Bass: the Physical Sensation of Sound

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: 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.

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.

 

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.  

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.

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.

 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

The 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.

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