Frequency compression (or frequency lowering) is a general term applied to attempts to lower the spectrum of the acoustic speech signal to better match the residual hearing of listeners with severe to profound high-frequency sensorineural impairment accompanied by better hearing at the low frequencies. This pattern of hearing loss is common to a number of different etiologies of hearing loss (including presbycusis, noise exposure, ototoxicity, and various genetic syndromes) and arises from greater damage to the basal region relative to the apical region of the cochlea (see noise-induced hearing loss; ototoxic medications; presbyacusis). The major effect of high-frequency hearing loss on speech reception is a degraded ability to perceive sounds whose spectral energy is dominated by high frequencies, in some cases extending to 10 kHz or beyond. Perceptual studies have documented the difficulties of listeners with high-frequency loss in the reception of high-frequency sounds (including plosive, fricative, and affricate consonants) and have demonstrated that this pattern of confusion is similar to that observed by normal-hearing listeners deprived of high-frequency cues through the use of low-pass filtering (Wang, Reed, and Bilger, 1978). Traditional hearing aids attempt to treat this pattern of hearing loss by delivering frequency-dependent amplification to overcome the loss at high frequencies. Such amplification, however, may not lead to improved performance and has even been shown to degrade the speech reception ability of some listeners with severe to profound high-frequency loss (Hogan and Turner, 1998).

The goal of frequency lowering is to recode the high-frequency components of speech into a lower frequency range that is matched to the residual capacity of a listener's hearing. Frequency lowering has been accomplished through a variety of different techniques. These methods have arisen primarily from attempts at bandwidth reduction in the telecommunications industry, rather than being driven by the perceptual needs of hearing-impaired listeners. This article summarizes and updates the review of the literature on frequency lowering published by Braida et al. (1979). For each of seven different categories of signal processing, the major characteristics of each method are described and a brief summary of results obtained with it is provided.