Should I apply non-linear frequency compression (NFC) for mild hearing losses?
Due to bandwidth limitations that exist for all commercially available ear-level hearing instruments, there is evidence that NFC can aid in the detection of very high-frequency environmental sounds and phonemes, in particular the /s/ phoneme for female talkers which peaks around 9kHz. As the third most common letter and one of the most important linguistic markers in the English language denoting plurality, possession and sometimes tense, audibility for /s/ is required for optimal understanding. In the case of mild hearing loss, the NFC parameters are set very lightly with no compression applied below about 5kHz. This is intended to provide the additional high-frequency access with minimal impact on signal quality.
Wolfe et al. (submitted, JAAA) studied the benefit of NFC for mild hearing loss in a group of 11 children. Data showed significant improvement in the detection of an 8kHz warble tone and the female production of /s/ when compared to performance with the same device with NFC off. It is important to recognize that most clinical tools, including speech material and electro-acoustic measurements, will not be sensitive to changes in audibility at 9kHz. In this study, the Phonak Phoneme Perception Test (PPT) and aided threshold testing using warble tones were the measures used that were sensitive to the benefits obtained with NFC.
This Ask the Expert was taken from the article and text course, Sound Bytes on SoundRecover – view the complete article for more information.
Wolfe, J., et al. (Submitted). Journal of the American Academy of Audiology.