broad band noise (dashed lines). Figure 3. Analyses of a vehicle pass-by noise recorded at diﬀer- ent distances (7 m, 25 m, 50 m, 100 m); 1/3Octave Modulation Spectrum.

Figure 2. Analyses of a vehicle pass-by noise recorded at different distances (7 m, 25 m, 50 m, 100 m); Level (A) vs. time, Loud- ness vs. time; Sharpness vs. time, Hearing Model Roughness vs. time. Figure 4. Analyses of a vehicle pass-by noise recorded at different distances (7 m, 25 m, 50 m, 100 m); Relative Approach. are also less dependent from the distance and from the SPL respectively [15]. The vehicle pass-by noise is further analyzed in Figures as pleasant in the context of environmental noise – pre- 3 and 4. The occurrence and the persistence in particular sumably the car pass-by noise is rarely evaluated as pleas- of disturbing patterns in the noise event can be observed, ant – the noise annoyance does not decrease in the same too. The modulation spectra clearly shows the unchanged way compared with the decline of the LAeq. particularities of the vehicle’s pass-by noise, although the The evaluation of tram noise is also often complex. LAeq decreases with distance (Figure 3, Figure 4). The next example, recorded in Brussels, shows the insuf- Moreover, the diagram depicting the “Relative Ap- ficiency of averaged SPL descriptions for the understand- proach”, a pattern measurement algorithm, underlines this ing of noise annoyance. Although the A-weighted Leq is conclusion. (Figure 4) This calculation method identifies almost identical, the evaluations, given by a small sam- changes in the short time spectrum with respect to fre- ple of test subjects, differ widely. Two curves squeal noise quency and time domain applying the difference calcula- events were compared. Tram 1 and tram 2 do not consid- tion between current signal values and estimated values. erably differ in LAeq (left 80,05 dBA / right 75,0 dBA to The estimated value is derived from the signal known up 80,3 dBA / 73,9 dBA) and loudness (46,2 sone / 30,4 sone to the current time. The determined difference can be in- to 49,3 sone / 35,0 sone). The difference in the sharp- terpreted as the short time signal change [12]. ness gives first information (3,29 acum / 2,31 acum to The diagrams show that specific acoustic properties 4,38 acum / 3,26 acum) about relevant features of the (tonal components, pattern in the time and frequency do- sound events for the evaluation. Tonal components as well main, etc.) attracting attention, remain nearly unchanged as temporal variations could also be identified. The Rela- in a case of a tenfold increase of the distance and de- tive Approach clearly shows pattern in the time domain, crease of LAeq. In case, those properties are not evaluated which finally led to the different evaluations (Figure 5).

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Figure 6. The Evaluation of Soundscapes.

To sum up, the examples mentioned above point out the necessity of new approaches, measurement techniques and new parameters in order to adequately grasp the particularities of a soundscape and to suﬃciently consider the mechanism of sound perception. Conventional parameters and analysis techniques are not suﬃcient. A new approach has to be applied involving aurally-accurate measurements and psychoacoustic analyses, where the characteristics of the human auditory apparatus are taken into account.

3. Evaluation of soundscapes

3.1. “Questions and answers” Human hearing can classify complex soundscapes into single sound events because of its binaural hearing and its consequential directional hearing and selectivity. Single contributions to a soundscape can be selected by human hearing and decisively influence the individual evaluation. Thus, if people are complaining about noise annoyance, the actual reasons for noise objection have to be explored first. Physical, psychoacoustic and binaural signal processing as well as cognitive aspects affect the evaluation of environmental noise. (Figure 6) Relevant questions may be: Which of the existing sound sources causes the noise annoyance? Which kinds of signal attributes like modulation or specific patterns in the time and frequency domain are creating the annoyance? Is the time structure responsible for the complaints? Are in- formative features relevant with respect to the annoyance? Is the noise unpleasant or conspicuous due to modulation or are there noticeable patterns in the time or frequency range? What kind of attitude and expectation has the listener? Answers to those questions can already give significant Figure 5. Tram curves squeal noise (left: tram 1, right: tram 2), information about the actual causes for complaints. On Relative Approach. the one hand, these answers already allow first successful measures in order to improve the perceived environmental noise quality [17]. The phrase sound/noise quality in the Impressive sound features contribute to overall judgments context of environmental noise is also frequently used by regardless of their sound pressure levels [16]. other authors [18].

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On the other hand, the acoustic analysis of the soundscape can be carried out in a more goal oriented manner, if the exposed persons give reasons for their annoyance.

3.2. Measurement of soundscapes – Aurally- equivalent sound measurement With conventional one-channel measurements it is not possible to sufficiently determine environment noise quality and the annoyance caused by a complex environmental sound situation constituted by several spatially distributed sound sources. Important aspects, which influence the subjective impression, are masking effects, sound impression, spatial distribution and complex phase relations. Conventional measurement and analysis technique only partly take into account these aspects. The use of binaural technology is recommended to bridge this gap. The artificial head accurately simulates acoustically relevant components, and it is able to obtain aurally-accurate binaural recordings of sound events. The playback of binaural recordings creates an auditory impression for the listener which is the same as it would have had been ex- perienced if the listener had been present at the original sound event [19]. The results of listening tests, realistically playing-back the noise situations, are highly valid and re- liable. Measurements with binaural technology are necessary if subsequent reproductions of noise are required, e.g. in the case of further examination in laboratories (listening tests). This insight has become widely accepted; the use of binaural technology and psychoacoustic analyses, considering both channels, in acoustic engineering win slowly recognition [20] (Figure 7).

3.3. Calculations models and analyses Generally, environmental noise is perceived by the sound perception mechanisms of human hearing. The perceived sound event can be described and analyzed in terms of psychoacoustic parameters, such as loudness, sharpness, roughness, and fluctuation strength. These parameters describe the environmental noise situation better than the simple A-weighted SPL and capture relevant, “ear- catching” phenomena of the noise. Different preliminary calculation models integrating several parameters have been developed to determine annoyance. For example, Zwicker calculated the unbiased annoyance with loudness, sharpness and fluctuation strength Figure 7. Artificial head technology used for environmental noise [21], Terhardt developed a model for the determination recordings. of “Wohlklang” (pleasantness of noise) using loudness, sharpness, roughness and tonality [22], Preis worked with annoyance loudness (time-averaged value of the difference have to be studied. The application of further parameters between loudness of the noise and background sound), in- – besides the known psychoacoustic parameters – can be trusiveness including sharpness, and distortion of informa- helpful regarding the description and evaluation of envi- tional content in order to determine noise annoyance re- ronmental noise as shown in some examples mentioned ception [23]. Further research must be carried out to deter- above. mine a model integrating the relevant parameters, which 3.4. Existing tools for the improved description of considers the sound perception mechanism and correlates environmental noise with annoyance evaluation respectively. Therefore, inter- action effects of different parameters (e.g. psychoacoustic) The intelligent use of already existing tools and analyses with respect to annoyance caused by environmental noise would be a step forward that would allow an improved

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The application of these opportunities – frequency fil- tering, variation of integration time – can already help to recognize disturbing patterns without much effort. Unfor- tunately, the legislators and decision makers often do not use these simple tools in the context of the evaluation of environmental noise. However, questions concerning annoyance of environmental noise cannot be satisfactorily answered by the pre- sented parameters alone. Both, the nature of the information in the acoustic environment and the attitude of exposed persons greatly affect the subjective impression and must be added to the (psycho-) acoustic analyses.

4. Conclusions Figure 8. Airplane take-off noise; level(A) vs. time (green, red- level(A) vs. time(Fast); blue, pink- level(A) vs. time(0,002 ms) Conventional noise-maps listing only SPL values aver- of frequency band(1,4 kHz to 1,6 kHz). aged over different time intervals cannot describe environmental noise quality adequately. Inevitably, such averaging measures are crude and hide the substantial exposure variation relevant for individual annoyance reactions [1]. One necessary measure is the increased application of artificial head technology in the environmental noise context. Only binaural recording technology, analog to human hearing, allows an objective sound recording and can reg- ister sound in relation to direction of the sound incidence. The binaural recordings allow the aurally-accurate reproduction of acoustic scenarios, which are directly compara- ble with respect to pleasantness or annoyance by listeners. To sum up, the soundscape approach can help to bridge the explained gaps, which cannot be overcome with conventional methods alone. In order to achieve the common purpose of improving soundscapes with respect to pleasantness and reduced annoyance, “environmental sound de- Figure 9. Airplane take-off noise; Level(A) vs. Time (zoom) sign” with a multi-dimensional understanding must be car- (green, red- Level(A) vs. Time(Fast); blue, pink- Level(A) vs. ried out. This implies that the ultimate causes for com- Time(0,002 ms) of frequency band (1,4 kHz to 1,6 kHz). plaints and high annoyance have to be identified, e.g. specific noise features caused by a certain sound source. Then, analysis of noise. For example, the application of a third promising measures as a kind of sound design are feasible octave analyzer and sound pressure level meter with vari- and, if possible, the modification of the responsible source. able integration time is a simple but suitable approach. Thus, physical aspects, psychoacoustic aspects, taking An integration time of 2 ms, which corresponds with the into account the human (binaural) signal processing, and human hearing time resolution, allows the detection of cognitive, psychological aspects, regarding variables like strong modulations and apparent variations [24]. In con- information content, acceptance of sound sources, atti- trast to constant sound pressure level curves (level vs. tude of the listener, have to be adequately considered. time) varying curves in the short-time domain usually lead “From acoustics and psychoacoustics we will learn about to a higher level of annoyance. In addition, the comparison the physical properties of sound and the way sound is of frequency selected levels to the total A-weighted SPL interpreted by the human brain. From society we will curve points out the relevance of prominent spectral bands. learn how man behaves with sounds and how sounds af- An example is given in Figure 8. There, a prominent tonal fect and change his behaviour. From the arts, [. . . ] we band was detected in the sound event, which possesses a will learn how man creates ideal soundscapes [. . . ]” [2]. distinctive time structure in the short time domain. The dif- Only the elaborate combination of those disciplines will ferent integration times used in the Figure display the dif- allow the identification of promising measures against un- ference of the curves, which possess different explanatory wanted sound or better against unwanted sound features. power for the description and interpretation of the noise The first step to meet this claim regarding the aspects of situation. The detected tonal band dominates the sound physical properties of the sound will be the extension from event with respect to the SPL and therefore, contributes averaging energy descriptions (based on SPL values) to significantly to the overall impression. (Figure 9) a more detailed acoustic description taking into account

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