A Method to Measure the Effect of Noise Reduction Algorithms Using Simultaneous Speech and Noise

Most hearing impairments reduce the ability to understand speech in the presence of background noise. Therefore, there is a great need to enhance the signal-to-noise ratio (S/N) in the hearing aid. It is, however, difficult to estimate the improvement in technical terms. No standardised methods are offered. Our approach is to present speech and noise simultaneously and make two measurements, one of them with the noise phase reversed. Taking the sum of the corresponding two output signals, or the difference, the output speech or the output noise can be extracted. Thus the gain can be calculated for each of them, although they are present at the same time and influence the signal processing of the hearing aid in a normal way. Five different hearing aids were tested with various speech and noise signals and S/Ns. Noise reduction up to 4 dB was measured with noise reduction algorithms (NRAs) activated. The NRAs in the hearing aids measured were more efficient at positive S/N and almost not useful at negative S/N at the particular levels used in the experiment. Fast compression can also influence the S/N.