Hertz, a field guide

A hertz is one cycle per second. That is the whole definition, and it is the same whether the thing cycling is an air pressure wave, a shaking floor, a radio signal or a heartbeat. What differs is how a body meets each band. This is a guide to the bands, walking upward from below hearing to above it, with the folklore separated from the measurements as we go.

Below 20 Hz: infrasound

The textbook floor of hearing is 20 Hz, but the floor is porous. Geoff Leventhall's review of infrasound notes that hearing thresholds have been measured down to 1.5 Hz, and that tones below 20 Hz are plainly audible if they are loud enough. The sensation loses its pitch and becomes a pressure, but it does not vanish (Leventhall, 2007). Surf, wind over buildings, large ventilation plants and slow machinery all live here. So does most of the mythology.

The number that circulates most is 7.83 Hz, the Schumann resonance. It is real. The cavity between the Earth's surface and the ionosphere rings electromagnetically, excited by lightning, with a fundamental near 7.8 Hz and higher modes above it. Balser and Wagner published the first clear observations in Nature in 1960 (Balser and Wagner, 1960). Two things follow. First, it is a radio phenomenon, a magnetic field measured in picoteslas, not a sound. Second, a loudspeaker playing a 7.83 Hz tone is not reproducing the Schumann resonance any more than humming reproduces a radio station. We have not found a controlled trial showing that 7.83 Hz played as sound does anything a person can measure, and we do not claim one exists.

0.5 to 80 Hz: the whole body moves

Engineers who worry about trucks, ships and helicopters use a different map. ISO 2631-1, the international standard for whole body vibration, evaluates exposure from 0.5 to 80 Hz, because that is the band in which a seated, standing or lying body moves along with whatever it is resting on. Its weighting curves are most sensitive between about 4 and 8 Hz in the vertical direction, where the trunk resonates on the spine (ISO 2631-1:1997). Michael Griffin's Handbook of Human Vibration, the standard text in the field, tabulates the rest: the head resonating relative to the shoulders in the 20 to 30 Hz region, the eyeball in the tens of hertz, and the chest wall somewhere between 50 and 100 Hz (Griffin, 1990). These are ranges, not switches. Bodies differ in mass and posture, and a resonance is a broad hill, not a peak.

20 to 100 Hz: the felt band

This is the band people mean when they say they can feel the bass. Yukio Takahashi's laboratory measured the body surface of volunteers exposed to pure tones at 25, 31.5 and 50 Hz at 100 decibels and found that the chest and abdomen moved measurably, and that the amount they moved predicted how strongly the subjects said they felt it (Takahashi, 2011). The lowest string on a four string bass, E1, sits at 41.2 Hz in standard tuning (HyperPhysics). A kick drum's fundamental lives just below it. Almost every dance floor since the 1970s has been an experiment in this band.

30 to 120 Hz: the clinical band

Vibroacoustic therapy, the clinical practice that grew out of Olav Skille's work in Norway in the 1980s, uses sinusoidal tones between 30 and 120 Hz played through a bed or chair. Lee Bartel and Abdullah Mosabbir's 2021 review of possible mechanisms records that Skille favoured single pitches at 40, 52, 68 and 86 Hz, slowly rising and falling in loudness about every seven seconds (Bartel and Mosabbir, 2021). Those numbers were chosen by experience rather than by theory, and the same review is candid that the mechanisms are still hypotheses. The band was picked because it is where a body can feel a tone clearly without the tone being loud.

40 Hz: the one number with real laboratories behind it

Forty hertz has a different kind of story, and it deserves care. In 2016 Li-Huei Tsai's group at MIT reported in Nature that flickering light at 40 Hz reduced amyloid in the visual cortex of mice bred to model Alzheimer's disease, by driving gamma rhythm activity in neurons and changing the behaviour of microglia (Iaccarino et al., 2016). In 2019 the same group reported in Cell that 40 Hz sound, and sound with light together, reached beyond the visual cortex in mice and improved memory tasks (Martorell et al., 2019). In 2022 they published feasibility and pilot results in a small number of people with mild Alzheimer's dementia, using an hour a day of combined 40 Hz light and sound at home. The device was safe and tolerated and there were early signals worth following, but the study was built to test feasibility, not to prove a treatment (Chan et al., 2022).

In Toronto, Bartel's group had already tested 40 Hz sound delivered through a chair in 18 people with Alzheimer's disease and found improvement on a cognitive screen across six sessions, in an exploratory pilot with no placebo arm (Clements-Cortes et al., 2016).

Then the limits. In 2023 a group at Columbia reported in Nature Neuroscience that 40 Hz light flicker did not entrain native gamma oscillations in the hippocampus of mouse models, and questioned the mechanism the earlier work proposed (Soula et al., 2023). No large randomized trial in people has reported. The honest summary is that 40 Hz is the one frequency with serious neuroscience behind it, that most of it is in mice, and that no one has yet shown that 40 Hz sound treats a disease in humans.

Forty hertz is the one number with real laboratories behind it. Most of the results are in mice.

100 to 1,000 Hz: where touch peaks and voices live

The skin's vibration sense does not fade as the felt band ends. It sharpens. The Pacinian corpuscles, the deep receptors that answer to vibration, are most sensitive near 250 Hz, according to the four channel model Bolanowski and colleagues measured in 1988 (Bolanowski et al., 1988). This is the band of most sung and spoken vowels, of a cello's middle register, and of the concert pitch A4 at 440 Hz. One octave up, at 880 Hz, is A5, the top of the AVA bed (HyperPhysics).

1,000 to 20,000 Hz: the ear takes over

Above a kilohertz the skin loses interest and the ear is at its best. The equal loudness contours in ISO 226 bottom out around 3,000 to 4,000 Hz, the region where the ear canal resonates and where the least pressure is needed to hear anything at all (ISO 226:2003). The upper edge, near 20,000 Hz in a child, recedes steadily with age (Purves, Neuroscience). Consonants, cymbals and the sense of air in a recording live up here. Nothing at these frequencies is felt through a bed, and nothing needs to be.

What this means on AVA

The bed at 29 Nassau Ave spans the guide above from the felt band to the edge of the ear's territory. Five body zones run from 41 Hz at the feet, the E1 of the double bass, up to 880 Hz at the head, with 33 transducers under eight wedges carrying the music between them. That range was chosen from the physiology in this piece: the trunk moves with the low band, the skin peaks in the middle, the ear handles the top. The projected visuals come from the same signal. We do not play 7.83 Hz, we do not promise anything about 40 Hz, and we choose the music first. This is wellness content, not medical advice.