Effects of intermodulation distortion on speech intelligibility

This investigation was designed (1) to investigate further intermodulation distortion in hearing aids, and (2) to determine the effect of this type of distortion on speech intelligibility. After four hearing aids were analyzed for intermodulation distortion, 10 CID sentences were recorded through each instrument and presented to both normally hearing and sensorineurally impaired listeners. Results indicated an inverse relationship between levels of intermodulation distortion and frequency, with the highest distortion levels found below 1 kHz. This low-frequency distortion significantly interfered with speech intelligibility for sensorineurally impaired subjects listening to sentences in quite. For both the normally hearing group and the sensorineural group listening in noise, no significant differences were found. The results were interpreted to suggest that the low-frequency intermodulation distortion resulted in an upward spread of masking which interfered with perception of high-frequency acoustic cues in sensorineural impaired listeners; however, with the addition of a competing signal, the subtle masking effect of intermodulation distortion were obscured. Implications for additional research are discussed.