19 Hz is the experimentally determined resonant frequency of the human eyeball — the frequency at which the eye vibrates most readily when exposed to external vibration. This was established by US Air Force research in which 15 volunteers were exposed to vertical vibrations between 5 and 50 Hz while their eye movements were monitored via corneal reflection.

At 19 Hz, the eye resonates in a way that can cause subtle visual artefacts — smearing, shadowy movements in peripheral vision, and brief distortions that the brain may interpret as seeing something that isn't there. This discovery has led some researchers to make the controversial claim that 19 Hz infrasound may be responsible for certain reports of paranormal experiences, particularly in enclosed spaces where standing waves can build up.

Infrasonic waves at this frequency have been detected in several locations with histories of reported hauntings, including medieval cellars, old hospitals, and industrial buildings. Delivered here as a binaural beat with a 210 Hz carrier (left: 200.5 Hz, right: 219.5 Hz). At headphone volumes, the eyeball resonance effect does not occur — it requires physical vibration of the body, not sound delivered through earphones.