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At 1 Hz the sound waves were breathing for the dog

Ask any audiophile or home theater enthusiast and they will
tell you that strong bass is crucial in achieving a full sound. The listening
experience has a hollowness without the weight and foundation of palpable low
frequencies, since strong bass is often described as being felt as much as it is
heard. In fact, it is often said that low enough frequencies can only be felt
and not heard at all. In this article we look at how bass is felt rather than
heard, and, after reviewing some of the research that has been done in this
area, we investigate the points at which low frequencies go beyond sound and
become a tactile sensation in an experiment of our own.

How Sound is Felt

In order to understand how sound is felt by the body, we
have to examine how the body feels anything to begin with. The human sensory
system is commonly thought of as a set of five senses: touch, taste, smell,
sight, and hearing. However, the truth is that these five senses are just
groups composed of many more specific senses. For example, the sense of what is
usually called “touch” is a combination of four different types of sense
receptors: mechanoreceptors (which senses pressure and vibration),
thermoreceptors (which senses temperature), nociceptors (which detects tissue
damage and causes pain), and proprioceptors (which senses where parts of the
body are relative to other parts). Going further, these four sensory systems
under the label of “touch” are also composites of numerous receptor types. For
the purposes of this article, the receptors we are interested in fall within
the mechanoreceptor group, and the particular mechanoreceptors we want to know
about have to do specifically with sensing vibratory pressure.

Of course, any system set up to sense vibration must have a
range in which to discern periodic motion, or in other words, a frequency
response. Our sense of hearing, which is also a form of mechanoreception, is
often said to have a frequency response of 20 Hz to 20,000 Hz (although that
frequency response is a simplification, and a more accurate depiction of normal
hearing response can be seen here). Likewise, mechanoreceptors throughout
our skin and inside our body have a frequency response range, although the
vibration that is normally measured is an object having contact directly with
the skin. While air has direct contact with the body, its vibratory motion is
not usually forceful enough, except when moving hair, to trigger
mechanoreceptors except at high sound pressure levels. For sound pressure
levels on the skin to be felt, they must be greater than the mechanical
thresholds of the mechanoreceptors as outlined in figure 1.

Fig 1 response of mechanoreceptors 

Fig. 1: Dashed lines represent
sensitivity threshold of different mechanoreceptors located in the palm of the
hand, and the dots represent the absolute sensitivity of all mechanoreception.
Threshold values are given in decibels (dB) referenced to 1 micrometer (μm) peak. (from Bolanawski, Gescheider, Verillo, & Checkosky,
1988). Reproduced with permission from AIP
Publishing LLC. Copyright 1988, Acoustical Society of America.”

Note how, much like our sense of hearing (seen in this chart), our sense of touch is relatively
insensitive at low frequencies. Also, note the total frequency band of
vibratory mechanoreception: 0.4 Hz to approximately 800 Hz, has a much more
limited frequency range than hearing. The threshold of sensing touch vibration
frequencies seem to follow the contours of the minimum audibility curve, and it
has been shown at levels 20-25 dB above the hearing threshold, it is possible
to feel vibrations in various parts of the body. In theory, to sense sound
‘non-auditorily,’ a sound wave would need to be powerful enough to displace
enough skin and be above the mechanical sensitivity threshold of the
frequencies in Fig. 1 to be felt. While the non-auditory sensations of sound
have not been studied enough to establish a ‘minimal sound pressure-induced
tactile sensations’ response curve, many studies have been done on the
physiological effects of sound on the body.

Studies on the Physiological
Sensations of Sound

Most of the experiments performed to measure the
non-auditory, physical sensations of sound have been done in two areas of
research: effects of noise pollution on health and deafness research. Those
studying the effects of noise pollution on health want to know how air pressure
waves may cause harm, and those studying deafness research want to learn how
much sound sensation on the body can be used by the hearing-impaired to
perceive their environment. Low frequency sounds are often the subject of
research in these areas, because low frequency noise is very pervasive in modern
life, and low frequencies have long been recognized to induce tactile
sensations, which is not the case with higher frequencies.

Much of what has been learned about which sound amplitudes
and frequencies affect human anatomy has been the result of experiments where
human subjects were simply blasted with a really loud noise, and their
physiological state was compared before and after the test noise. For example,
there have been studies commissioned by NASA for the Apollo program to see if
high amplitude noise levels could jeopardize a mission during launch by
incapacitating personnel as a result of exposing human subjects to 140 dB sound
pressure levels. Other experiments have gone much further, with some human
testing reaching 155 dB sound pressure levels, which,
for you home theater enthusiasts, has a power ratio 100,000 times that of THX’s
Reference Level peak of 105 dB.

Per our topic, let us take a look at some of the research
that studies the effects of loud bass on the human anatomy and how those
sensations are perceived. For our purposes, we will look at frequencies below
200 Hz, since that frequency band is usually segregated and designated as the
‘low frequencies’ in the scientific literature and ‘bass’ in music literature.
Let’s start our survey at the lowest frequencies and work our way up.

1 Hz

While one test found that none of the participants could
sense any vibration at 1 and 2 Hz even at 144 dB, one effect that might be possible for humans at this
extremely low frequency and extremely high amplitude is artificial respiration.
One Air Force study found decreased respiration in anesthetized animals
subjected to frequencies from 0.5 Hz to 8 Hz at sound pressure levels above 166
dB, and at 171 to 173, independent respiration ceased for large dogs, with
their chest being virtually motionless below 1 Hz. The animals were not
suffocating; what was occurring was the pressure waves were so large that air
molecules were being exchanged between the ambient air and the lungs of the
dog, so, in a manner of speaking, the sound waves were breathing for the
dog.   

2-10 Hz

An experiment with 25 subjects reported a subjective
“feeling of body sway” when exposed to 2-10 Hz tones above 130 dB, with the
effect most pronounced at 7 Hz. Vertical Nystagmus (involuntary movement of the
eye) was also reported. Another test that exposed subjects to 5-10 Hz tones at
150 dB reported nostril vibration. One tester subjected ten normal hearing and
ten deaf participants to a 6 Hz tone at 115 dB for 20 minutes and found changes
in EEG patterns (described as ‘diminished wakefulness) in the hearing
participants accompanied by changes in pulse and blood pressure. However these
effects were not found in the deaf subjects. Other tests in the 5-10 Hz range
found decreased respiration, depressed blood flow in the brain, and changes in
pulse and blood pressure. Subjective complaints of testing in this frequency
band included body vibrations, pressure in the ear, and an inability to
concentrate.

10-20 Hz

A test conducted on four participants found abdominal wall
vibrations for a 10-20 Hz narrow band noise at 150-154 dB. Another test found
chest and abdominal vibrations from 4-20 Hz at 132 dB and above. One study
conducted by the Air Force found the resonant frequency of the eyeball to be 18
Hz. It has been suggested that sound pressures at sufficient levels at the
resonant frequency of the eye can cause visual disturbances, and that locations
that have sound emissions at this frequency can sometimes be mistaken for being
‘haunted’ for this reason. In another test, a 17 Hz tone was shown to cause
anxiety in some people when it was used as an undertone in a concert
performance against a control performance that did not have the undertone. As
with the resonant frequency of the eyeball, it was speculated that locations
with sound emissions at this frequency might cause some to feel they are
‘haunted.’  

20-30 Hz

One test using tones from 1-30 Hz at amplitude levels from
125-144 dB reported voice modulation, and abdominal and chest vibration.

30-50 Hz

One series of testing conducted with tones from 31-50 Hz at
90-100 dB output levels compared how people perceived their sensations versus
actual vibration levels of different areas of their body by hooking up
accelerometers to the head, abdomen, and chest of their test subjects. It was
found that although the head itself was not measured to vibrate as much as the
abdomen and chest, head vibrations were perceived as being stronger, likely due
to auditory structures within the head. Chest vibrations were measured and
subjectively felt to be stronger than abdominal vibrations, and 50 Hz
frequencies were more effective at causing vibrations and vibratory sensations
than lower frequency sound at the same output level. Other tests conducted at
much more powerful levels past 140 dB reported respiratory rhythm changes,
gagging, chest wall vibrations, and perceptible visual field vibration.

50-100 Hz

In one test on three individuals, one of the subjects
reported a headache from being exposed to a 50 Hz tone at an astonishing 153 dB
output level. At higher frequencies of 60-73 Hz in the same test at 150-153 dB
output levels, other subjects reported coughing, substernal pressure, choking
respiration, pain on swallowing, salivation, hypopharyngeal discomfort, and one
subject reported testicular aching. At 100 Hz at 153 dB, mild nausea,
giddiness, subcostal discomfort, cutaneous flushing, and tingling was reported.
Pulse changes were also observed. The test was halted due to these alarming
responses. All test subjects suffered from evident post-exposure fatigue. In
another study, high-level low-frequency noise from aircraft engines were
reported to cause 63-100 Hz chest resonances.

100-200 Hz

Some testing has shown that noise-induced vibrations occur
higher than 100 Hz in the chest. In one round of testing, a 100+ Hz noise was
injected into subjects’ mouths and readings were taken of the frequencies where
the chest was most active. 129-143 Hz were found to be the most active
frequencies as measured on the chest wall, but their results also suggest that
noise induced vibration could be more severe from 150 to 200 Hz.

Fig 2 Compression Wave Diagram 

Figure 2: Diagram of sound wave as a
compression wave

Chest Punch!

One of the most prominent effects of high-level low
frequency sound is the so-called ‘chest punch’ or ‘chest slam’. The sensation
of chest vibration was reported over a broad range of low frequencies, although
it seems more commonly pronounced in mid-bass frequencies around 100+ Hz as
opposed to lower bass below 50 Hz. One experimenter explained this as being the
result of easier induction of vibration on the lung, which is “organized like a
balloon and linked to the atmosphere through an airway.” Another researcher
suggests that “thoracic
cavity resonances may have particularly important effects on sound transmission
at frequencies below approximately 250 Hz, where the magnitude of parenchymal
attenuation appears to be small.” Abdominal regions were much less
susceptible to vibration, and it was deduced that crowding of the internal
organs and tissue in the abdomen hinders the induction of vibration. Body fat
was also thought to dampen vibration and obstruct its propagation throughout the
body. It stands to reason from these findings that the greater one’s hard
tissue over soft tissue ratio is, the more affected they will be by sound
waves.

What about the body’s Resonant
Frequencies?
 

While resonant frequencies of the chest and eye look to have
been determined in vivo (in a living
subject), they seem to have only been estimated for other parts of the human
anatomy, at least as far as sound exposure has been tested. Since the human
body is so heavily damped with various soft tissues, the resonant frequencies are
bound to be at very low frequencies, with an estimated 4-8 Hz frequency range
for the body as a whole. Enormous sound pressure levels would be needed for
these resonances to become evident. For most of the body, excitation of these
resonances may be unlikely to be sensibly palpable at a point before the
pressure levels would pulverize the subject and death would occur.

 
Fig3 test rig

Figure 3: The test chamber!

To the Lab!

Now that we have surveyed some of the results of the
physiological and subjective sensations of low frequency sounds, let’s see how
well they match our own experience by running a test of our own. We gathered
nine participants, all adult males in ages ranging from 25 to 58, and subjected
them to a set of bass frequencies at three different output levels and had them
write down where in their body were they feeling any sensation, while we
recorded what output level was needed to reach that point. The results were
then tabulated, with areas of the body which needed less output to effect a
sensation given a higher weighted score than those body areas which only
responded to higher output levels, so anatomical areas with a higher score were
more affected than those with lower scores. The scoring system was weighted by
having the body areas that registered sensation at the lowest output level
count for three points, while those areas that registered sensation in the
middle output level count for two points, and those areas that would only
register sensation at the highest output level counted for one point. The test
sounds were ⅓ octave tones starting from 10 Hz and ending at 200 Hz. Each
frequency was played back in five successive pulses at one second per pulse and
repeated at three different volume levels, with each volume level ramped up by
6 dB and the starting loudness level averaging around 95 dB (C-weighted). While
the frequency response in the listening position was not perfectly flat, the
testing was done in a manner which insured all subjects were exposed to the
same output levels per tone, by confining the listening position to a very
small area and only testing two subjects at a time (see Fig. 3). The sound
playback equipment consisted of four large subs with 18” woofers powered by
4,800 watts of amplification. The results may not be as rigorously scientific
as they could be (to say the least), but some interesting patterns did emerge.

To make a large amount of frequency data digestible, we
divided it into three bands: ‘deep’ frequencies from 10 to 25 Hz, ‘mid bass’
from 31.5 to 80 Hz, and ‘upper bass’ from 100 to 200 Hz.  

Fig4 body chart 10-25 Hz 

Figure 4: Reported sensations from 10
to 25 Hz. Areas of body with higher scores were more commonly or severely
affected than lower scored body regions
.      

Fig. 4 is a graph of the deepest frequency segment of our
testing, 10 to 25 Hz, and we see many participants reporting a considerable
amount of activity in the head for this band. Comments include “pressure” and
“pulsing” with respect to head sensations. We also see the ears themselves were
felt to vibrate. As was mentioned before, auditory structures in the head may
intensify vibrations felt there. Perhaps low frequencies have some kind of
effect on the vestibular system? Also in this band, one test subject mentioned
feeling his nose vibrate.

Fig5 body chart 30 - 80 Hz 

Figure 5: Reported sensations from 31.5 to 80
Hz. Areas of body with higher scores were more commonly or severely affected
than lower scored body regions.

In Fig. 5 we see the results of test tones from 31.5 to 80
Hz, the bulk of the ‘subwoofer’ range. What is immediately clear is that the
chest is very sensitive to sound within this band, with most test subjects
reporting sensation there at some volume level. In light of the results of
previous research in the effects of sound on human anatomy, this is an
unsurprising outcome. Something else notable is the continued sensations on the
ear, which was not reported in previous studies. Perhaps the thin structure of
the ear and the stiffness of the cartilage combine to make it prone to
vibration at sufficient volume levels in lower frequencies.

Fig6 body chart 100-200 Hz 

 Figure
6: Reported sensations from 100 to 200 Hz. Areas of body with higher scores
were more commonly or severely affected than lower scored body regions.

Figure 6 seems to show the sensations were more evenly
distributed over the body. Indeed, two participants wrote down “whole body” on
some tones in this range. Perhaps this is because the skin’s mechanoreceptors
become more sensitive in these frequencies (as shown in Fig. 1), so more skin
area feels activity instead of just those body regions prone to vibration. 

Discussion
of Our Results

First, it should be stressed that these results were not
captured in an exacting laboratory setting. As was mentioned, some tones had
more output behind them than others, with a 10 dB null at 31.5 Hz and 80 Hz.
Also, the SPL meter used loses precision below 31 Hz, so the output recorded
there is not reliable. Furthermore, many of the test subjects had imbibed a few
beers by the time testing began, so it is difficult to determine how much of
the vibrotactile sensation was due to the test tones and how much was due to
alcohol. With that said, some trends emerged. The head looks to be more
responsive to deep bass vibrations, and the chest area was shown to be
sensitive to mid-bass sound, especially at the 50 Hz and 63 Hz tones. As was
said, more of the body felt vibration in upper bass, a region which is higher
than most subwoofers are normally set up to playback sound in, so anyone
interested in a highly tactile sound system should be sure their main speakers
are up to the task of high SPL bass as well as their subwoofers.

Something else to note is that, unlike the study cited above
which indicated chest resonance well above 100 Hz, our findings placed chest
vibration sensation well below that point. The reason for that may be that in
the previous study, sound was transmitted to the lungs through an open mouth.
It may be that in our testing, and in other testing which found maximal chest
vibration to be below 100 Hz, the test subjects did not have their mouths open
and thus the air wave passage to the lungs were more acutely attenuated. Could
an open mouth can allow more ‘chest punch’ at higher bass frequencies? Further
research is required in this area! 

Ways to Increase Tactile Feeling from
your Sound System

Buttkicker Tactile TransducerThe surest way to increase a tactile feeling from your sound
system is to simply increase the loudness level, particularly in the bass
region. Of course, that is the brute force method to gain a more physical
presence from the system, and there are other more exact solutions. One
solution is tactile transducers which are devices attached to the seat and
hooked up to the receiver’s subwoofer pre-out. Tactile transducers physically
shake according to the frequency of the signal they receive, which vibrates the
seat and thus the listener. Some tactile transducer brands have colorful names
such as ‘Bass Shaker’, ‘Buttkicker’, and ‘Earthquake’. They can have an
impressive effect; however, they are not a full substitution for the effect of
a high-level air pressure wave. To quote one review of published research on
low frequency noise and its effects, “The vibratory response of the body to
acoustic stimulation is different from its response to mechanical vibration
through the feet or seat. Low frequency acoustic stimulation acts over the
whole body surface
.” 

There may be other tricks into increasing the tactile
feeling of your bass. As was mentioned before, it could be that simply having
your mouth open may make a difference in how you feel sound. Keeping the room
warm may help as well, since one experiment found the skin to be more sensitive
to vibrations at 86°F than
at 59°F. Body fat has also been shown to dampen vibration and obstruct its
propagation throughout the body, so shedding some body fat may help to get a
more visceral feeling from your sound system. One precise way to easily bump up
the ‘feeling’ of your bass is to boost certain narrow bands in the bass region
instead of the entire frequency range. As was noted in our testing, 50-63 Hz
seemed to carry a very potent effect on the chest region, so giving that
frequency range a boost may give your system an extra kick.

A Word of Warning for Those Seeking
to Explore the Effects of High Level Bass

There seems to be a widely held assumption among audio
enthusiasts that loud bass frequencies does not cause hearing damage and that
only loud mids and treble must be guarded against. However, recent research has
shown that low frequencies may be having a greater effect on hearing than was
previously thought. An experiment in which 21 volunteers were subjected to 90
seconds of a 30 Hz tone at 120 dB SPL found a persistent effect on the cochlea
which lasted longer than the stimulus itself. While the results do not
definitively conclude that low frequencies can cause hearing loss, it opens to
the door to that possibility. Or, as this article states, “The changes aren’t directly indicative of hearing
loss, but they do mean that the ear may be temporarily more prone to damage
after being exposed to low-frequency sounds.” Adventurous
readers may want to keep this in mind before battering themselves with powerful
bass. We at Audioholics take responsibility only for our own noise-induced hearing
loss, not yours.

Concluding Remarks

When one considers that tactile sensation stimulates
portions of the auditory cortex in addition to the somatosensory cortex, it
isn’t surprising how closely touching is related to hearing. In fact, this
processing goes much further in deaf people who process touch vibrations in
areas of the brain normally used for hearing by a phenomenon known as cross
modal plasticity. As was mentioned before, the hair cells by which we hear
sound vibrations in the air are mechanoreceptors much like those that cover our
body that sense pressure and vibration. From this, it is hypothesized that ears
and hearing gradually evolved from pressure sensing on the skin. The relationship between touching and hearing is deep
and complex, and the next time you read about someone commenting about
‘feeling’ a piece of music, perhaps that comment may not be as metaphorical as
they realize. 

A big thanks
goes out to our test participants, and an extra big thanks goes to Mike Masunas
for coordinating the experiment and providing the testing environment.

References

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Horowitz, Seth S. The Universal
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