Impact of infrasound on the human cochlea

Low-frequency tones were reported to modulate the amplitude of distortion product otoacoustic emissions (DPOAEs) indicating periodic changes of the operating point of the cochlear amplifier. The present study investigates potential differences between infrasound and low-frequency sounds in their ability to modulate human DPOAEs. DPOAEs were recorded in 12 normally hearing subjects in the presence of a biasing tone with fB = 6 Hz and a level LB = 130 dB SPL. Primary frequencies were fixed at f1 = 1.6 and f2 = 2.0 kHz with fixed levels L1 = 51 and L2 = 30 dB SPL. A new measure, the modulation index (MI), was devised to characterise the degree of DPOAE modulation. In subsequent measurements with biasing tones of fB = 12, 24 and 50 Hz, LB was adjusted to maintain the MI as obtained individually at 6 Hz. Modulation patterns lagged with increasing fB. The necessary LB decreased by 12 dB/octave with increasing fB and ran almost parallel to the published infrasound detection threshold. No signs of an abrupt change in transmission into the cochlea were found between infra- and low-frequency sounds. The results show clearly that infrasound enters the inner ear, and can alter cochlear processing.