The effect of occluded ear impedances on the eardrum SPL produced by hearing aids

The purpose of the investigation was to determine the effect of eardrum and earmold impedance on eardrum sound pressure level (SPL) produced by a hearing aid receiver. Acoustic interactions between receivers and ears occurring at receiver resonances were explored and analyzed. Results with ear simulators showed that the effect of simulator eardrum impedance was increased in regions where ear and receiver impedance approached each other during resonances. Earmold acoustics were also significant factors at frequencies where their impedance were comparable to ear impedances. Resonant frequency shifts of 540 Hz in the region between 800 and 1500 Hz were observed for the range of simulator eardrum and earmold impedances employed, with SPL changes of 10 dB occurring at some frequencies. Results were different for each of the five receivers tested confirming the effect of the receiver characteristics on the eardrum SPL. A subsequent test was performed using real ears with results that agreed substantially with those obtained with the simulators.