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Ultrasonic hearing and the hypersonic effect

Posted by KlausR. (A) on January 7, 2009 at 01:14:26

As some of you possibly remember, the “hypersonic effect” has been discussed on these pages many moons ago. Back then I intended to look up the relevant literature and prepare an overview. This overview has been lying in the drawer since then, but a recent discussion on ultrasonic hearing reminded me so I went and finished. I’ve prepared a short version for posting purposes; those who are interested in reading the extended version simply drop me a mail.


Ultrasonic hearing and the hypersonic effect


It is generally accepted that audio frequencies above 20 kHz are beyond the audible range of humans. Absolute thresholds usually start to increase sharply above 15 kHz and reach about 80 dB SPL at 20 kHz. Threshold values at 24 kHz and above are more than 90 dB SPL. Some humans can perceive tones up to 28 kHz when their level exceeds about 100 dB SPL (Ashihara 2007)

Ultrasound whose frequency ranges up to at least 120 kHz can be perceived by bone conduction. Bone conduction requires an ultrasound source (e.g. ceramic vibrator) being mechanically coupled to the temporal bone (Nishimura et al. 2003).

High resolution audio formats such as SACD and DVD-A have frequency response up to 100 and 96 kHz, respectively. They have not experienced general acceptance in the market so that scientific research of ultrasonic perception seems to be performed only for reasons of medical use such as treatment of tinnitus, diagnosis of sudden deafness, Meniere’s disease, noise-induced hearing loss, and hearing aids (Nishimura et al. 2003).

Some research, however, is specifically performed in the context of music reproduction, so I thought it might be interesting to look into what this particular research has found so far. I’m extracting only the physiological experiments from the various papers, not the psychological and behavioural. The latter result in general in experiencing the sound more comfortable to the ear and in increased “comfortable” listening level when high frequency components are present. Some behavioural experiments were performed with the participants blindfolded and they were not informed of the purpose of the experiments.




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ULTRASONIC HEARING

Muruoka et al (1981) tried to find out what upper band limit audio equipment should have. According to their results 15 kHz was sufficient with highly trained listeners being able to discriminate a 20 kHz cutoff.

Ashihara et al. (2001) found that under conditions in which experimental artefacts had been adequately eliminated, ultrasounds would be extremely difficult to be perceived and that they may have little influence on the sound image and its localisation.

Ashihara’s paper has been discussed by Griesinger (2003). Griesinger also did some experiments himself: “Result: nothing significant is heard. No difference could be heard with and without the ultrasonics. When ultrasonics only were played at high levels, intermodulation products from the input signals were easily heard at levels consistent with amplifier distortion.”

Nishiguchi et al. (2003) found no significant difference between sound stimuli with and without frequency components above 21 kHz.

Hamasaki et al. (2004) compared sound stimuli with to stimuli without frequency components above 22 kHz. They found no significant correct responses except for two subjects, who perceived the differences between with and without higher frequencies band above 22 kHz only for a longer stimulus with the highest level of very high frequency components.

In experiments performed by Sugimoto et al (2005) the subjects judged various aspects of subjective sound quality, in particular REALITY. It was found that the presence of high frequency components enhanced the apparent REALITY. The sound was guided towards the ears by small tubes.

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Ashihara “Hearing thresholds for pure tones above 16 kHz”, The Journal of the Acoustical Society of America 2007, Volume 122, Issue 3, pp. EL52-EL57

Nishimura et al., “Ultrasonic masker clarifies ultrasonic perception in man”, Hearing Research 2003, Volume 175, pp.171-177

Muraoka et al., “Examination of audio-bandwidth requirements for optimum sound signal transmission”, Journal of the Audio Engineering Society 1981, p.2

Ashihara et al., “Detection threshold for tones about 22 kHz”, 110th AES convention 2001, preprint no. 5401

D. Griesinger, “Perception of mid frequency and high frequency intermodulation distortion in loudspeakers and its relationship to high-definition audio”, 24th International AES Conference 2003, Banff, Canada
www.davidgriesinger.com/intermod.ppt

Nishiguchi et al., “Perceptual discrimination between music sounds with and without very high frequency components”, 115th AES convention 2003, preprint no. 5876

Hamasaki et al., “Perceptual Discrimination of Very High Frequency Components in Musical Sound Recorded with a Newly Developed Wide Frequency Range Microphone”, 117th AES convention 2004, preprint no. 6298

Sugimoto et al, “Human perception model for ultrasonic difference tones”, Proceedings of the 24th IASTED International Conference, Feb 16-18, 2005, Innsbruck, Austria


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HYPERSONIC EFFECT

The phenomenon of increased brain activity in the presence of inaudible high frequency sounds has been termed “hypersonic effect” by Oohashi et al (2000).

There are several papers investigating the hypersonic effect:

Oohashi et al. (1991), “High frequency sound above the audible range affects brain electric activity and sound perception”, 91st AES convention, preprint no. 3207

Nakamura et al. (1999), “Analysis of music-brain interaction with simultaneous measurement of regional cerebral blood flow and electroencephalogram beta rhythm in human subjects”, Neuroscience Letters 275, p.222-226

Oohashi et al. (2000), “Inaudible high-frequency sounds affect brain activity: hypersonic effect”, Journal of Neurophysiology 83, p. 3548-3558
http://www.linearaudio.nl/Documents/high%20freq%20inpact%20on%20brain.pdf

Oohashi et al. (2002), “Multidisciplinary study on the hypersonic effect”, International Congress Series 1226, pp.27-42

Yagi et al. (2002), “Auditory display for deep brain activation: hypersonic effect”,
Proceedings of the 2002 International Conference on Auditory Display, Kyoto, Japan, July 2-5, 2002
http://www.icad.org/websiteV2.0/Conferences/ICAD2002/proceedings/Oohashi.pdf

Yagi et al. (2003), “Modulatory effect of inaudible high-frequency sounds on human acoustic perception”, Neuroscience Letters 351, p.191-195

Oohashi et al. (2006), “The role of biological systems other than auditory air-conduction in the emergence of the hypersonic effect”, Brain Research 1073-1074, p. 339-347

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In general, a bi-channel sound system was used with cross-over frequency at 26 kHz (170 dB/octave) or at 22 kHz (80 dB/octave). Sound stimulus was a 200 – 400 s extract of traditional Gamelan music of Bali. Test subjects were aged 19-43 years.

Two types of measurements were made:

EEG (electroencephalogram) at 12 scalp points (10-20 electrode system), data were determined in alpha (8-13 Hz) and beta (13-30 Hz) bands.

PET (positron emission tomography) measurement of regional cerebral blood flow (rCBF)

Presentation of stimulus:


1) FRS = full-range sound = HFC + LFC (High Frequency Components + Low Frequency Components)
2) high-cut sound (HCS) = only LFC
3) low-cut sound (LCS) =only HFC
4) baseline = no sound except for ambient noise

Results of the measurements were:

EEG

Alpha
Alpha-EEG was greater during FRS than during LFC, HFC or baseline. When sound was presented through earphones, no difference was found between FRS and LFC. When FRS was presented through earphones and LFC through speakers, alpha –EEG was significantly greater during FRS than during LFC alone. When head and body surface were insulated from exposure to HFC, the increase in alpha-EEG during FRS was markedly suppressed.

Beta
Beta power was significantly higher during music condition than during rest condition.

PET
Compared with resting, listening to music caused an increase in rCBF in the temporal regions bilaterally. An increase in rCBF in the bilateral superior temporal gyri was observed, including the primary and secondary auditory cortices. Increased rCBF in certain regions (brainstem and the lateral part of the left thalamus, bilateral superior temporal gyri, primary and secondary auditory cortices) was observed with FRS as compared to the other conditions

When HFC was compared with the baseline, no significant differential activation was observed anywhere in the brain, and neither the left thalamus nor the brainstem showed changes in rCBF.

The following conclusions were drawn:

Oohashi et al. (2002):
“Despite the fact that nonstationary HFC was not perceived as a sound itself, we demonstrated that the presentation of sounds that contained a considerable amount of nonstationary HFC (i.e. FRS) introduced various kinds of responses to listeners. In the physiological study, FRS significantly increased rCBF in the deep-lying brain structures, including the brainstem and thalamus, and enhanced the power of the spontaneous EEG activity of alpha range, compared with the same sound lacking HFC (i.e. HCS).
Although how inaudible HFC produces a physiological affect on brain activity is still unknown, we need to consider at least two possible explanations. The first is that HFC might change the response characteristics of the tympanic membrane in the ears and produce more realistic acoustic perception, which might increase pleasantness. An alternative explanation is that HFC might be conveyed through pathways, distinct from the usual air-conduction auditory pathway, affecting the central nervous system. It was reported that the vibratory stimulus of ultrasound modulated by the human voice activated the primary auditory cortex and was successfully recognized by people with normal hearing as well as those whose hearing was totally impaired. Although we cannot conclude that the neural mechanisms incorporating ultrasound hearing are the systems responsible for the hypersonic effect, it is notable that the ultrasound can reach to the central nervous system.”

Yagi et al. (2003):
“Taken together, the results of the present study demonstrate that an enhanced HFC increased the comfortable listening level and improved the subjective impression of the sound in association with an increase in the alpha-EEG. These results further suggest that the inaudible HFC has a modulatory effect on human sound perception and that such effect may not linearly increase as the intensity of the HFC increases, but has some optimum point.”

Oohashi et al. (2006):
“These data indicate that the hypersonic effect was evoked only when HFC was presented to the head and/or body surface. The point of the present experimental design is to focus on the fact that the hypersonic effect does not emerge at the presentation of HFC alone but it emerges only when HFC and LFC were simultaneously presented. The fact that absolutely no hypersonic effect was observed under this condition [LFC and HFC through earphones] demonstrates that the air-conducting auditory system does not respond to HFC.”

“The finding compares well with the previous report that the activation of the brainstem and thalamus serve as a neurophysiological basis of the hypersonic effect. It is reasonable to consider, therefore, that the hypersonic effect detected by the increase in the power of the alpha-EEG in the present study reflects the activation of the deep-lying brain structure, including the brainstem and thalamus. The experimental findings in this study cannot be explained by the air-conducting auditory system alone; they can be explained with less contradiction by assuming the existence of some hitherto unknown sensing mechanism somewhere on the body surface, even on the head. We must also consider the possible existence of an unrecognized sensing mechanism.”

The J. Neurophysiology article has been discussed on Audio Asylum and Kal Rubinson, Neuroscientist himself, mentioned some points of concern:

1. The full-range sound in Oohashi papers is an acoustic combination, not an electronic one, so that difference tones are possible (Kal refers to the paper by Sugimoto et al, 2005)

2. A time-lag between the presentation of the stimulus and the EEG-response has been observed. A tighter temporal correlation between signals and response would be desirable. Is the spectral distribution of the music uniform with time?

3. The mapped statistical data of thalamus and brainstem are unilateral and asymmetric, whereas the ears are known to project bilaterally to brainstem, thalamus and auditory cortex, as can be seen in the PET data for cortex activity. Oohashi indicates however that they found responses in the left thalamus.

http://www.audioasylum.com/audio/general/messages/459943.html

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