Hearing threshold, loudness, and annoyance of infrasonic versus non-infrasonic frequencies

Research related to perception, loudness, and annoyance of infrasound (frequency below 20 Hz) is limited compared to non-infrasound (20–20000 Hz). The purpose was to determine hearing threshold, equal loudness contours, equal annoyance contours, and other sensations apart from hearing. The laboratory experiment involved 19 normal hearing participants. Observed hearing thresholds within 4–8000 Hz agreed with previous findings supporting the adequacy of our methods. Equal-loudness contours for 20, 40, and 60 phon were determined within 4–1000 Hz. They emphasized the non-linear nature of hearing. The dynamic range of hearing is extremely suppressed at infrasonic frequencies: an increment of 5 dB at 4 Hz feels like an increment of 10 dB at 20 Hz and an increment of 20 dB at 1 kHz. Equal-annoyance contours were derived for 20, 40, and 50 phon within 4–1000 Hz. Because individual hearing thresholds varied up to 20 dB, an infrasonic tone still being inaudible for one participant could be loud or annoying for another participant. The finding may explain why some people perceive low frequency sound more annoying than the others. Other sensations apart from hearing (such as pressure in the ear, headache, and vibration sensation) were reported both for infrasound and non-infrasound. Thus, other sensations apart from hearing are not limited to infrasonic frequencies. The study findings emphasize that sound below 20 Hz should be treated similarly as sounds within 20–20000 Hz. Health effect assessment procedures would benefit from standardized hearing threshold below 20 Hz.