Sensory Unpleasantness of High-Frequency Sounds

Acoust. Sci. & Tech. 34, 1 (2013) #2013 The Acoustical Society of Japan PAPER

Sensory unpleasantness of high-frequency sounds

Kenji Kurakata1;, Tazu Mizunami1 and Kazuma Matsushita2 1National Institute of Advanced Industrial Science and Technology (AIST), AIST Central 6, 1–1–1 Higashi, Tsukuba, 305–8566 Japan 2National Institute of Technology and Evaluation (NITE), 2–49–10, Nishihara, Shibuya-ku, Tokyo, 151–0066 Japan ( Received 5 March 2012, Accepted for publication 2 August 2012 )

Abstract: The sensory unpleasantness of high-frequency sounds of 1 kHz and higher was investigated in psychoacoustic experiments in which young listeners with normal hearing participated. Sensory unpleasantness was defined as a perceptual impression of sounds and was differentiated from annoyance, which implies a subjective relation to the sound source. Listeners evaluated the degree of unpleasantness of high-frequency pure tones and narrow-band noise (NBN) by the magnitude estimation method. Estimates were analyzed in terms of the relationship with sharpness and loudness. Results of analyses revealed that the sensory unpleasantness of pure tones was a different auditory impression from sharpness; the unpleasantness was more level dependent but less frequency dependent than sharpness. Furthermore, the unpleasantness increased at a higher rate than loudness did as the sound pressure level (SPL) became higher. Equal-unpleasantness-level contours, which define the combinations of SPL and frequency of tone having the same degree of unpleasantness, were drawn to display the frequency dependence of unpleasantness more clearly. Unpleasantness of NBN was weaker than that of pure tones, although those sounds were expected to have the same loudness as pure tones. These findings can serve as a basis for evaluating the sound quality of machinery noise that includes strong discrete components at high frequencies.

Keywords: Sensory unpleasantness, High-frequency sound, Loudness, Sharpness, Equal-unpleasantness-level contour, Sound quality PACS number: 43.66.Cb, 43.66.Jh, 43.66.Lj [doi:10.1250/ast.34.26]

evaluating psychoacoustic effects of HF sounds have not 1. INTRODUCTION yet been established. 1.1. High-Frequency Sounds and Evaluation of Their For evaluation, the hearing threshold can be the first Perceived Quality criterion [5,6]. If the sound pressure level (SPL) of a tone is Some electric and electronic products, such as infor- higher than the threshold value, then the tone is judged to mation technology and telecommunications (ITT) equip- be audible for the population from which the threshold was ment, emit sounds including salient high-frequency (HF) obtained. However, comparison with the hearing threshold components of several thousand hertz. Other products provides no useful information for evaluating the auditory reported to emit HF noise are electric home appliances, impression of sounds above the threshold. medical devices [1], automobiles [2], and railway trains For tones accompanied by broadband noise, indices including the Shinkansen [3]. Those sounds are often such as the tone-to-noise ratio (TNR) and prominence generated from built-in inverters or mechanical devices ratio (PR) are used. Measurement methods for these two that rotate at a very high speed. Sources for some other HF indices are specified for ITT equipment in an international sounds were unidentifiable [4]. standard [5]. However, TNR and PR are not applicable Those HF noises can elicit an unpleasant impression properly, by definition, to tones without surrounding and are annoying for users of the product or equipment, or noise or when the noise is at a subthreshold level. The nearby people. Although such products generating HF latter case can often occur at high frequencies because sounds have become popular in daily life, methods for the hearing threshold increases as the frequency becomes higher than about 4 kHz. The level of noise is usually e-mail: [email protected] lower than the threshold. Furthermore, even when the

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Table 1 Frequency and sound pressure level combinations of experimental stimuli. Crosses denote the combinations used.

Frequency, kHz Sound pressure level, dB 1 2 4 5 6.3 8 10 12.5 16 18 80 75 70 65 60 55 50 45 40 30 20 10 0

prominence of two tones is equal, their effects of sen- In both experiments, young people aged about 20 years sory unpleasantness might differ depending on their with normal hearing, who were expected to be most frequencies. The relationship between those indices and sensitive to HF sounds, participated as listeners. sensory unpleasantness of discrete tones has not been Sensory unpleasantness in this study was defined as fully investigated. the sensation giving rise to the sentiment that ‘‘I would Loudness and sharpness [7] are sensory attributes of not want to hear the sound if I had to hear it continuously sounds that have been investigated widely for sound- for a long period.’’ Any loud sound might be annoying if quality evaluation and which might be related to the presented continuously. However, the degree of unpleas- auditory impression of unpleasantness. It has been argued antness might differ depending on the acoustic character- that loudness and sharpness are inversely related to sensory istics of the sound. pleasantness [7]. In addition, the perceived quality of some Whether a sound is perceived as unpleasant or not machinery noise is degraded when sharpness is high [8]. depends on the subjective relation of the listener to the However, having high loudness and sharpness is not always sound [7]. An unpleasant sound might not be as annoying the cause of an unpleasant impression of the sound. For as it could be if the listener is indifferent to the sound example, an appropriate increase in the amount of sharp- source (e.g., a person or a machine), and vice versa. To ness can give a sound the character of powerfulness [9], differentiate sensory unpleasantness from annoyance in this which is not a negative attribute of sounds. Consequently, respect, the sound source was not specified in the experi- acoustic correlates of sensory unpleasantness of HF sounds ments; the listeners were asked to concentrate on a having a high loudness or sharpness value must be inves- judgment based on their perceptual impression. tigated further. 2. EXPERIMENT I: SENSORY UNPLEASANTNESS OF PURE TONES 1.2. Objectives of This Study This study was conducted to measure the sensory 2.1. Measurement Method unpleasantness of HF sounds. The frequency, SPL, and Stimuli Pure tones with a frequency of 1, 2, 4, 5, 6.3, 8, bandwidth of stimulus sounds were varied systematically 10, 12.5, 16, or 18 kHz were used. Table 1 shows that the and their respective relationship with the calculated sharp- SPL was changed by 5 or 10 dB in several steps. The ness, loudness and other sound-quality metrics of those duration of tones was 10 s with a rise/fall time of 1 s so that sounds were analyzed. the transients at the beginning and end of tones would not Two psychoacoustic experiments measured the sensory elicit an unwanted impression of timbre change. unpleasantness of HF sounds. In the first experiment, pure Apparatus The signals were calculated digitally on a tones with various frequencies were used as stimuli. The personal computer with 24-bit resolution and 48 kHz effects of frequency and SPL on perceived unpleasantness sampling and were generated from a D/A converter (UA- were investigated. In the second, narrow-band noise (NBN) 5; Edirol, Roland Corp.). Then, they were fed into a control with various center frequencies and bandwidths was used. amplifier (SU-C1010; Technics, Matsushita Electric In- The unpleasantness of NBN was compared to that of pure dustrial Co., Ltd.) and a power amplifier (SE-A1010; tones to ascertain the effects of frequency and bandwidth. Technics, Matsushita Electric Industrial Co., Ltd.), and

27 Acoust. Sci. & Tech. 34, 1 (2013) were presented to the listener via a two-way concentric In the analyses, the median ME values of the listeners loudspeaker (i8; Tannoy Ltd.). were used. The values were obtained in the following way. Measurement was conducted in an anechoic room (1) The geometrical mean of the two estimates that the of 4.35 m (W) 6.00 m (D) 2.95 m (H) at AIST. The listeners gave for each stimulus tone was calculated for loudspeaker and the listener’s chair were set 3.0 m apart individual listeners. (2) The group median of the logarithm from each other on the room’s diagonal axis. The listener of the geometrical means was calculated for each stimulus sat on the chair with a headrest and faced the speaker level and frequency. The median was taken because some directly. The listener’s head movement was monitored with listeners failed to respond to the stimulus tones at the two stick-type sensors attached to the headrest: movement lowest levels and highest frequencies; the mean was not during measurement was restricted to about 1 cm. calculable for those tones. (3) For smoothing purposes, an Procedure The magnitude estimation (ME) method with- INEX function [12] was fitted to the median as a function out a modulus [10] was used to measure the unpleasantness of SPL. The INEX function has been applied successfully that listeners perceived. The pure tone stimuli were to the loudness growth function of the 1 kHz tone, taking presented to individual listeners one by one in random the place of the conventional power function [13]. Fitting order. After listening to each tone, they estimated the was done separately for each tone frequency. (4) The magnitude of sensory unpleasantness and verbally reported 1 kHz, 40 dB tone was chosen as a reference tone and the a positive number that they assigned to the degree of ME value for the tone was adjusted to one. The INEX impression. No limit was imposed to the range of numbers functions and ME values of other tones were shifted in that listeners might use. proportion to it. Listeners were given an explanation about the meaning According to the two-stage theory of magnitude of ‘‘unpleasantness’’ as described in Sect. 1.2. They were estimation, in which the first stage is sensory and the also instructed not to confuse unpleasantness with loud- second is cognitive and involves processes of judgment, the ness because some sounds might be soft but unpleasant, unpleasantness growth function obtained in the present although others might be loud but pleasant. experiment can be affected by the second stage and may About 30 trials were run for practice before the main not express the sensory function directly. However, in the trials started. Each listener gave two estimations for every case of loudness growth functions, the effect of the second stimulus tone in the main session. stage is known to be negligibly small (for review, see [10]). In addition to unpleasantness, listeners were asked to Therefore, the ME values given by the listeners in this estimate the loudness of 1 kHz tones in a separate session study were assumed to reflect their sensory magnitude using the same apparatus and procedure. Thereby, the properly and were adopted as they were. relationship of unpleasantness with loudness was inves- 2.2.2. Relationship with sharpness tigated. The unpleasantness for HF tones was regarded as Participants Study participants were 46 university stu- resembling the auditory sensation of sharpness. To exam- dents aged 19–24 years. Otoscopic examination, monaural ine the relationship between those two auditory impres- audiometry using a pure-tone audiometer, tympanometry, sions, the sharpness of the stimulus was calculated using a and a questionnaire about difficulties in hearing [11] were software package (Type 7698 ver. 15.1.0; Bruël & Kjær). administered to screen participants for otological abnor- Among several methods for calculating sharpness, Aures’ malities. Consequently, 38 students (18 men, 20 women) method [14] was employed, which corrected Zwicker’s were selected as listeners. Their average age was 21 years. method [7] to give improved level dependence, so that the The average hearing threshold level (HTL) at 1, 2, 4, and calculated sharpness can be compared with the present 8 kHz of their better ear was 0:3 dB. Those listeners dataset measured for the wide SPL range. were regarded as being sampled from a young, otologically Figure 1 shows median ME values of unpleasantness normal population. and calculated sharpness respectively, as a function of the SPL of stimulus tone, for stimulus frequencies of 1 kHz 2.2. Results and Discussion to 12.5 kHz. Data for 16 kHz and 18 kHz tones are not 2.2.1. Initial data processing presented in the figure because the software package used The correlation coefficient of the values assigned to did not provide reliable sharpness calculations for those the same stimuli in two trials was calculated to examine extremely high tones. the reliability of listeners’ responses. One listener showed The curves in the three panels illustrate clearly that, a markedly low coefficient (r ¼ 0:24). Therefore, her although unpleasantness and sharpness increased as the responses were regarded as unreliable. After deleting her SPL became higher, they are two different sensory data, those of the remaining 37 listeners were adopted for attributes. Unpleasantness increased more rapidly than the following analyses. sharpness did as the SPL of tone became higher. When SPL

28 K. KURAKATA et al.: UNPLEASANTNESS OF HIGH-FREQUENCY SOUNDS

(a) (a)

(b) (b)

(c)

Fig. 2 Relationship between ME values of sensory unpleasantness and loudness for tones (a) from 1 kHz to 5 kHz and (b) from 6.3 kHz to 12.5 kHz.

more level dependent, but less frequency dependent than sharpness. 2.2.3. Relationship with loudness Although sharpness is not closely related to unpleasantness, loudness might be because loud sounds are annoying in general. The relationship between sensory unpleasant- Fig. 1 Unpleasantness and sharpness as functions of ness and loudness was investigated by estimating the sound pressure level: (a) unpleasantness for tones from loudness as described below. (1) An INEX function [12] 1 kHz to 5 kHz, (b) unpleasantness for tones from 6.3 kHz to 12.5 kHz, and (c) sharpness for tones from was fitted to the group medians of ME values that the 1 kHz to 12.5 kHz. listeners assigned to the stimuli in the separate session for loudness measurement. (2) The MEs of loudness at frequencies other than 1 kHz were obtained using the increased from 20 dB to 80 dB, unpleasantness increased equal-loudness-level relation described in an ISO standard by a factor of a few tens, or one hundred or more for [15]. Tones having a different frequency but the same some frequencies, although sharpness increased only by a loudness level as a 1 kHz tone were given the same ME factor less than 2. In contrast to that, the effect of tone value as assigned to the 1 kHz tone. Data for 16 kHz and frequency on unpleasantness was much smaller than on 18 kHz tones were not adopted here either because the sharpness. For example, unpleasantness of the 12.5 kHz international standard does not cover those frequencies. tone was only slightly larger than that of the 1 kHz tone Figure 2 presents the unpleasantness ME’s as a at 40 dB, whereas sharpness of those two tones differed function of thus-estimated loudness ME. For the ME’s by a factor of 10. Consequently, sensory unpleasantness for each frequency, the power function expressed below is not determined solely by sharpness; unpleasantness is was separately fitted:

29 Acoust. Sci. & Tech. 34, 1 (2013)

Fig. 3 Equal-unpleasantness-level contours related to 1 kHz tones. Open circles connected with dotted lines lie out of the range of measurement and were obtained by extrapolation. The equal-loudness-level contours [15] (broken curves) and the threshold curve [16] (solid curve) are shown for comparison. Crosses show equal loudness levels of 20 phons and 40 phons at 16 kHz (see the text for details).

equals one. The 95% confidence interval of a was from b U ¼ a L ð1Þ 1.24 to 1.78 for 4 kHz tones and from 2.21 to 2.97 for or 10 kHz tones, both of which were significantly larger than one. The tones with those frequencies elicited greater log U ¼ log a þ b log L; ð2Þ unpleasantness than those with other frequencies even where U is the unpleasantness ME, L is the loudness ME, when their loudness was equivalent. and a and b are, respectively, a constant and the exponent 2.2.4. Derivation of equal-unpleasantness-level contours of the power function that varies depending on the tone The frequency dependence and level dependence of frequency. sensory unpleasantness and the relation to loudness can be It can be seen that the functions ran close to the made even clearer by drawing equal-unpleasantness-level diagonal line overall, indicating that unpleasantness in- contours (EUPLCs). Drawing EUPLCs has the advantage creased proportionally with increasing loudness. Closer that the cognitive factors associated with the ME method observation reveals that the increase of unpleasantness that were described in Sect. 2.2.1 can be canceled out if we was greater than that of loudness, as is evident from the assume that those factors affected the listeners’ judgments slope of the function. Furthermore, the slope differed in a similar manner regardless of stimulus frequencies; the depending on the tone frequency; higher-frequency tones contours represent the relative, not absolute, degree of generally showed a steeper slope. The 95% confidence unpleasantness of tones as a function of frequency. interval of b, the power exponent or the slope of the In deriving the contours, another assumption that function, was from 0.94 to 1.09 for 1 kHz tones. On the tones to which the same number had been assigned by other hand, that for 12.5 kHz tones was from 1.09 to 1.38, listeners in the experiment had the same unpleasantness which was significantly larger than one. The acceleration of irrespective of the tone frequency was made. Combinations unpleasantness functions at high SPLs and their frequency of frequency and SPL were identified using the INEX dependence are apparent also in Figs. 1(a) and 1(b). functions of unpleasantness and loudness presented in the The upward shift of the overall function from the previous sections. diagonal line in Fig. 2 indicates the magnitude of unpleas- Figure 3 shows some EUPLCs derived in this manner. antness relative to loudness. The size of the shift was large The equal-loudness-level contours (ELLCs) [15] were at 4 kHz and around 10 kHz, which was supported by a superimposed to compare the two families of contours. statistical analysis of the constant a in Eq. (1). When the The EUPLCs are lower than the ELLCs in general, function is exactly on the diagonal line, the constant a suggesting that pure tones with a frequency higher than

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1 kHz can elicit a stronger than expected sensation of (a) unpleasantness from the equal-loudness relation. That is, HF tones can be more unpleasant even when their loudness is equal to that of lower frequency tones. The differences between EUPLCs and ELLCs are largest at 4 kHz and around 10 kHz, as was demonstrated by the analysis of power functions described in Sect. 2.2.3. The tones at and around those two frequencies can yield a high degree of unpleasantness even when their loudness is low. ISO 226 does not specify equal loudness levels at frequencies higher than 12.5 kHz, but two studies [17,18] that were used for deriving the ISO contours measured those of 20 phons and 40 phons at 16 kHz. Average equal- (b) loudness-level values of those studies are presented in Fig. 3. Apparently, equal-unpleasantness levels and equal- loudness levels do not differ much at that frequency. 3. EXPERIMENT II: SENSORY UNPLEASANTNESS OF NARROW-BAND NOISE 3.1. Measurement Method Stimuli NBN with a center frequency of 1, 4, 8, 10, 12.5, or 16 kHz was used. The bandwidth was 1/3, 1/6, 1/12, or 1/24 octave. The SPL was kept constant at 40 dB, except Fig. 4 (a) Unpleasantness and (b) loudness for pure for the 16 kHz noise, whose level was 60 dB. In addition tones and narrow-band noise. Error bars in panel (a) to the noises, pure tones with the same frequency and SPL indicate the interval between the 75th percentile and were also used as reference sounds. The sound duration the median. was 10 s with a rise/fall time of 1 s so that the transients at the beginning and end of sounds would not elicit an unwanted impression of timbre change. 3.2. Results and Discussion Apparatus The signals were calculated digitally on a 3.2.1. Initial data processing personal computer with 24-bit resolution and 96 kHz One listener showed a markedly low correlation sampling. They were presented to listeners using apparatus coefficient (r ¼ 0:26) between the two trials for the identical to that used in Experiment I. same stimulus. Therefore, his responses were regarded as Procedure The procedure for the NBN measurement unreliable. After deleting his data, those obtained from the was fundamentally identical to that in Experiment I for remaining 28 listeners were adopted for the following pure tones: the ME method without a modulus was used; analyses. listeners estimated the magnitude of unpleasantness of Median ME values of the listener group were calcu- noises and pure tones presented one by one in random lated similarly, as conducted in Experiment I. (1) The order; then, they reported a positive number that they geometric mean of the two estimates that the listeners gave assigned to the degree of impression; every stimulus sound for each stimulus was calculated for each listener. (2) The was presented twice for each listener. median of the logarithm of the mean values was calculated. The listeners were given the same instruction about (3) The 1 kHz pure tone was chosen as a reference and its the meaning of ‘‘unpleasantness’’ and its difference from ME value was adjusted to one. The ME values of other loudness, as in Experiment I. sounds were shifted in proportion to it. After a short training session, each listener gave two 3.2.2. Effect of bandwidth estimations for every stimulus sound in the main session. Figure 4 portrays the median unpleasantness for the Participants Study participants were the 29 university stimulus sounds. It is observable in Fig. 4(a) that the students (13 men, 16 women) who participated in Experi- unpleasantness of NBN was lower than that of pure tones in ment I and who were screened for hearing abnormalities. spite of the fact that their loudness varied little with the Their average age was 21 years. The average HTL at 1, 2, change of bandwidth, as shown in Fig. 4(b). According to 4, and 8 kHz of their better ear was 0:8 dB, which was the results of the Friedman test (IBM SPSS Statistics 20; normal for their age. IBM Corp.), the difference in unpleasantness ME was

31 Acoust. Sci. & Tech. 34, 1 (2013) statistically significant (the 2 statistic varied from 24.8 were not related to the difference in loudness and sharp- for 12.5 kHz sounds to 51.1 for 4 kHz sounds; df ¼ 4, ness. Furthermore, producing the sensation of roughness p < 0:01). The difference was not significant for the 1 kHz or fluctuation strength did not lead to the increase of sounds (2 ¼ 3:31, df ¼ 4, p > 0:50) and 16 kHz sounds unpleasantness. The metrics that have often been used in (2 ¼ 3:14, df ¼ 4, p > 0:50). sound-quality analyses cannot be used to evaluate HF noise The insensitivity of 16 kHz sounds to the bandwidth unpleasantness. change is explainable as follows. At around 16 kHz, Sensory unpleasantness might be more closely related loudness decreases rapidly as the frequency increases, as to pitch strength, which can be labeled as faint pitch or expected from the rapid increase of the hearing threshold strong pitch on a scale. The pitch strength of NBN has (see Fig. 3). Consequently, the energy at the lower- been shown to be smaller than that of pure tones [19]. It is frequency edge of the noise band is dominant, whereas imaginable that the weaker pitch sensation could lessen the that at higher frequencies becomes less effective in penetrating impression that HF sounds could evoke, which perception, which yields a pure-tone-like impression of resulted in mitigating the unpleasantness of NBN. sound; the NBN might have been perceived as a pure tone Unfortunately, models for predicting the pitch strength by listeners. of sounds have not yet been established [20]. Neither has 3.2.3. Relationship with other sound-quality metrics the pitch strength of HF sounds been fully investigated. Although not portrayed in Fig. 4, Aures’ sharpness of Therefore, it is worth investigating the relation of unpleas- NBN changed very little when the bandwidth was widened antness with pitch strength so that the effect of spectral from 1/24 octave to 1/3 octave and could not explain the contents on unpleasantness will be clarified. experimental results. The values for NBN were almost 4. SUMMARY equal to that for the pure tone of the same frequency, according to the calculations using a software package Measurement results of sensory unpleasantness of high- (Type 7698 ver. 15.1.0; Bruël & Kjær), which was frequency sounds in the two experiments can be summa- expected from the derivation procedure of sharpness [7]. rized as described below. Other popular sound-quality metrics, roughness and (1) Sharpness (Aures’ sharpness) was not closely related fluctuation strength, are related to the magnitude of the to the unpleasantness of pure tones. The unpleasant- temporal variation of sounds. NBN has a time-varying ness was more level dependent and less frequency envelope and that variation can elicit the sensory impres- dependent than sharpness. sions of roughness and fluctuation strength [7]. On the (2) Pure tone unpleasantness increased as loudness other hand, pure tones, whose temporal envelope is increased. The growth rate of unpleasantness was constant over time, do not yield those sensations. higher for high-frequency tones. Those two sensory impressions can cause the degrada- (3) Comparison between equal-unpleasantness-level con- tion of sound quality, as was demonstrated by the studies tours and equal-loudness-level contours revealed that on the prediction of perceived annoyance on the basis of pure tones at 1 kHz and higher elicit a stronger the composed metrics including roughness and fluctuation impression of unpleasantness than expected from their strength [9]. If the unpleasantness of HF sounds were loudness. The two families of contours were most associated with roughness and fluctuation strength, NBN different at 4 kHz and around 10 kHz. would produce stronger unpleasantness than would pure (4) Unpleasantness of narrow-band noise was less than tones. However, the experimentally obtained results pre- that of pure tones having the same center fre- sented in Fig. 4(a) were quite opposite to that inference. quency and sound pressure level as the noise had; The reduction of unpleasantness of NBN might be nevertheless, those sounds did not differ in loud- explainable as follows: the NBN did elicit the sensation of ness. roughness and fluctuation strength to some extent, which (5) The unpleasantness of narrow-band noise did not could lead to the increase of unpleasantness. However, change greatly when their bandwidth varied from their broader spectrum than that of pure tones had an effect 1/24 octave to 1/3 octave. of lessening the penetrating impression that HF sounds (6) The difference in unpleasantness between pure tones could evoke, and decreased the unpleasantness. The and narrow-band noise was not explainable in terms latter effect was greater than the former one of roughness of loudness or sharpness. Furthermore, other sound- and, consequently, the overall impression of sensory quality metrics, roughness and fluctuation strength, unpleasantness of NBN became weaker than that for pure were not directly related to the unpleasantness. tones. Instead, pitch strength must be investigated further To summarize, the variation of sensory unpleasantness to ascertain a perceptual correlate with the effect of among pure tones and NBN with a different bandwidth bandwidth on unpleasantness.

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