A study about the improvement of the dynamic range of Schroeder plots by the method of the two impulse responses product

The most usual way to measure the reverberation time in rooms is based on the sound level decay obtained with the backward integration of the squared impulse response (Schroeder plot). One of the problems related to this type of measurement may be the lack of a sufficient signal to noise ratio for the measured impulse response, especially in the low octave bands. The intruding low frequency environmental noise reduces the useful dynamic range of the Schroeder plot. This paper reports a study about the possibility to ameliorate the useful dynamic range of the sound level decay when a random noise affects the impulse response. The considered technique is based on the acquisition of two impulse responses in sequence for the same source-receiver set. The backward time-integration of the product of these responses yields a sound level decay superposed with a random fluctuation which appears in the later part of the plot. This helps the separation of noise from the decay curve better than the usual operation with a single impulse response affected by random noise. The study was carried out with a noise-free synthetic impulse response mixed with various random noises.