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Cochlear anatomy shapes sensitivity to low-frequency sounds

[abstract] In accordance with standards on hearing, sound sensitivity is assumed to decrease smoothly as frequencies approaches infrasound. However recently, non-invasive measurements of the forward-middle-ear-transfer function (fMETF) in human subjects reveal a resonance feature near 55 Hz, where the slope changes rather sharply by 6 dB per octave. These transfer characteristics of the pressure in the ear canal to the differential pressure across the basilar membrane are presumably caused by the shunting effect of the helicotrema – a small connection between scala vestibule and scala tympani at the apical end of the cochlear. This study investigates whether this shunting effect has an effect on individual equal-loudness contours (ELC). We were able to obtain reliable fMETFs (20 – 250 Hz) from five human subjects. All data replicate the resonance feature and the slope change of the previous study. Surprisingly, the resonance is only reflected in the ELC of two of the five subjects. Nevertheless, the transition frequency of the slope appears to correlate between individual fMETF and ELC. Implication on hearing standards and low-frequency noise hyper-sensitivity will be discussed.

Torsten Marquardt
Christian Sejer Pedersen

Acoustics, Institut for Elektroniske Systemer

Proceedings from the 14th International Conference on Low Frequency Noise and Vibration and Its Control, June 9-11, 2010, Aalborg, Denmark