A laboratory investigation into the threshold of the oppressive or

vibratory feeling to low-frequency pure-tone

Makoto Morinaga 1 Kanagawa University 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama-shi, Kanagawa 221-8686, Japan Shigenori Yokoshima Kanagawa Environment Research Center 1-3-39 Shinomiya, Hiratsuka-shi, Kanagawa 254-0014, Japan Tomohiro Kobayashi Sakae Yokoyama Koichi Makino Tetsuya Doi Kobayasi Institute of Physical Research 3-20-41 Higashi-motomachi, Kokubunji-shi, Tokyo 185-0022, Japan

ABSTRACT The oppressive or vibratory feeling caused by low-frequency sound is a widely known sensation inherent to that type of sound. Previous studies have shown the frequency characteristics of the sound pressure levels that induced this feeling, finding that it tended to occur before other feelings. However, its threshold has not yet been investigated. In the present study, we conducted a laboratory experiment to investigate the threshold by the method of adjustment. The participants adjusted the sound pressure level of pure tones by themselves until they started to feel an oppressive or vibratory sensation. The frequencies of the stimuli were 20 Hz, 40 Hz, 50 Hz, 63 Hz, and 80 Hz, in which range the results showed that the higher the frequency, the lower the threshold. Also, the threshold differed greatly among the participants, and the higher the frequency, the more pronounced the difference. In future work, we intend to examine the validity of these results by using a different method.

1. INTRODUCTION

Many studies have been conducted to evaluate impressions of low-frequency sound. Tokita and Nakamura [1] reported the existence of an oppressive or vibratory feeling as a sensation unique to low-frequency sound, and they showed the frequency characteristics of the sound pressure levels that induced this feeling, finding that it tended to occur before other feelings; the results of that study have been cited by various others [2–4]. Using an experimental method like that used by Tokita and Nakamura [1], Morinaga et al. [5] conducted an experiment to provide up-to-date findings on the frequency characteristics of the sound pressure levels at which the oppressive or vibratory feeling due to low-frequency sound tends to occur. They showed that the results of the experiment changed depending on how the participants understood the terms “oppressive feeling” and “vibratory feeling,”

1 m-morinaga@kanagawa-u.ac.jp

and the oppressive or vibratory feeling at 160 Hz tended to occur at lower sound pressure levels compared with the previous estimates [1].

In the studies by Tokita and Nakamura [1] and Morinaga et al. [5], the stimulus was one-third octave band noise, and it was suggested that the level fluctuation of that noise affected the oppressive or vibratory feeling. Morinaga et al. [6] conducted an experiment using a method similar to the one used in previous studies, and the frequency characteristics of the sound that induced the oppressive or vibratory feeling were investigated by using low-frequency pure-tone stimuli. The results showed that the sound pressure levels at which 50% of the participants in the study experienced the oppressive or vibratory feeling were higher than those obtained by Morinaga et al. [5], except at 20 Hz, and it was suggested that the sensitivity to the oppressive or vibratory feeling is the highest at 50 Hz. From re-analyzing the data of Tokita and Nakamura [1], Tagusari et al. [7] proposed a mathematical model describing resonance and obtained a frequency weighting that had a peak at approximately 50 Hz.

The aforementioned studies showed the frequency characteristics of the sound pressure levels that induced an oppressive or vibratory feeling, finding that it tended to occur before other feelings, but the threshold for the feeling requires further investigation. Takahashi [8] is among the few to have examined a threshold for vibratory feeling, and the results showed that the threshold decreases with increasing frequency up to 80 Hz. Tokita and Nakamura [1] also examined the threshold for the oppressive or vibratory feeling, and their results are similar to those of Takahashi [8]. However, Takahashi [8] did not examine oppressive feeling, which is difficult to distinguish from vibration sensation. Also, in the study by Tokita and Nakamura [1], the target frequencies were only 20 Hz, 40 Hz, and 80 Hz, and the stimulus was one-third octave band noise, leaving room for further investigation. In the present study, to investigate the threshold for oppressive or vibratory feeling in detail, a laboratory experiment using pure-tone stimuli was conducted by the method of adjustment.

2. EXPERIMENTAL METHOD

The method of adjustment was used to investigate the threshold of oppressive or vibratory feeling. The experiment was conducted at the Kobayasi Institute of Physical Research in an experimental room designed especially for investigating exposure to low-frequency sound. As shown in Fig. 1, the ceiling of the room was equipped with 16 loudspeakers with a diameter of 0.38 m for reproducing low-frequency sound, and the room dimensions were 2.8 m (L) × 2.1 m (W) × 2.2 m (H). Ascending and descending series were played twice for each of the five different frequency stimuli. The participants adjusted the sound pressure level of the pure tones by themselves to the level at which they started to feel an oppressive or vibratory sensation. The sound pressure levels of overtones in each stimulus were small, so the participants did not feel the oppressive or vibratory sensation because of the overtone components.

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Room interior

16 loudspeakers in ceiling

Digital volume controller Figure 1: Photographs of the experimental room.

2.1. Stimuli Five pure tones of 20 Hz, 40 Hz, 50 Hz, 63 Hz, and 80 Hz were used as stimuli and reproduced through the 16 loudspeakers. Because the sound pressure levels varied with height from the floor, the participants’ ear positions were set at 1.15 m from the floor. As shown in Table 1, the ascending series for 20 Hz started at 70 dB, whereas those for 40 Hz, 50 Hz, 63 Hz, and 80 Hz started at 30 dB; the descending series for 20 Hz, 40 Hz, 50 Hz, and 80 Hz started at 100 dB, whereas that for 63 Hz started at 90 dB. The order in which the frequencies were presented was randomized for each participant. Table 1: Starting level s of stimulus for ascending and descending series at eac h frequency.

Ascending series Descending series 20 Hz 70 dB 100 dB 40 Hz 30 dB 100 dB 50 Hz 30 dB 100 dB 63 Hz 30 dB 90 dB 80 Hz 30 dB 100 dB

2.2. Procedure As shown in Fig. 2, the ascending and descending series were played twice for each of the five different frequency stimuli. Using a digital volume controller equipped with a rotary encoder, the participant in the experiment adjusted the sound pressure level by themself to the sound pressure level at which they began to feel the oppressive or vibratory sense. In the ascending series, the sound pressure level started at a low level at which no oppressive or vibratory sensation was felt at all. In the descending series, the sound pressure level started at a high level at which the oppressive or vibratory sensation was sure to be felt. After the experiment, we asked the participants some questions about the experiment for feedback, and we conducted a noise sensitivity test according to WNS-6B [9].

Frequency 1 Ascending 1 Descending 1 Ascending 2 Descending 2

Frequency 2 Ascending 1 Descending 1 Ascending 2 Descending 2

Frequency 3 Ascending 1 Descending 1 Ascending 2 Descending 2

Frequency 4 Ascending 1 Descending 1 Ascending 2 Descending 2

Frequency 5 Ascending 1 Descending 1 Ascending 2 Descending 2

Figure 2: Experimental procedure.

2.3. Participants The participants were 30 adults aged between 20 and 59 years old; the distributions of age and gender are given in Table 2. The participants were recruited by our laboratory colleagues via telephone or e- mail, and none were acoustic experts; having recruited widely, we assumed that the participants were representative of the general population. Before the experiment, we conducted audiometry using a pure-tone audiometer and calculated the average hearing levels for 125 Hz, 500 Hz, 1 kHz, 2 kHz, and 4 kHz [10]. Each participant had an average hearing level of 25 dB or less for all the frequencies including 125 Hz; the World Health Organization [11] states that a hearing level of over 25 dB marks the onset of mild hearing loss. Accordingly, we considered all the participants to have normal hearing ability. Ten of the participants were defined as having high sensitivity according to the noise sensitivity test.

Table 2: Distributions of age and sex of participants.

Age (years) 20–29 30–39 40–49 50–59

Female ( n =15) 4 3 4 4

Male ( n =15) 3 4 3 5

3. RESULTS AND DISCUSSION

Data from 29 of the participants were analyzed, excluding those from one participant who did not reproduce the correct sound pressure level because of an error by the experimenter.

3.1. Threshold for oppressive or vibratory feeling

The average of the threshold values obtained from the two playings of the ascending and descending series was calculated for each participant, and the averages among the 29 participants are given in Table 3 and compared with the results of previous studies. It is found that the threshold decreases with increasing frequency, and this result is the same as that in previous studies [1, 8].

Table 3: Averages of threshold for oppressive or vibratory feeling among participants. The threshold va lues attributed to Takahashi [8] were obtained by extrapolation from the relevant figure therein.

20 Hz 40 Hz 50 Hz 63 Hz 80 Hz

Present study 88 dB 73 dB 70 dB 67 dB 63 dB

Tokita and Nakamura [1] 87 dB 66 dB - - 65 dB

Takahashi [8] 88 dB 70 dB 69 dB 61 dB 58 dB

3.2. Individual differences in threshold A boxplot of the threshold of oppressive or vibratory feeling for all the participants is shown in Fig. 3, where the symbol “x” indicates the mean value and the horizontal line in a box indicates the median value; the top of a box is the 75th percentile value, and the bottom is the 25th percentile value. As can be seen, the difference is approximately 10–30 dB and tends to widen with increasing frequency.

Multiple regression analysis was conducted with the independent variables of frequency (dummy variable referenced to 20 Hz), sex, age (under 30 or not), and noise sensitivity (high or low) to examine the reasons for the vast individual differences, and the results are given in Table 4. It was found that noise sensitivity and sex were significant at the 5% level, and the thresholds of the more- sensitive or younger participants were lower than those of the less-sensitive or older participants. These results suggest that these individual factors caused the significant differences in threshold among the participants.

4. CONCLUDING REMARKS

In the present study, an experiment was conducted using the method of adjustment, and the threshold for the oppressive or vibratory feeling due to low-frequency sound was investigated. As was the case in previous studies, the threshold decreased with increasing frequency in the range of this experiment. Also, it was found that the threshold differed significantly among the participants because of individual factors such as noise sensitivity and gender; however, another reason might have been experimental difficulty. For more-robust experimental results, we plan to conduct further experiments with more participants and with methods other than adjustment.

max

75 percentile

median mean

ｘ

25 percentile

min

Figure 3: Ranges of participants’ thresholds for oppressive or vibratory feeling.

T able 4: Results of multiple regression analysis; an asterisk indicates significance at the 5% level.

Coefficient Standard error t -value p -value

Intercept 96.53 2.72 35.44 0.00

40 Hz* −14.57 3.17 −4.60 0.00

50 Hz* −17.89 3.17 −5.65 0.00

63 Hz* −21.46 3.17 −6.78 0.00

80 Hz* −24.57 3.17 −7.76 0.00

Noise sensitivity (high)* −8.08 2.12 −3.81 0.00

Sex (female)* −7.84 2.02 −3.89 0.00

Age (under 30) −3.46 2.01 −1.73 0.09

5. ACKNOWLEDGMENTS

We gratefully acknowledge Mr. Tomoya Kibishi, Mr. Yu Kuwabara, and Mr. Takato Oyagi, graduates of Kanagawa University, for their assistance in carrying out this experiment. 6. REFERENCES

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