The relationship between sound transfer functions and hearing levels.

The effects of individuals differences in sound transfer function (STF) from free sound field to the tympanic membrane on hearing levels was studied in the right and left ears of 55 young male and 45 young female subjects. Furthermore, canal volumes and lengths have been recorded. STFs were measured in 1/3-octave bands using a miniature microphone and an attached probe. STF measurements were performed in the 0.5-16 kHz frequency range. Audiograms were registered with linear frequency sweep from 0.25 to 8 kHz. Transfer function spectra and magnitudes as well as ear canal dimensions were compared to hearing levels. There was a significant relationship between the dimensions of the ear canal and hearing levels. Large, compared to small, ear canal volumes resulted in a shift of STFs towards lower frequencies. STF spectra and magnitudes had a significant effect on hearing levels. Subject with low-frequency-dominated STFs have higher hearing thresholds than subjects with lower magnitude STFs.