Frequency Compression

Abstract

Frequency-lowering strategies, which present highf requency sounds to lower-frequency regions, have been used in audio recording and entertainment for a long time— e.g., changing the key of a karaoke song to suit the vocal range of the singer— and explored in hearing aids since the 1960s. Now, two new studies investigate the limits of the approach, particularly for those with lower cognitive function. The rationale for frequency lowering is that many hearing aids cannot make sounds audible to their users because of the high-frequency roll-off of receivers, and individuals with high-frequency hearing loss or cochlear dead regions may not be able to make use of high-frequency sounds even if the sounds are audible. While many different frequency-lowering strategies for hearing aid use have been proposed over the years, there are common features; namely, that sounds in the lowfrequency region remain true to their frequencies, and only sounds in the high-frequency region are processed. The differences between the strategies include, but are not limited to:  Whether the frequency-lowered sounds have unique representation in a frequency region or are superimposed onto the lower-frequency regions with other naturally occurring sounds.  Whether all high-frequency sounds are lowered, or just certain sounds with predetermined features.  Whether only the frequency-lowered sounds are presented at the output, or the frequency-lowered sounds are presented along with the original sounds at the output.  The settings of other parameters, such as the frequencylowering threshold and the amount of lowering in the resultant signal. Frequency-lowering strategies implemented in commercially available hearing aids generally can be divided into three categories: 1. Frequency compression: High-frequency sounds (e.g., 4,000-8,000 Hz) are squeezed into a smaller frequency range (e.g., 4,000-6,000 Hz) above a certain frequency, called a frequency compression threshold (4,000 Hz in this example).