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Year : 2004
| Volume
: 6 | Issue : 23 | Page
: 37-57 |
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Hearing at low and infrasonic frequencies |
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H Moller, CS Pedersen
Department of Acoustics, Aalborg University, Denmark
Click here for correspondence address
and email
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The
human perception of sound at frequencies below 200 Hz is reviewed.
Knowledge about our perception of this frequency range is important,
since much of the sound we are exposed to in our everyday environment
contains significant energy in this range. Sound at 20-200 Hz is called
low-frequency sound, while for sound below 20 Hz the term infrasound is
used. The hearing becomes gradually less sensitive for decreasing
frequency, but despite the general understanding that infrasound is
inaudible, humans can perceive infrasound, if the level is sufficiently
high. The ear is the primary organ for sensing infrasound, but at levels
somewhat above the hearing threshold it is possible to feel vibrations
in various parts of the body. The threshold of hearing is standardized
for frequencies down to 20 Hz, but there is a reasonably good agreement
between investigations below this frequency. It is not only the
sensitivity but also the perceived character of a sound that changes
with decreasing frequency. Pure tones become gradually less continuous,
the tonal sensation ceases around 20 Hz, and below 10 Hz it is possible
to perceive the single cycles of the sound. A sensation of pressure at
the eardrums also occurs. The dynamic range of the auditory system
decreases with decreasing frequency. This compression can be seen in the
equal-loudness-level contours, and it implies that a slight increase in
level can change the perceived loudness from barely audible to loud.
Combined with the natural spread in thresholds, it may have the effect
that a sound, which is inaudible to some people, may be loud to others.
Some investigations give evidence of persons with an extraordinary
sensitivity in the low and infrasonic frequency range, but further
research is needed in order to confirm and explain this phenomenon. Keywords: low-frequency sound, infrasound, hearing thresholds, equal-loudness-level contours, binaural advantage, sensitive persons
How to cite this article: Moller H, Pedersen C S. Hearing at low and infrasonic frequencies. Noise Health 2004;6:37-57 |
Introduction | | |
It is traditionally said that the human hearing covers a certain frequency range, called the audible range or the audio frequency range. The lower limit of this range is usually given as 16 or 20 Hz, and the upper limit is typically said to be 16 or 20 kHz.
The
upper limit is fairly sharp in the sense that the hearing threshold
rises rather steeply above the upper limit - meaning that the hearing
almost "stops" at this frequency. The lower limit is more smooth, and
the hearing threshold follows a curve that gradually goes to higher
levels for decreasing frequency. As a surprise to most people (even to
many acousticians), the threshold curve continues below 20 and even 16
Hz, and - as it will be seen in the following sections - humans can
perceive sound at least down to a few Hertz. This applies to all humans
with a normal hearing organ, and not just to a few persons.
Since
the threshold curve goes up for decreasing frequency, it reaches quite
high sound pressure levels at the lowest frequencies. Even when rather
high sound pressure levels are needed to cause a perception, there are
many sources in our everyday environment that do produce audible sound
in this frequency range. Engines, compressors, ventilation systems,
traffic and musical instruments are examples of man-made sources, but
also natural sources exist like thunder, ocean waves and earthquakes.
Driving a car at highway-speed with an open window is a situation, where
many people expose themselves to perceivable levels of 10-20 Hz sound.
The
ear is most sensitive in the frequency range from 200-300 Hz to around
10 kHz, and this is the frequency range we mainly use in communication.
As a natural consequence it is also the frequency range, where most
hearing research has been made. However, it is important to have insight
in the hearing function also outside this frequency range, in
particular at frequencies below, since much of the sound that we are
exposed to in our everyday environment contains significant energy in
this range. The present article gives a review of studies of the hearing
function below 200 Hz, focussing on the hearing threshold and the
loudness function.
Terminology | | |
Sound with frequencies below 20 Hz is called infrasound, infra
being Latin and meaning below. Thus the term refers to the widespread
understanding that these frequencies are below the range of (audible)
"sound". As mentioned, this understanding is wrong, and the use of the
term infrasound for these frequencies has resulted in many
misunderstandings. Nevertheless, the term is widely used, and it will
also be used in this article. For sound in the frequency range 20-200
Hz, the term lowfrequency sound is used. Since there is no sharp
change in hearing at 20 Hz, the dividing into infrasound and
low-frequency sound should only be considered as practical and
conventional.
Sensation of sound at low and infrasonic frequencies | | |
Everyone
knows from his everyday environment the feeling of hearing sound at low
and infrasonic frequencies. The following are examples of typical
low-frequency sound sources: ventilation systems, compressors, idling
trucks and the neighbour's stereo. Infrasound at an audible level is
usually found on the car deck of a ferry and when driving a car with an
open window. However, infrasound is most often accompanied by sound at
other frequencies, so the experience of listening to pure infrasound is
not common.
The subjective quality of the sound varies with
frequency. In the low-frequency range pure tones still result in a tonal
sensation, and - like at higher frequencies - a sensation of pitch is
connected to the sensation. If the frequency is gradually lowered from
20 Hz, the tonal sensation disappears, the sound becomes discontinuous
in character and it changes into a sensation of pressure at the
eardrums. At even lower frequencies it turns into a sensation of
discontinuous, separate puffs, and it is possible to follow and count
the single cycles of the tone. Some early descriptions of these
phenomena were given by Brecher (1934) and by Wever and Bray (1936).
However, the lower limit of tonality has been known much longer, e.g. it
has influenced the building of musical instruments, where the largest
organ pipes are tuned to a frequency around 17 Hz.
Yeowart et al.
(1967) described pure tones above 20 Hz as smooth and tonal, at 5-15 Hz
a rough sound with a popping effect was reported, and tones below 5 Hz
were described as chugging and whooshing. Below 5 Hz a sensation like
"motion of tympanic membrane itself" was reported. The perception of
noise bands was investigated by Yeowart et al. (1969). For an octave
band around 125 Hz the random noise was perceived as banded noise, while
at 63 Hz the character changed into a sensation of a fluctuating tone.
The octave bands around 32 Hz and 16 Hz were described as traffic
rumble, at 16 Hz with a fluctuating flutter, while the band at 8 Hz was
described as a rough peaky tone. For the octave-band noise around 4 Hz
separate random peaks were perceived.
The early qualitative
descriptions are well in line with later descriptions in the literature
as well as with reports from numerous experimental subjects in the
authors' laboratory and with the authors' experience from exposure of
themselves.
It is mentioned by many authors and easily verified
in a laboratory with suitable equipment that the loudness of
low-frequency and infrasonic sound grows considerably faster above
threshold than sound at higher frequencies. Yeowart et al. (1967)
mentioned that at 4 Hz a 1 dB change in level was sufficient to cover
the whole range from inaudible to definitely detectable. The faster
growth of loudness is reflected in the equal-loudness-level contours,
where the distance between the curves decreases with decreasing
frequency (see separate section 'Studies of equal-loudness-level
contours'). An implication of this compression is that if a
low-frequency sound is just audible, then a relatively small increase in
level will result in a much louder sound.
The sensation mechanism | | |
It
has been a matter of interest, how we sense the lowest frequencies, and
the key question is, if we sense them with our ears and in the same way
as we sense higher frequencies.
There is no doubt that the ear
is the organ that is most sensitive to sound at these frequencies. This
is seen from the fact that hearing thresholds are the same, whether the
whole body or only the ears are exposed (see the section 'Do we sense
with our ears'). It is more difficult to determine whether the sensory
pathway belongs to the auditory system or not. Bekesy (1936) noted that
it is difficult to distinguish whether the sensation is of a pressure or
tactile nature, or of an auditory nature. He argued, though, that
touching two symmetrical places on for example the entrance to the
external meatus results in two separate sensations, while binaural
exposure to infrasound fuses into a single impression localized in the
middle of the head. Therefore he concluded that it is in fact an
auditory sensation. However, he also observed that at higher sound
pressure levels the auditory sensation is accompanied by a "true"
sensation of touch at each of the ears. If the level of the sound is
increased even further, a sense of tickling or prickling is observed.
That the sensation at low levels is auditory is further supported by the
fact that perception thresholds for deaf people are much higher than
for people with normal hearing (see section 'Non-auditory perception').
It
seems fair to conclude that the sense of hearing is the primary sense
for detecting sound at low and infrasonic frequencies. However, it has
often been proposed that we do not sense infrasound directly, but that
we simply hear higher harmonics produced by distortion in the middle and
the inner ear (see e.g. Johnson (1980)). If this were true, it would
then be reasonable to assume that the subjective quality of a 15-Hz tone
would be comparable to that of a tone or a combination of tones at
higher harmonics like 30 and 45 Hz. However, to the authors' knowledge
such similarity has not been reported, and in an informal listening test
with the authors and colleagues as listeners, such sounds were
perceived as clearly different in timbre, pitch and general quality.
Thus, the theory is not supported.
Modulation of hearing | | |
One
way in which the presence of infrasonic sound can be detected at levels
around or possibly below the hearing threshold is by modulation of
higher frequencies. The infrasound moves the eardrum and the middle ear
bones, and the displacement may be so large that their mechanical
properties and the transmission change. As a consequence, sounds at
higher frequencies are amplitude-modulated with the infrasound. This
effect is easily demonstrated in a suitable laboratory, and it
emphasises the need of very quiet conditions, when perception of
infrasound is studied.
Speech modulation | | |
Another
modulation effect is sometimes mentioned in connection with infrasound,
namely modulation of speech. Whereas the effect mentioned in the
previous paragraph relates to a person as a sound detector, this effect
relates to a person's generation of sound. When a person speaks in the
presence of infrasound, the pressure from the infrasound may create a
small pulsating airflow in the throat. This flow adds to the natural
flow from breathing and speaking, and it modulates the speech. The
effect is only noticed at high levels of infrasound.
Studies of hearing threshold | | |
The
threshold is most likely the single characteristic of the hearing that
is investigated most and best known. However, it is not trivial to
produce a well-controlled exposure at low frequencies, and many original
investigations have a bad coverage of this frequency region. The number
of investigations in the infrasonic region is even more limited.
Thresholds
are usually given in terms of the pressure of a free plane wave, in
which the listener is exposed horizontally and from the front. The
pressure is measured without the listener being present in the sound
field. A threshold given this way is called the minimum audible field,
or the MAF. Another possibility is to specify the threshold in terms of
the actual pressure at the eardrum during exposure - in principle
without specific requirements to the nature of the sound field. This is
called the minimum audible pressure, or the MAP.
At high
frequencies the presence or absence of a person has a substantial
impact on the sound field, and there is a significant difference between
the MAF and the MAP. Furthermore, the difference depends on the nature
of the sound field (e.g. free or diffuse), direction to sound source(s)
etc. At low frequencies, however, the listener's head and body have
little or no impact on a free plane wave, and it is expected that MAP
and MAF will have the same value.
Measurements of MAP may in
principle be carried out in any sound field. However, they are usually
done either in a pressure-field chamber that encloses the entire body of
the listener, or with the sound created in a cavity that is coupled to
the ear (or to both ears). If, in the latter case, the cavity is very
small, e.g. like that of a supraaural audiometric earphone,
physiological activity around the ear seems to result in noise under the
earphone that elevates the threshold, in particular at low frequencies
(see e.g. Anderson and Whittle (1971)). Therefore MAP measurements with
sound applied in very small volumes have not been included in the
following.
Sivian and White (1933) gave a review of earlier
studies of hearing thresholds. These investigations differ much in means
of exposure and calibration as well as experimental method, and they
are now mainly of historical interest. Nevertheless it is interesting to
see how close the results of at least some of these studies are to
threshold data obtained in more recent years. These early studies will
not be further reported here.
Common to all studies mentioned in
the following is that they have been made with sinusoidal tones, and
that the duration of the tones has been so long that the temporal
integration of the ear is expected not to have any impact on the result
(usually a duration of 0.5-2 s or longer).
Most studies have been
made in a free or an approximately free sound field (e.g. an anechoic
room) using an electrodynamic transducer (usually a loudspeaker) as
sound source. Data obtained under such conditions have been presented by
Sivian and White (1933) (100 Hz15 kHz, 14 subjects monaural, five
subjects binaural), Fletcher and Munson (1933) (60 Hz15 kHz, 11
subjects), Churcher et al. (1934) (100 Hz-6.4 kHz, 48 subjects),
Churcher and King (1937) (54 Hz-6.4 kHz, 10 subjects), Robinson and
Dadson (1956) (25 Hz-15 kHz, up to 120 subjects depending on frequency,
lowest frequencies measured in a duct), Teranishi (1965) (63 Hz-10 kHz,
51 subjects), Anderson and Whittle (1971) (50-1000 Hz, ten subjects),
Brinkmann (1973) (63 Hz-8 kHz, up to 58 subjects depending on
frequency), Betke and Mellert (1989) (40 Hz-15 kHz, up to 44 subjects
depending on frequency) (reported in more detail by Betke (1991)), Fastl
et al. (1990) (100-1000 Hz, 12 subjects), Watanabe and Moller (1990a)
(25-1000 Hz, 12 subjects), Takeshima et al. (1994) (31.5 Hz-20 kHz,
below 1 kHz: 17-69 subjects depending on frequency) (partly reported on
earlier occasions, e.g. by Suzuki et al. (1989)), Lydolf and Moller
(1997) (50 Hz-8 kHz, 27 subjects), Poulsen and Han (2000) (125 Hz-16
kHz, 31 subjects) and Takeshima et al. (2001) (31.5 Hz-16 kHz, below 1
kHz: seven to eight subjects). Most likely the study by Bellmann et al.
(1999) (40-160 Hz, 12 subjects) was also carried out in a free-field,
although it was not specifically reported.
Especially at the
lowest frequencies it is difficult to produce sufficiently high sound
pressure levels in a free field, and the walls of even the best anechoic
room become reflective. As a consequence no free-field data were
reported below 25 Hz, and most investigations did not even go down as
far as that.
Some investigators have produced the sound in a
pressure chamber connected to the outer ear(s) either directly or by
means of tubes. Data obtained under such conditions have been reported
by Brecher (1934) (6.7-15.1 Hz, one subject, monaural), Bekesy (1936)
(4.5-61 Hz, one subject, monaural), Corso (1958) (5-200 Hz, 15
subjects), Finck (1961) (25-50 Hz, five subjects, binaural), Yeowart et
al. (1967) (1.5100 Hz, six to ten subjects depending on frequency,
monaural) and Yeowart and Evans (1974) (5-100 Hz, five subjects,
binaural). In the study by Brecher (1934) the sound was generated by a
membrane driven by an eccentric wheel. Unlike other investigators,
Brecher kept the level constant and varied the frequency to obtain the
threshold. Bekesy (1936) excited the pressure chamber by either a
thermophone or a pistonphone. (A thermophone uses an amplitudemodulated
alternating current to produce temperature variations in a conducting
wire or foil. The surrounding air expands and contracts with the
modulation, thereby creating pressure variations at the modulation
frequency). The later studies used electrodynamic transducers to
generate the sound.
Another group of studies used a larger
pressurefield chamber that covered the entire body of the subjects.
This applies to studies by Whittle et al. (1972) (3.15-50 Hz, up to 58
subjects depending on frequency), Yeowart and Evans (1974) (2-20 Hz, 12
subjects), Okai et al. (1980) (8-50 Hz, 28 subjects), Yamada et al.
(1980) (8-63 Hz, 24 subjects), Nagai et al. (1982) (2-40 Hz, 62
subjects), Landstrom et al. (1983) (4-25 Hz, ten subjects), Watanabe and
Moller (1990b) (4-125 Hz, 12 subjects), Watanabe et al. (1993) (5-40
Hz, 20 subjects) and Lydolf and Moller (1997) (20-100 Hz, 14 subjects
plus nine added after publication). All studies made in whole-body
pressure-field chambers used electrodynamic loudspeakers to generate the
sound. Most studies had the loudspeakers mounted directly in the
chamber, while in two (Whittle et al. (1972) and Yamada et al. (1980))
the sound was generated in one box that was connected to the exposure
chamber by a tube. The two-box construction was used to reduce
high-frequency noise from the amplifier by acoustic filtering. The
exposure chamber used by Landstrom et al. (1983) had an opening to the
outside, thereby forming a Helmholtz resonator that was tuned to the
exposure frequency.
[Figure - 1],[Figure - 2],[Figure - 3]
show all the thresholds that have been reported above. Although mainly
frequencies below 200 Hz are considered in the present article, data up
to 1 kHz are shown. Monaural and binaural data are shown as observed
(i.e. with no correction), no distinction is made between data for men
and women, and no distinction is made between MAF and MAP. For studies
that have reported data for different age groups, the youngest group is
shown (Teranishi et al. (1965), Whittle et al. (1972)).
It is obvious from [Figure - 1],[Figure - 2],[Figure - 3]
that differences between investigations exist. However, one should have
in mind that the data are obtained in a period of 70 years with very
different techniques. Not surprisingly the largest discrepancies are
found in the low and infrasonic frequency region, because it is much
more difficult to produce the stimuli needed for this region. The demand
on higher sound pressure levels with less harmonic distortion (due to
the steep slope of the threshold curve) are difficult to meet as the
production of higher sound pressure levels usually causes more harmonic
distortion. Other differences between investigations can be found, e.g.
in background noise level, sound field, subjects (number, age, selection
process), psychometric method, instruction of the subjects, whether
mean or median threshold is reported, and number of repetitions.
The
differences between the investigations are so large that comparisons
across investigations of the results cannot give answers to questions
like the effect of gender, effect of age, monaural versus binaural
exposure, effect of sound-field, and differences between persons.
Therefore the following sections will deal with single investigations
that focus on these specific issues.
Significance of gender | | |
Most
investigations have included both male and female subjects. Robinson
and Dadson (1956) noted that there was no systematic difference between
thresholds of men and women, but they did not show data separately for
the two genders. Only Yamada et al. (1980) reported data separately. [Figure - 4]
shows their data for the two genders. Women seem to be around 3 dB more
sensitive than men except at 8 and 10 Hz, where men are around 2dB more
sensitive. The standard deviation between subjects is not specified, so
a statistical test cannot be performed on these data. However, large
differences between persons are mentioned in the study, and when the
relatively low number of subjects (16 men and eight women) is recalled,
it is most likely that the differences between genders are not
statistically significant.
Significance of age | | |
Several
investigations have studied thresholds for different age groups.
Robinson and Dadson (1956) had many subjects in a wide age range (16-63
years), and they concluded that there was no effect of age at
frequencies below 1 kHz. Consequently only data above this frequency
were reported separately for different age groups. Yamada et al. (1980)
mentioned threshold differences of 2-6 dB between people below and above
30 years, but he did not mention details about group sizes and age
ranges, and the only original data reported are for subjects around 20
years.
Teranishi (1965) reported data separately for five age
groups with 10 or 11 subjects in each group. Whittle et al. (1972)
reported data for two groups, one with mean age 30 years (23 subjects)
and one with mean age 47 years (35 subjects). The data from these two
investigations are seen in [Figure - 5]. This data suggests that up to 1000
Monaural versus binaural hearing | | |
It
is well accepted that binaural thresholds are slightly lower than
monaural thresholds. The difference is called the binaural advantage,
and it is said to be in the order of a few decibels, quite often around 3
dB. Some of the investigations already reported have studied the
binaural advantage at low and infrasonic frequencies.
Sivian and
White (1933) simply concluded that binaural thresholds were similar to
monaural thresholds for the person's best ear. This was observed for
only two subjects, and it was most likely too general and inaccurate.
Anderson and Whittle (1971) measured for the same 10 subjects both
monaural and binaural thresholds. Yeowart and Evans (1974) measured also
monaural and binaural thresholds for the same group of subjects (3-4
depending on frequency). The binaural thresholds were measured in two
situations, one with equal sound pressure at each of the two ears, and
one where a level difference was applied between the two ears
corresponding to the difference between ears in the monaural thresholds.
The binaural advantage as observed in these two investigations is
displayed in [Figure - 6]
(for Anderson and Whittle (1974) calculated by the present authors as
the difference between mean monaural and mean binaural thresholds). It
is seen that a binaural advantage around 3 dB is probably applicable
also at low and infrasonic frequencies.
Significance of sound field | | |
Whittle
et al. (1972) observed a large difference between their thresholds
obtained in a whole body pressure-field chamber and thresholds for
free-field exposure given in ISO R226:1961. In order to see whether this
was an effect of the sound field they also measured free-field
thresholds for their own subjects. Measurements were made in four
series, where the psychometric method and the set of included
frequencies varied. A difference of several decibels was seen between
thresholds obtained in the two sound fields. However, differences of the
same order of magnitude were seen between different series in the same
sound field, and no conclusion could be drawn about the effect of sound
field.
Watanabe and Moller (1990b) studied for a group of 12
subjects thresholds with exposure in a free field and in a whole-body
pressure-field chamber, keeping all other conditions constant. The
results are shown in [Figure - 7].
It is seen that there is a very good agreement between the two data
sets in the overlapping frequency region. Thus, the data give no reason
to suspect any effect of the sound field.
Do we sense with our ears? | | |
Connected
to the issue of the perception pathway is the question, whether the
same thresholds are obtained if the whole body or only the ears are
exposed. Yeowart and Evans (1974) measured thresholds in a whole-body
chamber and with a binaural earphone. The number of subjects was not the
same (12 and five respectively), and it is not stated whether there is
overlap between the groups. Nevertheless, psychometric method and
conditions in general were probably very similar. The data are seen in [Figure - 8].
It is seen that the agreement between the two data sets is very good.
This supports the assumption that also these low frequencies are
actually sensed by the ears.
Standardization of hearing thresholds | | |
The
first document that expresses an international agreement about the
human hearing threshold is ISO R226:1961. The document covered not only
the hearing threshold but also equal-loudness-level contours. Like all
later standards it does not cover frequencies below 20 Hz. The
bibliography of the document includes all relevant studies available at
that time (Sivian and White (1933), Fletcher and Munson (1933), Churcher
and King (1937), Robinson and Dadson (1956)), but data reflect only the
study by Robinson and Dadson (1956).
In 1987 ISO R226:1961 was
revised and issued as ISO 226:1987. The revision was a major editorial
renewal, but the data were unchanged, except that they were specified at
slightly different frequencies (the then new standard third-octave
frequencies), and the highest frequency had been lowered from 15 kHz to
12.5 kHz. The unused studies had been removed from the bibliography.
In
1996 a standard was issued that covered only the hearing threshold and
not the equal-loudnesslevel contours (ISO 389-7:1996). This was based
on data from Robinson and Dadson (1956), Brinkmann (1973), Betke and
Mellert (1989), Suzuki et al. (1989), Fastl et al. (1990), Vorlander
(1991) (only frequencies above 8 kHz), Watanabe and Moller (1990a) and
Watanabe and Moller (1990b). Deviations from previous standards were
small (max. 3.9 dB at 20 Hz). An explanatory overview of the aggregation
and processing of the data for the standard is given by Brinkmann et
al. (1994).
Most recently agreement has been obtained for a
complete set of hearing thresholds and equalloudness-level contours,
and a revised ISO 226 was issued in 2003 (ISO 226:2003). The hearing
threshold is based on the same investigations as ISO 389-7:1996 with the
addition of Teranishi (1965), Takeshima (1994), Poulsen and Thogersen
(1994) (only above 1 kHz), Takeshima et al. (2002) (only above 1 kHz),
Lydolf and Moller (1997), Poulsen and Han (2000) and Takeshima et al.
(2001). There are only small differences (max. 2.1 dB, at low
frequencies max. 0.6 dB) between the threshold in this document and in
ISO 389-7:1996. In order to avoid two different thresholds being
standardized (although they are close), a formal revision has been
initiated to make the thresholds of ISO 389-7 identical to those of ISO
226:2003.
The threshold of the most recent standard (ISO 226:2003) is included for reference in the following figures.
Proposed normal hearing threshold below 20 Hz | | |
s
no standardized hearing threshold exists for frequencies below 20 Hz,
it is adequate at this place to propose a normal threshold for the lower
frequencies, based on the existing data. [Figure - 9]
shows the most recent investigations of hearing thresholds that have
data in the infrasonic frequency range, together with the hearing
threshold of ISO 226:2003. (The monaural data from Yeowart et al. (1967)
have been adjusted to binaural conditions by subtraction of 3 dB).
Some
investigations have obtained values that are clearly too high in the
30-100 Hz range, but there is a remarkably good agreement between
investigations in the 5-20 Hz range. Below 5 Hz there are very few
investigations, and unfortunately they differ somewhat.
In [Figure - 10] the bold dashed line shows a second-order polynomial regression curve as an approximation to the data of [Figure - 9].
As seen it does not connect precisely to the curve of ISO 226:2003.
There are data that agree well with the standard (Yamada et al. (1980)
and Watanabe and Moller (1990)), but other data are higher. It is not
possible from the existing data material to give a definitive solution
in the area around 20 Hz. The proposed curve is also somewhat uncertain
below 5 Hz, where more data would be needed to give more conclusive
values. Despite these uncertainties, the curve is probably correct
within a few decibels, at least in most of the frequency range.
The
thin dashed line gives the more coarse linear regression (approximation
of a straight line). The slope of the line is 11.9 dB per octave which
is very close to the 12-dB-per-octave slope of the G-weighting filter
for infrasound (ISO 7196:1995). The thin dashed line corresponds to a
G-weighted sound pressure level of approximately 97 dB.
Individual differences | | |
Several hearing threshold studies have reported standard deviations between subjects. A summary of these is given in [Figure - 11].
In
general the standard deviations between subjects are in the order of 5
dB nearly independent of frequency, maybe with a slight increase at
20-50 Hz. Only the study by Sivian and White (1933) shows considerably
higher values (in the range 200-1000 Hz), a result that is most likely
due to the experimental conditions in this early study.
Nagai et
al. (1982) reported that out of 62 subjects 39 had a threshold that
followed the general trend with increasing threshold for decreasing
frequency, whereas the threshold of the remaining 23 subjects did not
increase further below 5 Hz. For the latter group the threshold was
claimed to flatten out or even decrease with decreasing frequency. For
the same subjects no flattening was observed in hearing thresholds for
low-pass-filtered white noise, where data were similar to those of the
rest of the subjects.
Especially sensitive persons | | |
A
few studies mention persons with extraordinary high hearing sensitivity
at low frequencies. Okai et al. (1980) report of two subjects being
especially sensitive to lowfrequency sound, and Yamada et al. (1980)
report of one subject. In addition, a subject has been observed in our
laboratory with a repeatable, very low threshold (Lydolf, unpublished
1997). [Figure - 12]
shows three of these cases compared to the ISO 226:2003 and the
proposed normal threshold at infrasonic frequencies from above. (One of
Okai's two subjects seems normal when compared to these data and is not
shown in the figure). Assuming that the hearing threshold is normal
distributed around the mean with a standard deviation of 5 dB, then the
probability for a person to have a threshold around 20 dB below the mean
- as seen in this figure - is extremely low, and most likely another
explanation than the natural spread should be sought.
Extraordinary
sensitivity to low-frequency sound might be explained by abnormalities
in the person's hearing organs. A theoretical example could be an
abnormally small aperture in the helicotrema at the apex of the cochlea.
For lowfrequency sound the helicotrema acts like a kind of pressure
equalization vent for the perilymph in the cochlea, equalizing the
pressure between the scala tympani and the scala vestibuli. If the
helicotrema is unusually narrow or blocked, it cannot equalize the
pressure fast enough, and an unusually high pressure will build up
between the scala tympani and the scala vestibuli. The result is a
greater mechanical excitation of the basilar membrane, and thus a higher
sensitivity to these sounds is expected. For examples of simulations of
the effect of the size of helicotrema see e.g. Schick (1994).
Hearing threshold microstructures | | |
nother
explanation for an apparently high sensitivity to low-frequency sound
might be found in so-called microstructures in the individual hearing
threshold. Frost (1987) showed that the hearing threshold as a function
of frequency is not a smooth continuous line, but has peaks and dips of
sometimes several decibels spread over the frequency spectrum. The
irregularities were reported to be repeatable and not the result of
experimental spread. An example showing microstructures in two persons'
hearing thresholds is given in [Figure - 13].
Although these particular persons do not have an especially good
hearing, the microstructure is clearly seen. It is evident that for some
persons the phenomenon of microstructures may lead to an extreme
sensitivity at particular frequencies.
Thresholds for non-sinusoidal sound | | |
Only
few threshold measurements exist for lowfrequency non-sinusoidal
sound. Yeowart et al. (1969) measured thresholds for
octave-bandfiltered random noise with center frequencies in the range
4-125 Hz and pure-tone thresholds for the same subjects. For center
frequencies down to 32 Hz they found no significant difference between
pure-tone thresholds and octave-band noise thresholds. In the range 4-16
Hz they found a significantly lower threshold for octaveband noise in
the order of 4 dB. An explanation could have been that it is the higher
frequency end of an octave band that is most audible, and comparison is
then to be made with the threshold at that frequency rather than at the
centre frequency of the noise band. With this explanation, the
difference will be largest in the frequency range with the highest slope
of the hearing threshold, i.e. 20-63 Hz. This was however not the range
where the difference was seen, and the theory was thus not supported.
This led to the idea, that for frequencies from 16 Hz and down, it might
be the individual peaks in the sound pressure that we detect. Yeowart
et al. (1969) modelled the hearing with appropriate time constants of
the loudness perception and showed that the peak-detection theory could
explain the 4 dB lower noise thresholds. The theory is in agreement with
the subjective impression of sensing the individual oscillations at the
lowest frequencies.
Nagai et al. (1982) made measurements with
lowpass-filtered white noise with a lower limit of 2 Hz and upper limits
of 5, 10, 20 and 40 Hz. Furthermore pure-tone thresholds were found for
the same subjects. These measurements show the opposite pattern as that
observed by Yeowart et al. (1969). For the random noise with upper
limits of 20 and 40 Hz the threshold was lower than the pure-tone
threshold (7-10 dB), but for the 2-5 Hz random noise the threshold was
higher than the pure-tone threshold (about 6 dB).
Generally
low-frequency and infrasonic sounds from everyday life are not pure
tones alone, but rather combinations of different random noises and
tonal components. It is however, impossible to make thresholds for all
imaginable combinations of sounds that exist, and as seen above there is
no final conclusion about possible higher or lower sensitivity to noise
bands than to pure tones. Anyway, differences seem to be relatively
modest, and the pure-tone threshold can with a reasonable approximation
be used as a guideline for the thresholds also for nonsinusoidal
sounds.
Field measurements of hearing thresholds | | |
All
the investigations reported in the section 'Studies of hearing
threshold' have been carried out in the laboratory. Tsunekawa et al.
(1997) carried out an interesting study, where they found hearing
thresholds using sound that occurred naturally in the field. They used
the sound under two bridges, inside an automobile and beside some
cooling towers. Of course, their resolution in frequency was determined
by the frequencies that occurred naturally. While they recorded the
sound they asked subjects to indicate, when the sound was audible and
when it was not. They only used responses, when later analyses showed
that the sound was sufficiently pure.
The results are given in [Figure - 14]
together with the standardized threshold for frequencies above 20 Hz
and the proposed normal hearing threshold for frequencies below 20 Hz.
It is interesting to see how close their results are to the results
obtained in the laboratory.
Non-auditory perception | | |
As
mentioned in the section 'The sensation mechanism', various attempts
have been made to determine the way we sense the low and infrasonic
frequencies. An investigation by Landstrom et al. (1983) deserves
special attention. Hearing thresholds were measured for 10
normal-hearing subjects (five of each gender). Furthermore vibrotactile
thresholds were measured for the same subjects and for 10 subjects with
complete perceptive or sensoryneural deafness. The vibrotactile
sensation was described as soft vibrations in different parts of the
body, mostly in the lumbar, buttock, thigh and calf regions.
The results from Landstrom et al. are given in [Figure - 15].
It is seen that the vibrotactile thresholds are very similar for the
hearing and the non-hearing groups. This suggests that the hearing
subjects were really able to distinguish between the two sensations. The
findings also support the idea that the sense of hearing is the primary
sense for detecting the presence of sound at low and infrasonic
frequencies. On the other hand, the results suggest that an additional
way of sensation connected to vibration occurs at levels that are only
20-25 dB above the hearing threshold.
Spontaneous reactions from
subjects and visitors in the authors' laboratory as well as their own
experience suggest that vibrotactile sensations and a feeling of
pressure may also occur in the upper part of the chest and in the throat
region.
Studies of equal-loudness-level contours | | |
Loudness is a measure of the subjectively percieved intensity of sound. The unit of loudness level
is phon, and for a given sound it has the same numerical value as the
sound pressure level (in dB relative to 20 µPa) of an equally loud
reference sound. The reference sound consists of a frontally incident,
sinusoidal plane wave at a frequency of 1 kHz. An equalloudness-level
contour is a curve in the sound pressure level versus frequency plane
that represents tones of the same loudness level. Most studies are made
with the reference tone held at a constant level, while some
psychometric procedure is used to find the level of the test tone that
makes the two tones appear equally loud to the subject. A few studies
have used fixed levels of the test tone and varied the level of the
reference tone, in which case interpolation is needed to obtain
equal-loudnesslevel contours.
Initially, it should be mentioned
that Kingsbury (1927) was one of the first to attempt measurements of
equal-loudness-level contours. However, he used a monaural earphone, and
no attempt was made to calibrate it to free-field conditions, thus his
results will not be further reported here. Churcher et al. (1934) also
made some early studies of loudness, but they used a reference tone of
800 Hz and a mixture of freefield and earphone exposures, thus their
results will also not be reported further.
One of the best known
studies of equal-loudnesslevel contours is the early one by Fletcher
and Munson (1933). They reported data for the frequency range 62 Hz-16
kHz and loudness range 10-120 phon, based on measurements with 11
subjects. The measurements were performed using earphones, but since
these were calibrated to free-field conditions, their data are
considered relevant and will be included in the following. (In the
review of hearing thresholds given above, studies that used audiometric
earphones were excluded due to the risk of interference from
physiological noise. This is not considered a problem for loudness
comparisons, which take place at levels somewhat above threshold).
Most
studies have determined points of equalloudness-level directly
according to the definition, i.e. through comparisons of the test tone
and the reference tone in a free or an approximately free field. This
applies to the studies of Churcher and King (1937) (54 Hz-9 kHz, 10-90
phon, up to 30 subjects depending on frequency and level), Betke and
Mellert (1989) (100 Hz-1 kHz, 30 phon; 50 Hz-12.5 kHz, 40, 50 and 60
phon, 28 subjects), Suzuki et al. (1989) (125 Hz-8 kHz, 40 and 70 phon,
23 subjects; 63 Hz-12,5 kHz, 20 phon, ten subject), Fastl et al. (1990)
(100 Hz-1 kHz, 30, 50 and 70 phon, 12 subjects), Watanabe and Moller
(1990a) (25 Hz1 kHz, 20, 40, 60 and 80 phon, 12 subjects), Lydolf and
Moller (1997) (50 Hz-1 kHz, 20, 40, 60, 80, 90 and 100 phon, 27
subjects), Takeshima et al. (1997) (31.5-12.5 kHz, 20, 40, 50, 60, 70
and 90 phon, 9-30 subject depending on frequency and loudness level),
Bellmann et al. (1999) (100 Hz-1 kHz, 60 phon, 12 subjects) and
Takeshima et al. (2001) (50 Hz-16 kHz, 20, 40 and 70 phon, eight
subjects).
For the lowest frequencies it is a practical problem
to create sound in the same room as the reference tone (anechoic room)
at sufficiently high level without significant harmonic distortion. It
will be noted that none of the freefield studies mentioned in the
previous paragraph had frequencies below 25 Hz, and most studies did not
even go that far down. Furthermore, it is often mentioned that it is
difficult for subjects to compare tones that are very distant in
frequency. Some investigators have overcome these problems by making
indirect loudness matches to the 1 kHz reference tone. Points of equal
loudness are determined at a low-frequency anchor point of for example
100 Hz through direct comparisons with 1 kHz in an anechoic room. Then
the 100 Hz points are used as new references for loudness matches in a
pressure-field chamber, where large sound pressure levels can be
produced at the lowest frequencies.
Studies that used exposures
in pressure field in combination with individual anchor points
determined in free field comprise those of Kirk (1983) (2-63 Hz, 20, 40,
60, 80 and 100 phon, anchor points at 63 Hz, 14 subjects), Moller and
Andresen (1984) (2-63 Hz, 20, 40, 60, 80 and 100 phon, anchor points at
63 Hz, 20 subjects), Lydolf and Moller (1997) (20-100 Hz, 20, 40, 60, 80
and 100 phon, anchor points at 100 Hz, 14 subjects plus three added
after publication) and Bellmann et al. (1999) (16-160 Hz, 60 phon,
anchor points at 100 Hz, 12 subjects).
Two studies used
experimental designs equivalent of using non-individual anchor points.
Robinson and Dadson (1956) measured equal-loudness relations for the
frequency range 25 Hz-15 kHz (up to approximately 130 phon and up to 120
subjects depending on frequency). Free-field conditions were used for
the higher frequencies, while a suitably terminated duct was used for
the lowest frequencies. At the lowest frequencies they used reference
tones of 50 or 200 Hz that were converted into phon by means of
interpolation in the data material from the free field. Whittle et al.
(1972) used a pressure field for their experiments (3.15-50 Hz, up to 32
subjects depending on frequency). They used a reference tone at 50 Hz
at three levels (60, 73 and 86 dB) without measuring the connection to 1
kHz. Subsequently they used ISO 226:1961 to find the standardized
loudness levels of their reference tones and labelled the contours
accordingly (33.5, 53 and 70.5 phon).
[Figure - 16],[Figure - 17],[Figure - 18]
show the equal-loudness-level contours measured in the investigations
mentioned above. It should be noted that the data from Fletcher and
Munson (1933) and Robinson and Dadson (1956) are not original data, but
data interpolated between original data points. For the data by Whittle
et al. (1972) the authors have taken the liberty of plotting them as 20,
40 and 60 phon, respectively, since these loudness levels seem more
reasonable than the original labels of 33.5, 53 and 70.5 phon when
comparing with the other data in the same frequency area.
The
figures clearly show large differences between equal-loudness-level
contours from different investigations. These differences are not only
in the low-frequency region but also at higher frequencies.
Standardization of equal-loudness-level contours | | |
The
first international standard about equalloudness-level contours is ISO
R226:1961. The contours in this were solely based on the study by
Robinson and Dadson (1956), despite the fact that also other studies
were present at that time. As already mentioned in the section on
standardization of hearing thresholds, the document was revised and
issued as ISO 226:1987, however without changes in data.
Virtually
all other investigations show data that are significantly higher than
those of Robinson and Dadson (1956) in the frequency area below 1 kHz.
The difference has been ascribed to the different psychometric methods
used. The data from Robinson and Dadson seem significantly biased
towards lower levels. Awareness of bias problems and the use of
computerized adaptive psychometric methods in later studies have
provided data that are believed to be more reliable.
Most
recently agreement has been obtained for a complete set of hearing
thresholds and equalloudness-level contours, and a revised standard has
been issued (ISO 226:2003). Below 1 kHz the equal-loudness-level
contours are based on the investigations by Kirk (1983), Moller and
Andresen (1984), Betke and Mellert (1989), Suzuki et al. (1989), Fastl
et al. (1990), Watanabe and Moller (1990), Lydolf and Moller (1997),
Takeshima et al. (1997), Bellmann et al. (1999) and Takeshima et al.
(2001).
[Figure - 19]
shows the standardized equalloudness-level contours for the frequency
range below 1 kHz, and the difference between the two old and the new
standard is obvious.
Proposed normal equal-loudness-level contours below 20 Hz | | |
No
standardized equal-loudness-level contours exist for frequencies below
20 Hz, and only four investigations provide data in this frequency
region. Whittle et al. (1972) and Moller and Andresen (1984) produce
quite similar contours, and the two points provided by Bellmann et al.
(1999) at 60 phon, 16 and 20 Hz, fit well with these. The contours by
Kirk (1983) deviate considerably, and the authors take the liberty of
disregarding these data in the following. The contours from the three
other investigations are shown in [Figure - 20].
Based on these data the authors have presented their best guess of
general contours of 20, 40, 60 and 80 phon for frequencies below 20 Hz
in [Figure - 21].
However, these contours should be taken with great reservation because
of the sparse amount of data and the uncertainty connected to the exact
phon values they should be labelled with. On the other hand it seems
beyond any doubt that the contours are very close in this frequency
region.
More definite contours at low and infrasonic frequencies -
in particular at high loudness levels - require that more experimental
data become available. Unfortunately, it is not a trivial task to
produce the high sound pressure levels needed without significant
harmonic distortion.
Conclusion | | |
The
human perception of sound below 200 Hz has been reviewed, and on the
basis of results from various investigations it is possible to draw some
general conclusions.
The hearing becomes gradually less
sensitive for decreasing frequency, but there is no specific frequency
at which the hearing stops. Despite the general understanding that
infrasound is inaudible, humans can perceive sound also below 20 Hz.
This applies to all humans with a normal hearing organ, and not just to a
few persons. The perceived character of the sound changes gradually
with frequency. For pure tones the tonal character and the sensation of
pitch decrease with decreasing frequency, and they both cease around 20
Hz. Below this frequency tones are perceived as discontinuous. From
around 10 Hz and lower it is possible to follow and count the single
cycles of the tone, and the perception changes into a sensation of
pressure at the ears. At levels 20-25 dB above threshold it is possible
to feel vibrations in various parts of the body, e.g. the lumbar,
buttock, thigh and calf regions. A feeling of pressure may occur in the
upper part of the chest and the throat region.
There is a
reasonable agreement between studies of hearing thresholds. For
frequencies down to 20 Hz, a normal threshold has been standardized by
ISO, and the present article presents a proposed normal threshold one
decade further down in frequency. The proposed curve corresponds roughly
to a G-weighted sound pressure level of 97 dB. More data are needed to
give a more conclusive curve.
It cannot be finally concluded
whether thresholds for noise bands are the same as puretone thresholds.
Below 20 Hz it is possible that the peak sound pressure determines the
sensation. The differences are small, though, and it seems reasonable to
use the pure-tone threshold as a guideline also for non-sinusoidal
sound.
The hearing threshold is the same for men and women.
Degradation with age takes place only above 50 years. The threshold is
the same in free and pressure field. Like at higher frequencies, the
binaural advantage is around 3 dB, and the standard deviation between
individuals is around 5 dB. However, there is evidence of individuals
that have a hearing that is much better than normal (several times the
standard deviation away from the mean). It has also been shown that the
hearing threshold may have a microstructure that causes a person to be
especially sensitive at certain frequencies. These two phenomena may
explain observations from case studies, where individuals seem to be
annoyed by sound that is far below the normal threshold of hearing. It
should be stressed that the explanation has not been confirmed in
specific cases.
Thresholds are the same, whether the whole body
or just the ears are exposed, thus is can be concluded that the
sensation takes place in the ears even at frequencies below 20 Hz.
However, it is not totally clear, whether the sensory pathway for
infrasound is the normal pathway for hearing. The observation that deaf
people can only detect infrasound through vibrotactile sensation - and
for that they have the same threshold as normal-hearing persons -
suggests that the normal auditory system is used. A hypothesis that
these frequencies are heard in terms of harmonic distortion in the ear
is not supported.
In addition to direct detection, infrasound may
be detected through amplitude modulation of sound at higher
frequencies. This modulation is caused by the movement of the eardrum
and middle-ear bones induced by the infrasound, which results in changes
of transmission properties. At very high levels, modulation of speech
can occur due to a pulsating airflow in the throat caused by the sound.
The
perceived intensity of the sound rises more steeply above threshold
than at higher frequencies. This is especially pronounced for
frequencies below 20 Hz, where a sound only few decibels above threshold
may be perceived as quite intense. Combined with the natural spread in
thresholds, this may have the effect that a sound, which is inaudible to
some people, may be loud to others. The compression of the dynamic
range of the auditory system is reflected in the equal-loudness-level
contours. Such contours have been standardized for frequencies down to
20 Hz, but there is a reasonable agreement between data also below this
frequency, and contours have been proposed down to 2 Hz. However, this
is based on only few investigations and more data are needed.
Acknowledgements | | |
The
authors want to thank their local and international colleagues for
fruitful discussions about our perception of low frequency sound. Many
discussions have taken place in an international collaboration on the
loudness function, the NEDO project, which was only made possible thanks
to the effort of Professor Yoiti Suzuki, Tohoku University, Japan. We
also want to thank our former colleague, Morten Lydolf, who started the
aggregation of the large amount of data and made it available to us. The
work was supported by the Danish Technical Research Council and Aalborg
University.[52]
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Correspondence Address: H Moller Department of Acoustics, Aalborg University Fredrik Bajers Vej 7 B5, DK-9220 Aalborg Ø Denmark
Source of Support: None, Conflict of Interest: None | Check |
PMID: 15273023
[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7], [Figure - 8], [Figure - 9], [Figure - 10], [Figure - 11], [Figure - 12], [Figure - 13], [Figure - 14], [Figure - 15], [Figure - 16], [Figure - 17], [Figure - 18], [Figure - 19], [Figure - 20], [Figure - 21] |
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