Comparison of two frequency lowering algorithms for digital hearing aids

A considerable percentage of listeners with severe hearing loss have audiograms where the losses are high for high frequencies and low for low frequencies. For these patients, lowering the speech spectrum to the frequencies where there is some residual hearing could be a good solution to be implemented for digital hearing aids. In this paper we have presented two different frequency–lowering algorithms: frequency compression and frequency shifting. Preliminary results have shown a slight better performance of the frequency shifting method relatively to the frequency compression method. I. INTRODUCTION There are several kinds of hearing impairment. The origin of the sensorineural hearing losses can be due to defects in the cochlea, auditory nerve or both. These problems reduce the dynamic range of hearing. The threshold of hearing is elevated, but the threshold of discomfort (at which the loudness become uncomfortable) is almost the same as for normal–hearing listeners, or even may be lower. For some range of frequencies, the threshold of hearing is so high than it is equal to the threshold of discomfort, i.e., it is impossible for the listener hearing any sound at those frequencies. Hearing loss is more common for high–frequency and mid–frequency sounds (1 to 3 kHz) than for low– frequency. Frequently, there are only small losses at low frequencies (below 1 kHz) but almost absolute deafness above 1.5 or 2 kHz. These facts lead researchers to lower the spectrum of speech in order to match the residual low–frequency hearing of listeners with high–frequency impairments. Slow playback, vocoding, and zero–crossing rate division are some of the methods that have been employed in the last decades. All of these methods involve signal distortion, more or less noticeable, generally depending on the amount of the frequency shifting. Many of the lowering schemes have altered perceptually important characteristics of speech, such as temporal and rhythmic patterns, pitch and durations of segmental elements. Hicks et al. [1] have done one of the most remarkable investigations about frequency lowering. Their technique involve pitch–synchronous, monotonic compression of the short–term spectral envelope, while at the same time