Hearing thresholds for pure tones between 16 and 30kHz were measured by an adaptive method. The maximum presentation level at the entrance of the outer ear was about 110dB SPL. To prevent the listeners from detecting subharmonic distortions in the lower frequencies, pink noise was presented as a masker. Even at 28kHz, threshold values were obtained from 3 out of 32 ears. No thresholds were obtained for 30kHz tone. Between 20 and 28kHz, the threshold tended to increase rather gradually, whereas it increased abruptly between 16 and 20kHz.

It has been warned since the 1960s that very high-frequency noises could cause subjective effects, such as discomfort and fullness in the ears, malaise, nausea, vestibular dysfunction, tinnitus, and persistent headaches. Extraordinarily high-level ultrasounds may also induce temporary threshold shifts.1 Although a number of damage risk criteria and maximum permissible levels such as that introduced by Health Canada2 have been proposed since the 1960s, these tentative recommendations were based on scant experimental and survey data.1 

Absolute thresholds for pure tones have been studied by many groups of researchers.3–11 The absolute threshold usually starts to increase sharply when the signal frequency exceeds about 15kHz. It reaches about 80dB SPL at the frequency of 20kHz.3,6,8,11 Above 20kHz, however, only limited data have been reported. According to recent studies,12,13 ultrasounds seem to be inaudible as long as their level does not exceed about 85dB SPL.

To determine thresholds at very high frequencies, stimuli have to be presented at extremely high levels. It is not easy, however, to present pure tones at a level above 80dB SPL with a good resolution. Factors that affect the maximum measurable threshold are the resolution of the signal, performance of the D/A converter, amplifiers, and loudspeakers. In particular, sufficient linearity of loudspeakers is definitely needed.12,14

Henry and Fast4 used a sound delivery system that could deliver constant stimuli up to 124dB SPL, and reported that most listeners had detected tones up to 24kHz. They noted that thresholds increased abruptly as the signal frequency changed from about 14 to 20kHz. Above 20kHz, however, thresholds increased less rapidly. In Henry and Fast’s study, however, the characteristics of acoustical stimuli were not fully described. They did not specify the amount of subharmonic distortions; they only referred to harmonic distortions. Listeners in their experiment might have been responding to low-frequency distortions or noises.

Ashihara et al.15 made an attempt to measure threshold of hearing for pure tones up to 28kHz. In their study, white noise was used to mask subharmonic distortions. They could obtain threshold values from some listeners for a 24kHz tone. They also noted that hearing threshold increased gradually for tones from 20 to 24kHz.

These studies show that some listeners can perceive tones up to at least 24kHz. The highest frequency examined in Henry and Fast’s study was 24kHz. Ashihara et al. could not obtain threshold values above 26kHz. The highest presentation level in their study was 99dB SPL. Therefore, it is still an open question if tones above 26kHz would be audible or not when their level exceeded 100dB SPL.

The purpose of the present study is to obtain thresholds for tones up to 30kHz. A transformed up-down method combined with a two-alternative forced choice (2AFC) procedure16 was employed in the present study. The hearing threshold was measured at every 2kHz between 16 and 30kHz. To prevent listeners from detecting subharmonic distortions in the lower frequency range, pink noise was used as a masker. To evaluate the masking effect caused by the pink noise, masked and the absolute thresholds were measured at 250Hz, 1, 4, and 12kHz.

Eight males and 8 females participated. None of them had a history of otological disease. Their ages ranged between 19 and 25 years. They were paid for their participation. Necessary information about the experiments was given to them and a written informed consent was obtained from each participant prior to the experiment. The study was approved by the Ethics Committee of National Institute of Advanced Industrial Science and Technology.

In the free-field measurement of the hearing threshold, the distance between the signal source and the listening point is recommended to be at least 1m.17 In the present study, however, a signal source was placed at a distance of 50cm from the listener’s ear to provide sufficient level at the listening point. A listener sat on a chair with the back of his or her head attached to a headrest in an anechoic room. The listener was instructed not to move his or her body during the measurement.

Two sound sources were used in the measurement. They were a signal source and a masker source. The signal source was either a super-tweeter (PIONEER PT-R100) or a loudspeaker (DENON SC-A33) and the masker source was a loudspeaker (DENON SC-A33). The signal source was set on a three-dimensional (3D) stage placed on either the right or left side of the listener. The stage was adjusted so that the signal source directly faced to the entrance of the listener’s outer ear and the distance between the signal source and the entrance of the outer ear was 50cm as can be seen in Fig. 1. The masker source was at a distance of 120cm from the midpoint of the listener’s head and it directly faced to the listener’s face as shown in Fig. 1. A liquid crystal display was placed in front of the listener for instructions and a visual feedback.

FIG. 1.

Front view (left) and side view (right) of the listener. A 3D stage was adjusted so that the signal source directly faced to the entrance of the listener’s ipsilateral ear and the distance between the signal source and the entrance of the listener’s outer ear was 50cm. The masking noise was presented by the masker source that directly faced to the listener’s face. The distance between the masker source and the midpoint of the listener’s head was 120cm.

FIG. 1.

Front view (left) and side view (right) of the listener. A 3D stage was adjusted so that the signal source directly faced to the entrance of the listener’s ipsilateral ear and the distance between the signal source and the entrance of the listener’s outer ear was 50cm. The masking noise was presented by the masker source that directly faced to the listener’s face. The distance between the masker source and the midpoint of the listener’s head was 120cm.

Close modal

Digitally synthesized sinusoids were used as signals. The signal was generated by a D/A converter (EDIROL UA-1000) at a sampling rate of 96kHz and 16bit resolution. The signals at 16kHz and above were presented by a super-tweeter (PIONEER PT-R100) via a high-pass filter (PIONEER DN-100). A loudspeaker (DENON SC-A33) was used for the signals at 250Hz, 1, 4, and 12kHz. The signal level was calibrated with a 12in. microphone (B&K type 4133) placed at a distance of 50cm from the signal source when the listener was absent. The power spectra of the tones at the listening point are shown in Fig. 2. Although harmonic distortions were quite eminent at frequencies higher than the signals, subharmonic distortions in the lower frequency side of the signals were not larger than 15dB SPL. It was confirmed that for any frequencies, subharmonic distortions never exceeded 20dB SPL.

FIG. 2.

Signals at the listening point. The power spectra of the signals at 20kHz (left) and 28kHz (right) are shown. The signals were recorded at a distance of 50cm from the signal source when the listener was absent. Their level was 110dB SPL. It was confirmed that there were no subharmonic distortions larger than 20dB SPL.

FIG. 2.

Signals at the listening point. The power spectra of the signals at 20kHz (left) and 28kHz (right) are shown. The signals were recorded at a distance of 50cm from the signal source when the listener was absent. Their level was 110dB SPL. It was confirmed that there were no subharmonic distortions larger than 20dB SPL.

Close modal

In the measurement of the hearing threshold, signal tones were amplitude modulated by a sinusoid of 2Hz. They were, therefore, supposed to be heard as intermittent tones. The duration of the signal was 2000ms.

A low-pass filtered pink noise was used to mask distortions in the lower frequency range. When the signal frequency was 16kHz, pink noise low-pass filtered at 12kHz was used, otherwise pink noise low-pass filtered at 15kHz was used. The level of the masker was fixed at 60dB SPL at a distance of 120cm from the masker source. The masker duration was 2500ms, including linear onset and offset ramps of 250ms each. The masker was generated by a D/A converter (EDIROL UA-1000) at a sampling rate of 96kHz and 16bit resolution.

The threshold was measured by a three-down one-up transformed up-down paradigm combined with a 2AFC procedure. Two test intervals of 2500ms were presented to the subject. Both intervals contained the masker but only one of them contained the signal. A silent interval between the two test intervals was 300ms. Duration of the signal was 2000ms. The masker always started 250ms prior to the signal onset and ended 250ms after the signal offset. Subjects were asked to judge which test interval contained an intermittent tone and respond by pressing a key within 8s. A visual feedback was given immediately after every response. The level of the stimulus varied adaptively according to a three-down one-up transformed up-down method so that the threshold was estimated automatically. The level of the masker was fixed.

A single run consisted of eight reversals. The threshold value was defined as the mean level at the last four reversal points. The minimum step size was 1dB. If the level exceeded the maximum level of presentation before eight reversals were completed, the run automatically terminated and no estimation was made. As mentioned earlier, masked and absolute thresholds were also measured for tones at 250Hz, 1, 4, and 12kHz. Absolute thresholds were measured without using pink noise.

For all ears, masked threshold values at 12kHz and below were higher than 20dB SPL indicating that any distortions smaller than 20dB SPL would be masked by the pink noise. As mentioned earlier, subharmonic distortions were always lower than 20dB SPL in the present study. It can be said that pink noise sufficiently masked distortions in the present study.

Table I shows hearing threshold values for tones between 16 and 30kHz. It also shows the absolute threshold values for tones at 250Hz, 1, 4, and 12kHz. Because 16 listeners participated and both sides were examined for each listener, the number of the measured ears was 32. Threshold values were measurable for most ears at 20 and 22kHz. They were obtained from half of the ears at 24kHz and from about one-third of the ears at 26kHz. Although no threshold values were obtained for a tone at 30kHz, they could be obtained from 3 ears out of 32 at 28kHz. Above 24kHz, threshold values were always higher than 90dB SPL.

TABLE I.

Threshold of hearing for pure tones. Threshold values for tones at 12kHz and below are the absolute threshold values, otherwise the threshold values were measured with pink noise as the masker.

Frequency (kHz)Maximum level of presentation (dB SPL)MinimumThreshold values (dB SPL) medianMaximumNumber of valid dataNumber of tested ears
0.25 80 0.7 7.2 20.4 32 32 
80 9.4 1.6 6.2 32 32 
80 13.8 5.9 7.0 32 32 
12 80 3.6 9.7 25.1 32 32 
16 110 22.1 41.8 84.0 32 32 
18 105 28.0 64.0 99.5 32 32 
20 110 66.4 89.9  29 32 
22 111 87.6 102.7  25 32 
24 110 91.9   16 32 
26 112 95.3   10 32 
28 111 101.3   32 
30 110    32 
Frequency (kHz)Maximum level of presentation (dB SPL)MinimumThreshold values (dB SPL) medianMaximumNumber of valid dataNumber of tested ears
0.25 80 0.7 7.2 20.4 32 32 
80 9.4 1.6 6.2 32 32 
80 13.8 5.9 7.0 32 32 
12 80 3.6 9.7 25.1 32 32 
16 110 22.1 41.8 84.0 32 32 
18 105 28.0 64.0 99.5 32 32 
20 110 66.4 89.9  29 32 
22 111 87.6 102.7  25 32 
24 110 91.9   16 32 
26 112 95.3   10 32 
28 111 101.3   32 
30 110    32 

Figure 3 shows the maximum, median, and the minimum values of hearing threshold. It can be seen that the hearing threshold increased abruptly as signal frequency increased from 12 to 20kHz. The actual threshold curve is not known here for threshold values at 16kHz and above, especially the minimum values at 16 and 18kHz, might be affected by the masker. Still there seems to be a steep increase of threshold between 16 and 20kHz. Above 20kHz, however, it increased relatively slowly. This is consistent with the findings of Henry and Fast4 and Ashihara et al.15 

FIG. 3.

Hearing threshold for tones. Hearing threshold values are shown as a function of the frequency. The minimum, median, and the maximum values are represented by open triangles, open diamonds, and closed triangles, respectively.

FIG. 3.

Hearing threshold for tones. Hearing threshold values are shown as a function of the frequency. The minimum, median, and the maximum values are represented by open triangles, open diamonds, and closed triangles, respectively.

Close modal

Although it has been repeatedly observed that the thresholds of hearing start to increase abruptly at about 14kHz, what is responsible for this steep increase is not fully understood. Buus et al.18 proposed three explanations for this steep increase of thresholds: (1) inefficient transmission of acoustic energy to the inner ear, (2) decreasing sensitivity of auditory channel tuned to high frequencies, and (3) running out of channels or the end of cochlea. Their tentative conclusion was that the abrupt increase of thresholds seemed to reflect the characteristics of the last (highest) auditory channel.

Yasin and Plack19 suggested that the high-frequency limitation in humans would be imposed in part by the middle ear attenuation. Frequency characteristics of the middle ear have been studied and the amplitude at the stapes is known to fall off by 1215dB/ octave above 1kHz.20–23 In these studies, however, no reliable data are presented above 10kHz, probably because the signal to noise ratio also falls off at high frequencies. Although further investigations are needed to clarify what the sharp increase of thresholds above 14kHz represents and why the threshold curve changes its slope at around 20kHz, the present results can be interpreted as follows.

The characteristic frequency (CF) of the last auditory channel of the cochlea is between 14 and 18kHz as suggested by Buus et al.18 The threshold curve above this frequency may reflect a combined characteristic of the upper side slope of the last auditory channel’s tuning curve and the middle ear attenuation. The psychophysical tuning curve usually has a sharp dip around its CF and a shallower skirt at frequencies away from the CF. If this shallower skirt extends to the ultrasonic regions and the level of the ultrasound is sufficiently high, a part of the sound energy may activate the last auditory channel and thus the sound can be detected. The threshold, therefore, starts to increase rapidly above the CF of the last auditory channel and increase somewhat slowly at much higher frequencies.

Thresholds of hearing for pure tones between 16 and 30kHz were measured. The maximum measurable level was more than 100dB SPL. Although no threshold was obtained for a 30kHz tone, it was obtained from 3 out of 32 ears at 28kHz. The threshold values at 24kHz and above were always more than 90dB SPL.

The present results show that some humans can perceive tones up to at least 28kHz when their level exceeds about 100dB SPL. These findings would be useful for providing criteria for industrial and commercial use of ultrasounds.

The present data, however, may contain some errors. The signal level was calibrated in the absence of the listener. The actual sound pressure level of the signals at each ear is not known. Difference in size and shape of the heads and earlobes might have caused deviations that would not be negligible. Because the distance between the signal source and the listening point was not enough, a small movement of the head might seriously affect the sound pressure level. In addition, the contralateral ear canal was not sealed in the measurement. The data, therefore, may not precisely represent the actual hearing threshold values of the particular ear. Further investigations are needed to provide more accurate estimation of the hearing threshold values.

1.
B. W.
Lawton
, “
Damage to human hearing by airborne sound of very high frequency or ultrasonic frequency
,” Contract Research Report 343, Health and Safety Executive (HSE books, Suffolk,
2001
).
2.
Health Canada, “
Guidelines for the safe use of ultrasound. II. Industrial and commercial applications
,” Minister of National Health and Welfare, EHDTR-158, Canadian Communication Group, Ottawa,
1991
.
3.
S. A.
Fausti
,
R. H.
Fray
,
D. A.
Erickson
,
B. Z.
Rappaport
,
E. J.
Cleary
, and
R. E.
Brummett
, “
System for evaluating auditory function from 8,000-20,000Hz
,”
J. Acoust. Soc. Am.
66
,
1713
1718
(
1979
).
4.
K. R.
Henry
and
G. A.
Fast
, “
Ultrahigh-frequency auditory thresholds in young adults: Reliable responses up to 24kHz with a quasi-free-field technique
,”
Audiology
23
,
477
489
(
1984
).
5.
D. M.
Green
,
G.
Kidd
, Jr.
, and
K. N.
Stevens
, “
High-frequency audiometric assessment of a young adult population
,”
J. Acoust. Soc. Am.
81
,
485
494
(
1987
).
6.
P. G.
Stelmachowicz
,
K. A.
Beauchain
,
A.
Kalberer
, and
W.
Jesteadt
, “
Normative thresholds in the 8- to 20-kHz range as a function of age
,”
J. Acoust. Soc. Am.
86
,
1384
1391
(
1989
).
7.
K.
Betke
, “
New hearing threshold measurements for pure tones under free-field listening conditions
,”
J. Acoust. Soc. Am.
89
,
2400
2403
(
1991
).
8.
H.
Takeshima
,
Y.
Suzuki
,
M.
Kumagai
,
T.
Sone
,
T.
Fujimori
, and
H.
Miura
, “
Threshold of hearing for pure tone under free-field listening conditions
,”
J. Acoust. Soc. Jpn. (E)
15
,
159
169
(
1994
).
9.
T.
Poulsen
and
L. A.
Han
, “
The binaural free field hearing threshold for pure tones from 125Hz to 16kHz
,”
Acust. Acta Acust.
86
,
333
337
(
2000
).
10.
M.
Sakamoto
,
M.
Sugawara
,
K.
Kaga
, and
T.
Kamio
, “
Average thresholds in the 8 to 20kHz range in young adults
,”
Scand. Audiol.
27
,
169
172
(
1998
).
11.
K.
Kurakata
,
K.
Ashihara
,
K.
Matsushita
,
H.
Tamai
, and
Y.
Ihara
, “
Threshold of hearing in free field for high frequency tones from 1 to 20kHz
,”
Acoust. Sci. & Tech.
24
,
398
399
(
2003
).
12.
K.
Ashihara
, “
Audibility of complex tones above 20kHz
,”
Proceedings Internoise 2000
(Nice,
2000
).
13.
K.
Ashihara
, “
The higher limit of the audible area for complex sounds
,”
Proceedings of the 15th Triennial Congress of International Ergonomics Association
, pp.
524
537
(Seoul,
2003
).
14.
K.
Ashihara
and
S.
Kiryu
, “
Audibility of components above 22kHz in a complex tone
,”
Acust. Acta Acust.
89
,
540
546
(
2003
).
15.
K.
Ashihara
,
K.
Kurakata
,
T.
Mizunami
, and
K.
Matsushita
, “
Hearing threshold for pure tones above 20kHz
,”
Acoust. Sci. & Tech.
27
,
12
19
(
2006
).
16.
H.
Levitt
, “
Transformed up-down methods in psychoacoustics
,”
J. Acoust. Soc. Am.
49
,
467
477
(
1971
).
17.
ISO 8253-2
, “
Acoustics - audiometric test methods. - 2 Sound field audiometry with pure tone and narrow-band test signals
,” International Organization for Standardization, Geneva Switzerland,
1992
.
18.
S.
Buus
,
M.
Florentine
, and
C. R.
Mason
, “
Tuning curves at high frequencies and their relation to the absolute threshold curves
,” in
Auditory Frequency Selectivity
, edited by
B. C. J.
Moore
and
R. D.
Patterson
(
Plenum
, New York,
1986
).
19.
I.
Yasin
and
C. J.
Plack
, “
Psychophysical tuning curves at very high frequencies
,”
J. Acoust. Soc. Am.
118
,
2498
2506
(
2005
).
20.
M.
Kringlebotn
and
T.
Gundersen
, “
Frequency characteristics of the middle ear
,”
J. Acoust. Soc. Am.
77
,
159
164
(
1985
).
21.
M. S.
Vlaming
and
L.
Feenstra
, “
Studies on the mechanics of the normal human middle ear
,”
Clin. Otolaryngol.
11
,
353
363
(
1986
).
22.
R. L.
Goode
,
K.
Nakamura
,
K.
Gyo
, and
H.
Aritomo
, “
Comments on ‘Acoustic transfer characteristics in human middle ears studied by a SQUID magnetometer method’ [J. Acoust. Soc. Am. 82, 1646–1654 (1987)]
,”
J. Acoust. Soc. Am.
86
,
2446
2449
(
1989
).
23.
H.
Kurokawa
and
R. L.
Goode
, “
Sound pressure gain produced by the human middle ear
,”
Arch. Otolaryngol. Head Neck Surg.
113
,
349
355
(
1995
).