Below 20Hz, sound becomes infrasound — below the threshold of human hearing.
But your microphone doesn't care. Earthquakes, storms, industrial machinery,
traffic rumble, HVAC systems — all of it sits down here in the infrasonic band,
constantly shaking the air and your walls. You just can't tell.
Infrasound at 18.98Hz has been linked to feelings of unease, dread, and "presence".
In 1998, Vic Tandy found that a lab he worked in produced 18.98Hz standing waves from a fan —
staff reported ghost sightings and nausea. The frequency matches the resonant frequency
of the human eyeball. Military researchers have explored infrasound as a non-lethal weapon.
Some haunted houses have been found to contain infrasonic resonances from faulty plumbing.
—
Hz · dominant infrasonic frequency
waiting
—level dBFS
—peak Hz
—band energy
0 – 20 Hz · infrasonic spectrum
~0.1 HzOcean swell / seiche oscillations
~1–3 HzBuilding sway in wind
~4–8 HzTraffic rumble / HVAC duct resonance
~12–14 HzIndustrial machinery / large fans
~18–19 HzEyeball resonance / "ghost frequency"
▶ why can microphones hear what we can't?
Human ears have resonant structures tuned to roughly 20Hz–20kHz. The basilar membrane in your
inner ear simply doesn't vibrate efficiently below ~20Hz. But microphones are flat membranes —
they respond to any pressure wave. The Web Audio API's FFT analyser can resolve down to around
1–2Hz with a 32768-sample FFT at 44.1kHz (giving ~1.3Hz per bin).
The challenge: phone microphones are designed for speech (300Hz–3.4kHz is their sweet spot),
and their frequency response rolls off sharply below 100Hz. We're working near the noise floor,
so results will vary heavily by phone and environment. But real infrasound sources — big fans,
nearby traffic, ventilation ducts — can be surprisingly strong.
Try: stand near an AC unit, a fridge compressor, or a window facing a busy road.
The infrasonic output from these can reach 80–90dB SPL right at the source.