Calculating the magnitude of transfer functions for hearing aids by insertion gain measurements

Abstract

While the hearing loss is one of the most common problem in our lives, it is also the most compensable disability. In general, hearing impairment can be fixed by a hearing aid which amplifies sound levels at different frequencies. A transfer function of a hearing aid determines how much the input signal will be amplified at certain frequencies. However, this function may vary depending on whether the hearing aid is worn or not. Therefore, when a hearing aid is in use, expected results are mostly not achieved if the calibration is done without wearing it. This phenomena is known as a fitting problem in literature and due to the altered transfer function artifacts a small incidence of people having hearing abnormalities prefer using a hearing aid. In this thesis, to compensate the fitting problem, we designed and developed a hearing aid analyzer which uses a Real Ear Measurement (REM) method. Determining the electroacustic performance of hearing aid in situ is essential for ideal fitting and it varies according to ear shape, ear canal, and hearing sensitivity. In REM, an insertion gain measurement is obtained by inserting a probe microphone into the ear canal and calculating the gain between input and output signals. Fine tuning is performed using the transfer function based on insertion gain measurement. REMs allow determining individual based, actual characteristics of hearing aids. The superiority of our system comes from the fact that it is capable of measuring the transfer function while the hearing aid is in situ. Moreover, our design is battery powered and small in diameter which makes it portable. Finally, our system complies with international standards.|Keywords : Insertion gain measurement, Hearing aid fitting, DSP5509A, AIC33.