Found this online re infrasound, i.e. Very Low Frequency sound:
"Infrasound frequencies around 18-19 Hz—below normal human hearing range (20-20,000 Hz)—can cause anxiety, fear, disorientation, and even visual hallucinations. Researcher Vic Tandy discovered this accidentally when experiencing dread and seeing apparitions in his laboratory. Investigation revealed a malfunctioning fan generating 18.98 Hz infrasound causing his eyeballs to resonate, creating peripheral vision distortions interpreted as ghosts. Human eyeball resonant frequency is approximately 18 Hz; when stimulated, vision distorts creating illusion of shadowy figures. Many haunted locations show elevated infrasound from wind patterns, ventilation systems, or structural vibrations. Natural sources include earthquakes, avalanches, waterfalls, and severe weather producing frequencies causing unease in animals and humans. Some researchers propose infrasound explains paranormal experiences—old buildings with drafty corridors and loose windows generate these frequencies. Elephants and whales communicate using infrasound below human hearing, with calls traveling hundreds of kilometers. Military weaponry research explored infrasound as psychological warfare, though extreme intensities required for weapon applications cause physical damage. This invisible sound explains some supernatural encounters through pure physics."
www.bbc.co.uk
'There is more to these caves than bats and cheese'
