What sound is, and why you can feel it

Sound is a push. A loudspeaker cone moves forward and squeezes the air in front of it a little tighter than the air around it. It moves back and the air thins. That tight, thin, tight, thin pattern leaves the cone and crosses the room at about 343 metres per second, the figure physics departments use for the speed of sound in air at 20 degrees Celsius (HyperPhysics). No air travels with it. Each molecule shoves its neighbour and settles back where it was. What crosses the room is the pattern, not the air.

Hertz counts how many of those pushes arrive each second. One hertz is one full cycle, a squeeze and a release, every second. A tone at 41 hertz presses on your eardrum 41 times a second. A tone at 880 hertz presses 880 times a second. In the tuning system nearly all Western music uses, those two numbers have names: 41.2 Hz is E1, the lowest open string on a four string bass, and 880 Hz is A5, one octave above the note an orchestra tunes to (HyperPhysics, equal tempered scale). They are also the bottom and the top of the AVA bed, which is why this piece keeps returning to them.

How long a note is

A wave has a length as well as a rate. Divide the speed of sound by the frequency and you get the distance from one squeeze to the next. At 41 Hz that distance is about 8.4 metres, longer than the listening room is wide. At 880 Hz it is about 39 centimetres, roughly the width of your shoulders. Same air, same speed, same physics. A twentyfold difference in frequency gives a twentyfold difference in length.

That is why the two ends of the spectrum behave so differently indoors. Short waves bounce off walls and cast acoustic shadows, which is why you can point a tweeter. Long waves wrap around furniture and bodies and fill a space as one slow pressure that rises and falls everywhere at once. Down at the bottom of hearing you do not locate the sound. You are inside it.

What the ear does with it

Human hearing runs from roughly 20 Hz to 20,000 Hz, and the top of that range falls with age (Purves, Neuroscience). It is not flat. The international standard for equal loudness, ISO 226, maps how much pressure a tone needs at each frequency to sound as loud as a reference tone at 1,000 Hz. The curves bottom out in the region of 3,000 to 4,000 Hz and climb steeply below 100 Hz (ISO 226:2003). A 40 Hz tone needs far more energy than a 1,000 Hz tone to seem equally loud. Bass is expensive. Every subwoofer ever built is a monument to that curve.

The lower boundary is softer than the textbook number suggests. Geoff Leventhall's review of infrasound points out that tones below 20 Hz are audible at high enough levels, that hearing thresholds have been measured down to 1.5 Hz, and that the popular idea of a hard cutoff at 20 Hz is simply wrong (Leventhall, 2007). What changes as you go down is the character of the sensation. Pitch drains away, and what is left is a pressure that seems to arrive in the chest rather than the ears.

Skin is a second ear

The reason is that the ear is not the only organ tuned to pressure waves. The skin carries several kinds of mechanoreceptor, and two of them are built for vibration. Meissner corpuscles, close to the surface, respond to low frequency flutter in the tens of hertz. Pacinian corpuscles, larger, deeper and layered like an onion, respond to vibration across a wide band and are most sensitive around 250 Hz (Purves, Neuroscience). In 1988 Bolanowski, Gescheider, Verrillo and Checkosky mapped the four channels of touch with psychophysical tests on the hand and showed that the Pacinian channel alone sets the detection threshold across most of the band where music keeps its bass and low mids (Bolanowski et al., 1988).

So a 40 Hz wave that lands quietly on the eardrum, because the ear is insensitive down there, can land loudly on the skin, because the skin is not. The felt sensation of bass is not the ear failing. It is the skin succeeding.

Bone conducts too. Sound reaches the inner ear by two routes: through the air and the eardrum, and through vibration of the skull itself, which moves the fluid of the cochlea directly. Stenfelt and Goode reviewed the physiology of bone conducted sound and its clinical use in hearing aids anchored to the skull (Stenfelt and Goode, 2005). Bone conduction is why your own voice sounds deeper to you than it does on a recording, and why a transducer pressed against the body can reach the hearing system without a speaker cone anywhere in the room.

Measuring the body as it vibrates

Yukio Takahashi, at Japan's National Institute of Occupational Safety and Health, spent years with accelerometers taped to volunteers. In one study eight men were exposed to pure tones at 25, 31.5 and 50 Hz at 100 decibels while the movement of the body surface was recorded at five points. At the chest and the abdomen, the vibration the instruments measured matched the vibration the subjects said they felt. At the head it did not, which suggests the head senses low frequency sound by a different mechanism than the trunk (Takahashi, 2011). An earlier paper from the same lab found that the unpleasantness people reported from loud low tones tracked the measured acceleration of their own chest wall (Takahashi et al., 2005).

This is the plain mechanical account of why bass is felt. Below about 100 Hz the wavelength is long enough that the whole trunk moves as one mass, the ear's sensitivity is low, and the sensitivity of the skin and the organs behind it is not. The engineering standard for whole body vibration, ISO 2631-1, covers 0.5 to 80 Hz for the same reason: that is the band in which a seated or lying body moves along with whatever is moving beneath it (ISO 2631-1:1997).

Music has relied on this for as long as there have been drums. Sebastian Merchel and Ercan Altinsoy's work on the auditory tactile experience of music, collected in the open access book Musical Haptics, shows that adding a vibration channel derived from the low end of a recording changes how listeners rate a concert, at levels well below anything that would be called loud (Merchel and Altinsoy, 2018). We hear a bass drum. We also feel it, and the two sensations are fused before we can tell them apart.

The felt sensation of bass is not the ear failing. It is the skin succeeding.

What this means on AVA

The AVA bed at 29 Nassau Ave is built along this spectrum. Thirty three transducers sit under an octagon of eight wedges, divided into five body zones that run from 41 Hz at the feet to 880 Hz at the head. The feet get the eight metre wave, which the trunk carries as one slow movement. The head gets the 39 centimetre wave, which the skin reads as texture. The music is one signal. The body receives it in five bands, and the projected visuals are drawn from the same signal at the same moment. None of this treats a frequency as medicine. It treats the body as what the physiology says it is: an instrument with more than one way of hearing. This is wellness content, not medical advice.