Perceptual Discrimination of Very High Frequency Components in Musical Sound Recorded with a Newly Developed Wide Frequency Range Microphone

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1 Audio Engineering Society Convention Paper 6298 Presented at the 117th Convention 24 October San Francisco, CA, USA This convention paper has been reproduced from the author's advance manuscript, without editing, corrections, or consideration by the Review Board. The AES takes no responsibility for the contents. Additional papers may be obtained by sending request and remittance to Audio Engineering Society, 6 East 42 nd Street, New York, New York , USA; also see All rights reserved. Reproduction of this paper, or any portion thereof, is not permitted without direct permission from the Journal of the Audio Engineering Society. Perceptual Discrimination of Very High Frequency Components in Musical Sound Recorded with a Newly Developed Wide Frequency Range Microphone Kimio Hamasaki 1, 2, Toshiyuki Nishiguchi 1, Kazuho Ono 1 and Akio Ando 1 1 NHK Science & Technical Research Laboratories, Tokyo, , Japan {hamasaki.k-ku, nishiguchi.t-gy, ono.k-gs, ando.a-io}@nhk.or.jp 2 Kyushu University, Graduate School of Design, Fukuoka, , Japan ABSTRACT Subjective evaluation tests on perceptual discrimination between musical sounds with and without very high frequency (above 2 khz) components have been conducted. To make a precise evaluation, the test system is designed to exclude any influence from very high frequency components in the audible frequency range. Moreover, various sound stimuli are originally recorded by a newly developed very wide frequency range microphone, in order to contain enough components in very high frequency range. Tests showed that some subjects might be able to discriminate between musical sounds with and without very high frequency components. This paper describes these subjective evaluations, and discusses the possibility of such discrimination as well as the high resolution audio recording of music. 1. INTRODUCTION It is generally accepted that humans cannot perceive sounds in a very high frequency range over 2 khz. Thus, the upper limit of the frequency range in conventional digital audio formats such as CD, DAT and digital audio broadcasting is typically set at about 2 khz [1]. Nevertheless, some papers have discussed the influence of such inaudible high frequency components in musical sounds on the auditory sense or brain activity in recent years [2]-[6], and digital audio formats such as SACD and DVD having a frequency response of close to 1 khz have recently become available. If such extension of the frequency range affects the perception of sound, it must be caused by reproduction of very high frequency components or some other factor. If very high frequency components are the major factor, recording and reproduction of very high frequency components would be important. However, if some other factor is dominant, it would be independent of frequency range and efforts to improve sound quality

2 should be focused on such factor rather than on frequency range. Accordingly, in a previous paper [7], subjective evaluation tests to study perceptual discrimination between musical sounds with and without very high frequency components above 21 khz were reported. The sound stimuli of various pieces of music used for the evaluation tests were newly recorded by authors to maintain the highest quality for proper sound reproduction, and most of the subjects for the tests were selected from among professional audio experts and musicians. The results did not show a significant possibility of discriminating between sounds with and without very high frequency components. Through further consideration of these results, the following issues on stimuli and subjects emerged: stimuli recorded by conventional microphones did not include enough high frequency components and professional audio experts were not necessarily proper for such subjective evaluation tests. Therefore, subjective evaluation tests for examining the perception of very high frequency components were conducted with newly recorded sound stimuli and with a new listening panel including many young students of a musical university. Digital Filtering DAW SADiE ATEMIS Cool Edit Pro HP LP HP LP 1kHz 1kHz 21kHz Original Source Time Code > 9dB Controller Laguna Hills SYSTEM1E mute On/Off D/A dcs 954 Master Clock dcs 992 D/A dcs EXPERIMENT SYSTEM AND DESIGN 2.1. Experiment system In order to make a strict subjective evaluation test, the sound reproduction system for the test was designed to exclude any leakage or influence of very high frequency components in the audible frequency range, whereas some previous studies had been unable to exclude such leakage or influence completely. The test system consisted of two completely independent sound reproduction systems, one for the audible frequency band, and the other for the very high frequency band as shown in Figure 1. Each system had independent sound equipment, namely D/A converters, power amplifiers, loudspeakers, and power supply units. The sound source used as stimuli for the subjective evaluation tests also consisted of two frequency bands (below and above 21 khz) that were divided by 124- tap FIR digital low-pass and high-pass filters, which had very sharp roll-off characteristics as shown in Figure 2. The cut-off frequency for the low-pass filter was 2 khz, and that of the high-pass filter was 22 khz. The transition bandwidth of both filters was 1 khz and the rejection level was set over 9 db. Accordingly, the sound sources were divided almost perfectly into an audible frequency band (below 21 khz) and a very high frequency band (above 21 khz) without any overlap in Mute Unit Current CSP168 Custom AC1V AC1V Control Computer Remote Controller Amp. Amp. Super Tweeter PIONEER PT-R9 Clean Power Supply Accuphase PS-12V Clean Power Supply Accuphase PS-12V Speaker B&W Nautilus 81 Figure 1: Diagram of experimental system AES 117th Convention, San Francisco, CA, USA, 24 October Page 2 of 9

3 the frequency domain, and each frequency band was recorded independently on a different track of a digital audio workstation. Of course, these two different frequency bands were precisely synchronized. During the subjective evaluation tests, each frequency band was reproduced from different tracks and amplified independently. A mute unit was inserted between the amplifier and D/A converter of the very high frequency band. Throughout the evaluation tests, the sound of the audible frequency band was always reproduced and the sound of the very high frequency band was muted or reproduced according to the test sequence. As this method excluded any influence of inter-modulated distortion by the very high frequency band components within the audible frequency band [8]-[9], it was possible to make an absolute comparison between sounds with and without very high frequency components. The digital equipment in the test system was operated at a sampling rate of 192 khz and with 24-bit resolution. The evaluation tests were conducted in a listening room that was designed based on ITU-R BS1116. Loudspeakers were placed in a two-channel stereophonic arrangement, and the distance between each loudspeaker and a subject was set at 3 m. The height of the subject s ear was adjusted to the height of the super tweeter which reproduced the very high frequency band. The overall frequency response of this test system is shown in Figure 3. At around 8 khz, the reproduction level dropped to approximately 1 db from the reference level of 1 khz. The sound pressure level of the standard reproduction level of each stimulus was set at approximately 8 db(a) at its peak level, and the subject was allowed to adjust this level within 75 to 8 db(a) Recording of sound stimuli Sound stimuli were originally recorded in two-channel stereo with linear PCM of 192 khz sampling rate and 24-bit resolution, so that the recording conditions, selection of musical instruments as sound sources and characteristics of sound stimuli could be easily controlled. In previous experiments [7], sound stimuli were recorded using conventional condenser microphones designed for music recording use. Because the frequency responses of those microphones were limited up to around 4 khz, it was not possible to capture the very high frequency components sufficiently. In order to conduct the experiments properly, the frequency range of the recording system Gain (db) Response (db) low-pass filter high-pass filter Figure 2: Characteristics of band-dividing filters -2 Microphone: B&K Measuring Amp.: B&K 261 FFT Analyzer: AP System 2 Cascade Figure 3: Overall frequency response of experimental system AES 117th Convention, San Francisco, CA, USA, 24 October Page 3 of 9

4 should be wide enough to cover the entire frequency range of actual musical instruments. As the digital recording system of 192 khz sampling can record up to 1 khz, the frequency range of the microphone should also extend up to 1 khz. Some conventional condenser microphones designed for measurement have such a wide frequency range, however, the signal-tonoise ratio is too poor for musical recording. In recent experiments involving recording sound stimuli, a newly developed very wide frequency range microphone [1] was used, which was designed to have a very wide frequency range up to 1 khz by taking advantage of the diffraction effect on its membrane. Figure 4 shows the frequency response of this new microphone. This microphone can be used for recording a broad range of music except pop music, because its self noise level is below 2 db(a) and its directional pattern in omni Experimental design The pair test method was applied to the subjective evaluation experiments. A pair of sound stimuli labeled A and B was presented repeatedly, and there were two types of A-B sets as follows: (1) Either of A or B reproduced only the audible band without the very high frequency band, while the other reproduced both the audible and very high frequency bands. (2) Both A and B reproduced only the audible band without the very high frequency band. The subjects were asked to state whether A and B are the same or not. 3. SUBJECTIVE EVALUATION EXPERIMENT 3.1. Experiment Stimuli Three types of stimuli were used in the subjective evaluation tests as shown in Table 1. Stimulus 2 was played by a Japanese traditional musical instrument, Chikuzen-Biwa, which looks like a lute but is played with a bigger and harder plectrum. The duration of each stimulus was set between 9 s to 12 s, because it was reported that the influence of very high frequency components above the audible frequency range would be observed as a change of alpha waves on an electroencephalogram after presenting the stimulus for several tens of seconds and that the influence would remain for around 1 s. It was also reported that the duration of stimulus might affect the results of subjective listening tests [2], [3]. As mentioned above, the pair of stimuli A and B did not include a combination of both stimuli with very high frequency band in order to shorten the duration of reproduction of the very high frequency band in the whole test in consideration of the continuous influence of very high frequency components on the subjects hearing during the experiment. Figure 5 shows the characteristics of each sound stimulus. It shows measured or calculated sound pressure level of both the audible frequency band and very high frequency band at the listening position, and Sensitivity (dbv/pa) Figure 4: Frequency response of newly developed very wide frequency range microphone No. Title Musical Instrument Experiment 1 Experiment Faure: String Quartet Violins, Viola, ViolinCello a, b, c "Heike Monogatari" "Chikuzen-Biwa", Female narration J.S. Bach: France Suite No.2 - Air Harpsichord Table 1: Sound stimuli AES 117th Convention, San Francisco, CA, USA, 24 October Page 4 of 9

5 Level (db SPL ) Relative Level (db) Level (db SPL ) Relative Level (db) No. 1-1 Reproduction part below 21 khz above 21 khz distortion below 21 khz No Time (s) (a) No.1-1, 2-1 Faure String Quartet No. 1-2 Reproduction part No. 2-2a No. 2-2b No. 2-2c 1 8 below 21 khz above 21 khz 6 4 distortion below 21 khz Time (s) (b) No.1-2, 2-2a, b, c Chikuzen-Biwa the typical amplitude spectrum on each stimulus. The sound pressure level of the very high frequency band at the listening position was calculated according to the frequency characteristics of the reproduction system for this experiment Subjects Seven students of a university of music, two composers, one musical player, and three music teachers were selected for the listening panel for this test. Eight of the 13 subjects were female and the youngest subject was 19 years old and the oldest was 51 years old. The subjects were asked to listen to a pair of stimuli with 3 s interval, and after whole reproduction of the pair of stimuli, the subjects were asked to state whether the two stimuli were the same or different. All subjects listened to 32 pairs of each stimulus and made evaluations Results Figure 6 shows the results of experiment 1 in which the Subject Correct Response (%) No. Age Gender male 2 2 female 3 2 female 4 21 female 5 22 female (p =1.%) Level (db SPL ) Relative Level (db) No. 1-3 Reproduction part No below 21 khz 6 4 above 21 khz 2 distortion below 21 khz Time (s) (c) No.1-3, 2-3 Harpsichord 6 24 female 7 25 female 8 29 male 9 3 male 1 31 male female female male (p =.35%) Sound Stimulus No Figure 5: Analyses of sound stimuli Figure 6: Results of experiment 1 AES 117th Convention, San Francisco, CA, USA, 24 October Page 5 of 9

6 13 subjects had to evaluate each stimulus 32 times. The mathematical evaluation of the test data is based on a stochastic model of the binominal distribution. For each evaluation, the subject had to state whether stimulus A and B were the same or not. Thus, the probability of giving the correct answer was always.5 and each answer could be regarded as independent under the same condition for each evaluation, and so probability distribution of the rate of correct answers follows the binominal distribution. Accordingly, the significance probability (p value) of correct answers can be found from the cumulative distribution function of the binominal distribution. Since the rate of correct answers by subjects who can discriminate the difference should be higher, a one-sided test with 5% significance level can be applied to the analysis. Based on this test, evaluation results on stimulus 1-2 (Chikuzen-Biwa) by subject 2 (p = 1.%) and subject 9 (p =.35%) showed a significant probability. Subject 2 was a 2-year-old female student of a university of music and subject 9 was a 3-year-old male student of the same university Experiment Stimuli Experiment 1 shows the possibility of perceptual discrimination of very high frequency components in musical sound. However, the duration of stimulus is generally not as long as that in experiment 1 for such test design. Some of the subjects also reported that it was too long to memorize the first stimulus for comparing with the second stimulus. Thus, the duration of stimuli was set at around 2 s in experiment 2, and the influence of duration on the subjective evaluation was studied. As shown in Table 1 and Figure 5, five different stimuli were used for this test: they were extracted from the part with the greatest abundance of high frequency components of each stimulus of experiment 1, and 3 of the 5 stimuli were different parts of stimulus 1-2 played by Chikuzen-Biwa, which yielded a significant correct response from two subjects Subjects A 2-year-old female student, a 24-year-old female musician and a 3-year-old male student served as the listening panel for this test. Two of the 3 subjects gave a significant correct response in experiment 1. All subjects were asked to evaluate 4 pairs of each stimulus Results The results in Figure 7 show no significant correct response with 5% significance level in this test. Subjects 2 and 9, who gave correct answers with more than the significance probability in experiment 1, also had a lower correct rate than the significance probability for every stimulus in this test. This indicates that the duration of stimulus might affect the discrimination between with and without very high frequency components. Subject Correct Response (%) No. Age Gender female 6 24 female 9 3 male Figure 7: Results of experiment 2 Sound Stimulus No a 2-2b 2-2c MEASUREMENT OF HEARING THRESHOLD FOR PURE TONES ABOVE 2 KHZ According to the results of experiment 1, two subjects surely discriminated between musical sounds with and without very high frequency components. The other subjects, however, gave correct responses at around 5%, i.e. they could not discriminate. This result raises the question, do the two subjects have a higher hearing threshold above 2 khz than the other subjects? To discuss this question, the hearing threshold of these two subjects for pure tones above 2 khz was measured. As a conventional audiometer cannot measure such high frequency threshold, hearing thresholds were measured by using a loudspeaker for reproducing very high frequency stimuli in a free sound field, actually in an anechoic chamber. By combining a 2I2AFC (2 Interval 2 Alternative Forced Choice) procedure with 3-down 1- up transformed up-down method [11], thresholds as high as 9 db SPL were measurable [12]. The procedure of measurement was as follows. Two intervals of 2 s were presented with a 2 s intermission to AES 117th Convention, San Francisco, CA, USA, 24 October Page 6 of 9

7 a subject. In either of the two intervals, a pip tone of a certain frequency and 25 ms duration was presented four times. The subject was then asked to answer in which interval she or he had heard the pip tone. If a subject did not give the correct answer, the sound pressure level of the pip tone was raised, whereas if a subject gave the correct answer three times in succession, the level was reduced. The level was raised or reduced by 4 db until the first reversal and subsequently was raised or reduced by 2 db. On the 5th reversal, measurement was finished and the mean of the presented sound pressure level at the 4th and 5th reversal was decided as the hearing threshold at a certain frequency. When the maximum sound pressure level exceeded 9 db SPL, measurement was stopped. Table 2 shows the results and indicates that both subject 2 and subject 9 could not hear the pure tone above 22 khz within 9 db SPL. Subject No > >9 >9 (db SPL) Table 2: Hearing threshold measured with loudspeaker reproduction in a free sound field 5. NONLINEAR DISTORTION OF A LOUDSPEAKER The results of hearing threshold showed that the subjects who had discriminated between the musical sounds with and without very high frequency components, could not hear the pure tone in the very high frequency band. So how could they have perceived the difference between musical sounds? As previously stated, the experiment system was designed very carefully in order to exclude any leakage or influence of very high frequency components in the audible frequency range. It is, however, also impossible to exclude any nonlinear distortion entirely from conventional loudspeakers [13], [14]. To examine why they could perceive the difference, we measured the level of nonlinear distortion within the audible frequency range, which was produced by a super tweeter for reproducing the very high frequency components. The experiment system was set in an anechoic chamber, and the very high frequency band of the three stimuli shown in Table 1 were only reproduced by a super tweeter (Pioneer PT-R9), and then the level of nonlinear distortion within the audible frequency range was measured. This measurement was done with a microphone (B&K ), microphone amplifiers (B&K 2636 and Current CSP-353), and A/D converter (dcs 94). A measuring microphone was set at a distance of 19 cm from the super tweeter, because the level of nonlinear distortion is too small to be measured at the listening point compared with the self noise level of the measuring microphone. Measured data was filtered by LPF the same as in Figure 1, and then was converted to the sound pressure level at the listening point by calculating sound attenuation with distance. The level of distortion at the listening position is also shown in Figure 2. On stimulus 1-2, Chikuzen-Biwa, the maximum level of distortion is from 2 to 25 db SPL, and was far less on the other stimuli. Compared with the average level of actual sound in the audible frequency band, this distortion level is very small and may be neglected in view of the masking effect. If the subjects can discriminate the differences with this nonlinear distortion as a clue, there is no reason why they could not discriminate the differences with the same stimulus of a shorter duration, 2 s. Although the two subjects could clearly discriminate between the stimulus of the Japanese traditional musical instrument, the Chikuzen-Biwa, with and without very high frequency components, it is not yet understood how they could do so. 6. SUMMARY AND CONCLUSIONS Subjective evaluation tests of perceptual discrimination between musical sounds with and without very high frequency components were conducted in order to determine the necessary bandwidth for recording music. To make a precise evaluation, the test system was strictly designed to ensure that the very high frequency components did not affect the audible frequency band. Sound sources used for stimuli were primarily divided into the audible frequency band (below 21 khz) and very high frequency band (above 21 khz), and each band was represented independently. Sound stimuli were re-recorded with a newly developed very wide frequency range microphone in order to sufficiently capture the very high frequency components. The pair AES 117th Convention, San Francisco, CA, USA, 24 October Page 7 of 9

8 test method was used in the subjective evaluation experiments. This paper discussed the two kinds of subjective evaluation test. The duration of sound stimuli was different in each test: the shorter duration was around 2 s, and the longer one was around 2 minutes. The results of these evaluation tests showed that two subjects achieved a to significant correct answer rate at the 5% significance level for one sound stimulus, Chikuzen- Biwa, of long duration, but that no significant difference was found for the other sound stimuli of short and long duration. To examine the results, the nonlinear distortion level within the auditory band caused by reproduction of the very high frequency band was measured. From this measurement, 25 db SPL of nonlinear distortion within the audible frequency band was observed in the sound stimulus for which the two subjects achieved a significant rate of correct answers. The hearing threshold above 2 khz of the two subjects was also measured and it was found that their hearing threshold was below 22 khz. According to these results, the truth can be stated only as follows: two subjects, who could not hear the pure tone above 22 khz, perceived the differences between with and without higher frequency band above 22 khz only for a longer stimulus with the highest level of very high frequency components. We have no hypothesis or scientific reasons that can explain this finding; additional precise studies are required in order to have further discussions. It also remains impossible to indicate the necessary frequency bandwidth for recording music. However, it might be worth recording music by high resolution audio systems with very wide frequency range, because the possibility that such very high frequency band might affect human perception cannot be entirely discounted. Of course, the high resolution audio recording needs large storage area of audio data in comparison with conventional digital recording system, and it requires the very accurate handling in order to produce best sound quality. 7. ACKNOWLEDGEMENTS The authors would like to thank Sanken Microphone Co., Ltd. for their assistance in producing a very wide frequency range microphone, and the musicians for recording a wonderful musical performance to be used as sound stimuli. 8. REFERENCES [1] T. Muraoka, M. Iwahara, et al., Examination of Audio-Bandwidth Requirements for Optimum Sound Signal Transmission, Journal of Audio Engineering Society, Vol. 29, PP. 2-9 (1981) [2] T. Oohashi, E. Nishina, et al., High-frequency sound above the audible range affects brain electric activity and sound perception, AES 91st Convention, Convention Paper 327 (1991) [3] T. Oohashi, E. Nishina, M. Honda, et al., Inaudible High-Frequency Sounds Affect Brain Activity: Hypersonic Effect, J. Neurophysiology, pp (2) [4] S. Yoshikawa, S. Noge, M. Ohsu, et al., Sound Quality Evaluation of 96kHz Sampling Digital Audio, AES 99th Convention, New York, USA, Convention Paper 4112 (1995) [5] K. Ashihara, S. Kiryu, K. Kurakata, et al., Perceptual effects caused by high- and ultrasonicfrequency components in musical sounds, AES 9th Regional Convention, Tokyo, Japan, Preprint, pp (21) (in Japanese) [6] K. Kurakata, N. Nakamura, A. Shibasaki, et al., Perceptual effects of high- and ultrasonicfrequency components in musical sounds, AES 9th Regional Convention, Tokyo, Japan, Preprint, pp (21) (in Japanese) [7] T. Nishiguchi, K. Hamasaki, et al., Perceptual discrimination between musical sounds with and without very high frequency components, AES 115th Convention, New York, USA, Convention Paper 5876 (23) [8] S. Kiryu, K. Ashihara, Problems in High-sampling Audio, Technical Report of IEICE, EA99-1, pp (1999) (in Japanese) [9] K. Ashihara, S. Kiryu, Detection threshold for tones above 22 khz, AES 11th Convention, Amsterdam, The Netherlands, Convention Paper 541 (21) (in Japanee) AES 117th Convention, San Francisco, CA, USA, 24 October Page 8 of 9

9 [1] M. Iwaki, A. Ando, et al., A design of wide frequency range microphone using diffraction effect, Technical Report of IEICE, Tokyo, Japan, EA23-5, pp (23) (in Japanese) [11] H. Levitt, Transformed up-down methods in psychoacoustics, The Journal of the Acoustical Society of America, Vol. 49, pp (1971) [12] K. Ashihara, K. Kurakata, et al., Hearing threshold for pure tone above 2 khz, The Acoustic Society of Japan, Transactions on Technical Committee of Psychological and Physiological Acoustics, H (23) (in Japanese) [13] S. Kiryu, K. Ashihara, et al., Broader bandwidth of signals affects nonlinearity distortions of loudspeakers, The 1999 Spring Meeting of the Acoustical Society of Japan, proceeding pp (1999) (in Japanese) [14] D. Griesinger, Perception of mid frequency and high frequency intermodulation distortion in loudspeakers, and its relationship to high-definition audio, AES 24th International Conference, Banff, Alberta, Canada, Presentation Slides, (23) AES 117th Convention, San Francisco, CA, USA, 24 October Page 9 of 9