This paper investigates the perceptual significance of residual phase distortion due to an approximate equalization of the nonminimum-phase room response from a sound source to a microphone in a reverberant room. It is shown that disrupted phase relationships introduced by a minimum-phase equalization filter may have a detrimental effect on perceived sound quality. The subjective assessment of phase distortion on speech signals is related to an objective error criterion, newly introduced in this paper. An alternative approach to the minimum-phase/all-pass decomposition based on iterative flattening of the room transfer function (RTF) magnitude is also presented, which overcomes potential numerical problems and provides more insight into subjective aspects of magnitude and phase equalization in the reduction of acoustic reverberation. Factors contributing to the results and practical implications for equalization are discussed.

This paper investigates the robustness of sound equalization using a room response inverse filter with respect to changing or uncertain source or microphone positions. It is shown that due to the variations of the transfer function from point to point in a room, even small changes in the source or microphone position of just a few tenths of the acoustic wavelength can cause large degradations in the equalized room response. The robustness problem is especially acute at high frequencies, which are known to carry some important attributes of the speech signal. The spatial extent of equalization, derived from the statistical-average properties of sound transmission in rooms, is illustrated by computer simulations which corroborate the theoretical results presented.

This paper examines the sensitivity of sound equalization to source or microphone position changes in a reverberant room. It is demonstrated that even small displacements from the reference (equalization) point, of the order of a tenth of the acoustic wavelength, can cause large degradations in the equalized room response. The general theory developed in this paper, which implies that the sound equalization in practical environments may be an ill-posed problem, is verified by the simulation results averaged over different source and microphone positions.