Chapter 11

Applications (IV) – Noise Annoyance

Noise is a ubiquitous feature of modern life that is frequently the cause of psychological irritation and stress. We conducted a series of experiments to probe various aspects of noise perception and annoyance. Sections 11.1 and 11.2 discuss experiments in a laboratory that were conducted for annoyance judgments of noise based on both the temporal and spatial factors of environmental noise. In Section 11.3, long-time effects of aircraft noise during pregnancy on postnatal reactions of babies during sleep were investigated. Results show a “prenatal habituation” of babies to aircraft noise, but not music. These differential responses might be explained by the temporal factors extracted from the ACF. In Section 11.4, effects of noise and music on two different mental tasks of children are discussed. Results of two different tasks could be related to the specialization of cerebral hemispheres.

11.1 Noise Annoyance in Relation to Temporal Factors

11.1.1 Annoyance of Band-Pass Noise

This study examined annoyance of pure tones and band-pass noises with center frequencies of 1000 and 2000 Hz under equal-SPL conditions. Control of ACF factors of the noise source was realized by use of a 2068 dB/octave sharp filter to create bandwidths of 0, 40, 80, 160, and 320 Hz. The scale value of annoyance was obtained using paired comparisons tests (PCT). Results show that annoyances of pure tones and band-pass noises inside the critical band are not constant, and annoyances of pure tones are greater than those of sharply filtered noises within the critical band. Remarkably, the annoyance of band-pass noise increases with increasing effective duration of the ACF, τe, which represents the repetitive feature of the source signal within the critical band.

Changes in frequency, duration, and bandwidth all affect perceived loudness of a stimulus even though the level of the stimulus remains fixed (Yost, 2000). Loudness of the noise remains constant as the bandwidth of the noise increases until the bandwidth reaches the critical band. Loudness then increases with increasing bandwidth under the same SPL conditions (Zwicker et al., 1957; Greenwood, 1961a,b; Scharf, 1962; Zwicker and Scharf, 1965). However, it has been found that the loudness of

Fig. 11.2 Measured ACF factors of the band-pass noise source signal as a function of the bandwidth. (a) Delay time of the first maximum peak of ACF, τ1. (b) Amplitude of the first maximum peak of ACF, φ1. (c) Effective duration of ACF, τe. Different symbols indicate different frequencies: (◦ ): 1 kHz, ( ): 2 kHz

rally presented using headphones (Sennheiser HD-340, Wedemark, Germany). All stimuli were fixed exactly at 74 dBA by measuring (0). The SPL was calibrated by using a dummy head with half-inch condenser-type microphones at both ears. The IACC was kept constant at unity, because the signals fed to both ears were identical.

Eight 21to 23-year-old subjects with no histories of hearing disorders participated in the experiment. They were seated in a dark, soundproof room with a comfortable thermal environment where they listened to the sound stimuli. Paired comparison tests were performed for all combinations of the pairs of pure tone and band-pass noise, that is, 15 pairs (N(N – 1)/2, N = 6) of stimuli interchanging the order in each pair per session, with the pairs presented in random order. A total of 10 sessions were conducted for each subject. The duration of the stimuli was 2.0 s, the rise and fall times were 50 ms, the silent interval between the stimuli was 1.0 s, and the interval between pairs was 3.0 s, which was the time allowed for the subject

to respond by pushing one of two buttons. They were asked to judge which of two sound signals was more annoying.

The relationship between the scale value of annoyance and the filter bandwidths is shown in Fig. 11.3. For the center frequency of 1000 Hz, the averaged scale value of annoyance for the pure tone was significantly greater than those of sharply filtered noises under the condition of equal SPL (p < 0.01). For the center frequency of 2000 Hz, the scale value of annoyance for the pure tone was significantly greater than that of the band-pass noises with 40, 80, and 160 Hz bandwidths (p < 0.05). There was agreement among subjects that the most annoying stimulus was the pure tone or band-pass noise with a 320-Hz bandwidth. However, relatively large individual differences were observed of the noise with a 320-Hz bandwidth. The previous research suggests that tonal components increase the perceived annoyance and noisiness of broadband noise (e.g., Kryter and Pearsons, 1965; Hargest and Pinker, 1967; Fujii et al., 2002).

The loudness of the band-pass noise with center frequencies of 1000 and 2000 Hz was also investigated. The same subjects who had participated in the annoyance tests participated in the loudness test. The auditory stimuli were presented in the same way as in the annoyance tests. The subjects were asked to judge which of two sound signals was the louder signal. The relationship between the scale values of loudness and the filter bandwidths is shown in Fig. 11.4. For the band-pass noise centered on 1000 Hz, results of loudness as a function of its bandwidth are similar to those of Section 6.4 with different subjects. It is reconfirmed that the loudness for the pure tone was significantly greater than that of the band-pass noise, and loudness increased with increasing τe, although one subject in the study had increasing loudness from the pure tone to the widest bandwidth. The averaged scale value of loudness for the band-pass noise with the bandwidth of 320 Hz was significantly greater than those for the other band pass noises for both center frequencies of 1000 and 2000 Hz. This is consistent with previous research by Zwicker et al. (1957), Greenwood (1961a,b), Scharf (1962), and Sato et al. (2002). In this study, three of the eight subjects had identified increasing loudness with increasing bandwidth. The increase of loudness and annoyance by the pure tone was not found in two subjects for the noise centered on 2000 Hz, as indicated with an open triangle and square in Fig. 11.3b. Almost all subjects showed an increase of loudness above the critical band. However, the increase of annoyance above the critical band showed individual variations. The increase in annoyance for the pure tone (Fig. 11.3) was more evident than that of loudness (Fig. 11.4; see also Fig. 6.16). There are many more individual differences in the annoyance judgment than those in the loudness judgment. Loudness and annoyance produced different results. This is consistent with the findings obtained by Berglund et al. (1975), Hellman (1982, 1984), and Kuwano et al. (1988).

The ACF factors τe or φ1, which show the tonal component included in noise, had a significant effect on annoyance (Fujii et al., 2002). In this study, annoyance increased as the τe of the sound source increased within the critical band. Therefore, factors extracted from the ACF, such as τe, may be useful for evaluating noise. It was found that the annoyance of the pure tone and the band-pass noise with equal

Fig. 11.4 Scale value of loudness as a function of the bandwidth of band-pass noise. (a) The center frequency: 1 kHz. (b) The center frequency: 2 kHz. Each symbol represents a single subject. The line represents the mean scale value of eight subjects

11.1.2 Annoyance of Traffic Noise

The purpose of this section is to discuss the annoyance of traffic noise in relation to temporal factors, which are extracted from the ACF of noise. In order to avoid effects of spatial factors, the single loudspeaker located in front of a subject reproduced the noise recorded in a listening room.

The measured SPL and three ACF factors of the noise are shown in Fig. 11.5 as a time function (Fujii et al., 2002). Thick lines and thin lines show two extremes of

Fig. 11.5 Two extreme examples of measured SPL and three ACF factors as a function of time. Solid lines show factors of tonal noise (a motor bike), and dashed lines show factors of un-tonal noise (a passenger car)

noise: one has a clear pitch as a tonal and the other has a weak pitch as a nontonal because of a small value φ1 near 0.2, as defined in Section 5.2. The value of τ1 varied discretely 1 ms and 10 ms, meaning that perceived pitch varied 1000 Hz and 100 Hz for both noises. The strength of perceived pitch increases in proportion to the value of φ1 (Section 6.2). For the tonal noise, the φ1 value reaches a maximum around 0.7. Thus, a strong tonal noise is heard having a pitch of 100 Hz (τ1 = 10 ms). When the τ1 value varies with a high φ1 value, we perceive a variation of the pitch. If the φ1 value for the un-tonal noise remained constant around 0.2, in spite of the variation of τ1, then perceived pitch for the un-tonal noise is weak enough, and it is hard to discriminate pitch fluctuation.

In addition, the so-called previous standard measures may be extracted from the running ACF: (1) LL = mean SPL (dBA) due to ll(0) and rr(0); (2) variance σ2 of the SPL; (3) maximum SPL; (4) minimum SPL; (5) the SPL values exceeded 10% of the time (L10); (6) 50% of the time (L50); (7) 90% of the time (L90); and (8) equivalent sound level LAeq. As is indicated in Table 11.1, however, most of these standard measures were highly intercorrelated and thus not orthogonal. Clearly, all of these factors contain information about the overall sound level and its variability. Therefore, only the median (L50) as a representative of the mean SPL and LAeq, and the variance of the SPL (Var_SPL) as a representative of maximum SPL, minimum SPL, L10, and L90 were selected in the subsequent analysis.

The calculated power spectrum of the unfiltered signal and the autocorrelation function of the A-weighted filtered signal for the tonal and nontonal (atonal, untonal) noise are depicted in Fig. 11.6. For the tonal noise, there are several discrete

noise equal, an 0.5-s rise and fall time was added to all stimuli. To characterize the acoustic properties of a stimulus, we used the median and variance of each factor.

To find the effects of such fluctuations of noise level and its quality, we added the variance of SPL and ACF factors to the variables for describing the annoyance. The single loudspeaker was used to keep the spatial properties of the sound field constant. The subjects sat 1.0 m in front of the loudspeaker. Ten subjects (nine males and one female) participated in the experiment. They were between the ages of 23 and 27, in good health, with normal auditory acuity. Annoyance was judged by the PCT. All possible pairs from the nine sounds (36 pairs) were presented to the subject in a random order in one session. After the presentation of paired stimuli, the subject was asked to judge which of the two noises were more annoying. All subjects had four series of sessions, giving a total of 144 comparisons.

The scale value of annoyance for all the subjects was averaged, and the correlation coefficients were calculated between the annoyance and the median value and variance of the ACF factors. The correlation matrix between the ACF factors and the scale value (SV) of annoyance is shown in Table 11.2. Contrary to the assumption, perceived annoyance was not correlated with the SPL in this study due to the fact that the limited range of the SPL among the stimuli was 5.0 dBA. Instead, the variance of the SPL had greatly affected annoyance. In other words, Var_SPL, τe, and Var_τ1 are more effective on the scale value of annoyance than SPL in the range of about 5 dB. The values of τe and φ1 were significantly correlated with annoyance (r = 0.56 and 0.57, respectively, p < 0.05). This result shows that the noise having a strong tonal component was perceived to be more annoying than the nontonal noise. The comments of subjects also indicated that they judged a sound having a clear pitch to be more annoying.

Table 11.2 Correlations between the ACF factors (median and variance) and annoyance

p < 0.01, p < 0.05. Symbol Var represents the variance defined by σ2 = 1/n (xi – m)2, where xi is the value of data i, m is the mean value, and n is the number of the data.

Previously, on the evaluation of the perceived noise level for the tonal noise as used in the experiment, a number of tone corrections were used. For the sake of convenience, a value was added to the perceived noise level (PNL) to give the tone-corrected perceived noise level (PNLT). However, the calculation for this correction is lengthy, and its accuracy is not well established (May, 1978). Instead, as