What's the difference between "compression" and "expansion" in hearing aids?
If you have ever been confused by the definitions of compression and expansion, you're not alone. Part of the reason for this is that from an audiological viewpoint, they each seem to do something opposite of what the name implies. Wide Dynamic Range Compression (WDRC) makes a broader range of sound intensities audible for the user. Although it is compressing the wide range of sound intensities into the limited dynamic range of the hearing aid user, it can also be thought of as expanding the range of sounds that are made audible. Expansion, on the other hand, reduces audibility for very soft sounds. Also commonly known as "microphone noise reduction", expansion is intended to keep the hearing aid from amplifying very soft sounds which are not of interest to the wearer, such as internally generated noise or very low level environmental sounds. Amplification of such sounds can be annoying to the hearing aid wearer in quiet situations.
Both expansion and WDRC can be illustrated by input/output curves or gain curves. If the gain of a hearing aid with WDRC is measured at different input levels, gain will decrease as the input level increases. Thus, the gain curve for a 50 dB SPL input will be higher than the one for a 65 dB SPL input, which in turn is higher than the one for an 80 dB SPL input. Simply put, as the input level increases above the compression kneepoint, the gain decreases.
Below the compression kneepoint, the hearing aid provides the maximum amount of gain. This is the linear portion of the gain curve, and all input levels are amplified with the same amount of gain.
But what happens when there is expansion in the hearing aid? Expansion also has a kneepoint, and it will be lower than the WDRC kneepoint and the linear gain portion in each frequency region. Sounds below the expansion kneepoints are amplified with less gain. Below the expansion kneepoint, gain increases as the input level increases. This provides the least amount of gain for very low input levels, for internal hearing aid noise and external noise such as fans. In this way, expansion is the opposite of compression.
Tammara Stender, Au.D., CCC-A is a Senior Audiologist at GN ReSound, where she plans and conducts clinical trials for newly developed hearing aid technology and prepares documentation for released products. Her research interests include hearing aid benefit and satisfaction, the occlusion effect and spatial localization abilities with hearing aids. She received her Master of Science degree from Vanderbilt University, and her Doctor of Audiology degree from the University of Florida.
Learn more about ReSound at www.gnresound.com or on the ReSound web channel