Verification of Estimated Output Signal-to-Noise Ratios From a Phase Inversion Technique Using a Simulated Hearing Aid

Purpose: The signal-to-noise ratio (SNR) for speech presented in background noise may vary after being processed by digital hearing aids with nonlinear sig-nal processing algorithms, such as wide dynamic range compression (WDRC). A phase inversion technique has been previously developed to assess the out-put SNR of hearing aids. However, systematic validations of this technique have not been conducted. This study aims to validate the phase inversion technique.Method: A simulated hearing aid with multichannel WDRC was implemented, from which the output SNRs, computed via shadow filtering, for connected speech in background noise were directly computed. The agreement between the shadow fil-ter output SNRs and those estimated using the phase inversion technique for the same stimuli was utilized to validate the phase inversion technique. The back-ground noise was 2-or 20-talker babble noise, and the speech stimuli were pre-sented at SNRs of -10 to +10 dB at the input of the simulated hearing aid. The simulated hearing aid was configured to provide amplification for four representa-tive audiograms, and the WDRC was set to be fast or slow acting. To investigate the effects of additive noise, independent of the presented noise stimulus, on the phase inversion estimated output SNR, the same simulated hearing aid was imple-mented with an additive Gaussian noise at its input (45 and 60 dB SPL).Results: Results showed that the phase inversion technique could either overestimate or underestimate output SNR, depending on the test condition; the estimation errors tended to coincide with temporal landmarks, such as natu-ral pauses between consecutive sentences or fricatives; and increasing the sim-ulated noise led to poorer estimates of output SNR.Conclusions: Results imply that the accuracy of the phase inversion technique is dependent on the test conditions. Thus, the phase inversion technique should be used with caution, and its validity should be evaluated further.