Using two measurements with simultaneous speech and noise presentation, Hagerman and Olofsson have suggested a time-domain method to estimate the speech and noise signals at the output of a hearing device. The method, which uses a simple phase-inversion scheme, has gained popularity in hearing-aid research, although receiving only limited validation. In this work, we present an evaluation of this signal-separation method using simulated measurements with different hearing aids and listening conditions. Estimates of the speech and noise spectra from the phase-inversion method are compared to those obtained using the coherence function. New measures of speech and noise distortion are proposed as tools to evaluate the phase-inversion method. Additionally, we analyze the intelligibility predictions computed from the recovered spectral estimates, while accounting for the proposed speech distortion measure. Under additive-noise conditions, the phase-inversion method provides ideal signal separation without suffering any biases at low signal-to-noise ratios. For conditions involving automatic gain control, compressive output limiting, and peak clipping, the intelligibility predictions based on the phase-inversion method are found to agree with relevant findings from the literature.

In this paper, we present a comparative evaluation of two speech and noise estimation methods commonly used with nonlinear hearing devices: the coherence function used for spectral estimation, and a noise phase-inversion scheme used to perform signal separation. The speech and noise spectra estimated at the output with both methods are compared for normal-hearing subjects and for hearing-impaired subjects using linear and nonlinear hearing aid processing. The spectra are found to be similar except for very low SNR conditions. However, Speech Intelligibility Index (ANSI S3.5-1997 [1]) estimates are relatively unaffected by differences in the spectra obtained with each method.