Long-term signal-to-noise ratio at the input and output of amplitude-compression systems.

We present measurements showing that the long-term signal-to-noise ratio (SNR) at the output of an amplification system that includes amplitude compression may be higher or lower than the long-term SNR at the input, dependent on interactions among the actual long-term input SNR, the modulation characteristics of the signal and noise being mixed, and the amplitude compression characteristics of the system under test. The effects demonstrated with the measurements shown here have implications for choices of test methods when comparing alternative hearing aid systems. The results of speech-recognition tests intended to compare alternative systems may be misleading or misinterpreted if the above interactions are not considered.