Comparing Fast, Slow and Adaptive Compression
Pittman, Pederson, and Rash (2014) report that over the last 25 years the noted benefits of compression have included a reduced need to adjust the volume control, listeners preference for compression, improved tolerance for loud sounds, and better adjustment to amplification. They note that two compression parameters are well known and well understood (compression kneepoint and compression ratio). However, attach and release times are less understood, even though “evidence suggests that they may significantly affect successful communication for some hearing aid users….” They note that “fast” and “slow” compression release times refer to the time it takes for the circuit to return to the nominal gain value. “Fast-acting” compressors usually have release times of 100 milliseconds or less, whereas “slow-acting” compressors may have release times of 500 to 2000 milliseconds.
Pittman, Pederson, and Rash note that slow release times may provide inadequate amplification for low-level inputs (immediately after a loud input) and fast-release times may distort the acoustic waveform and may decrease the signal-to-noise ratio. Recent evidence indicates that people with lower cognitive ability may have more difficulty with fast-acting compression. Adaptive compression detects large changes (increases or decreases) in the input level to trigger the appropriate release time. That is, when there are no significant changes in input, or a large decrease in input level is detected, longer release times are used. When a large increase in input level is detected, short release times are used. Pittman, Pederson, and Rash compared three amplitude compression release times (slow, fast, and adaptive) with regard to the ability of children and adults (15 children with hearing loss and 20 children with normal hearing, and 26 adults with hearing loss [ages 50 to 78 years] and 21 younger adults [ages 21 to 30 years] with normal hearing) to accurately categorize words and environmental sounds in difficult listening environments.
Not surprisingly, people with hearing loss performed worse than people with normal hearing and “listeners with normal hearing achieved optimal performance with slow acting compression.” However, they also reported “listeners with hearing loss achieved optimal performance with adaptive compression” and “amplitude compression significantly affects perception of speech and environmental sounds.” They concluded “listeners with hearing loss may derive significant benefit from hearing instruments that use adaptive amplitude compression, especially in complex listening environments.”
For More Information, References, and Recommendations
Cox RM, Zu J. (2010) Short and Long Compression Release Times: Speech Understanding, Real-World Preferences, and Association with Cognitive Ability. Journal of the American Academy of Audiology. 21(2):121-138.
Pittman, AL., Pederson, AJ. & Rash, MA. (2014): Effects of Fast, Slow, and Adaptive Amplitude Compression on Children’s and Adults’ Perception of Meaningful Acoustic Information. Journal of the American Academy of Audiology. Volume 25, Pages 834 to 847.
Spirakis SE. (2011) Auditory Neuropathy Spectrum Disorder and Hearing Aids: Re-Thinking Fitting Strategies. Hearing Review. October.