Compression: Historical Development & Use Today (Q1 2017)-Compression: Historical Development & Use Today (Q1 2017)

Compression: Historical Development & Use Today (Q1 2017) - Article

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Compression is a common technique used in hearing aids to accommodate different types of sensori-neural hearing loss. Compression is a gain issue, affecting both input and output. The challenge in hearing aid fittings is to increase the signal-to-noise ratio for the client, making speech louder and distinct from background noise. Compression can be categorized as either sensory or neural based on the type of hearing loss. Sensory compression is used for mild-to-moderate hearing loss, while neural compression is used for severe hearing loss. Different types of compression were developed and implemented in analog hearing aids and have been adapted for digital hearing aids. Compression can be visualized on an input/output (I/O) function, with the knee-point indicating when compression begins and the compression ratio indicating the amount of compression provided. Output limiting compression (OLC) and wide dynamic range compression (WDRC) are two common compression schemes used in hearing aids. OLC has a high knee-point and compression ratio, while WDRC has a low knee-point and compression ratio. WDRC gradually reduces gain for more intense inputs, while OLC provides linear gain for soft and moderate inputs and dramatically reduces gain for intense inputs. In digital hearing aids, compression types are combined, and the knee-points and compression characteristics can be adjusted for each channel. Compression also has dynamic aspects, including attack and release times, which regulate the response of the hearing aid to changes in sound intensity over time. Clinicians typically do not adjust dynamic compression characteristics, but rather focus on frequency response adjustments.

Keywords

Compression

Hearing aids

Sensori-neural hearing loss

Signal-to-noise ratio

Sensory compression

Neural compression

Analog hearing aids

Digital hearing aids

Output limiting compression

Wide dynamic range compression