Massarani P., Muller S.

This work presents aspects about simultaneous reverberation times measurements at a number of points inside rooms. The reverberation time values are determined from impulse responses measured by an indirect technique, using sweep sine excitation signals. The first part of the paper deals with the sweep generation, considering the room behaviour and the electroacoustical sound source response. Doing so, a high and frequency-independent signal-to-noise ratio can be achieved in a broad frequency band, while at the same time keeping constant time amplitude. A unique deconvolution signal reference for all the channels can be determined from the designed sweep signal. The impulse response from each channel can be filtered by standard fractional octave bands and then back-integrated to yield filtered energy decay curves. The filtered impulse responses can have very different decay rates. Because of that, the integration limits have to be considered carefully, avoiding the well know bias errors from the impulse response noise energy. Procedures to detect the noise in the impulse response can be applied before the back-integration in order to establish a fixed integration interval for all channels. The paper also considers a method for averaging the filtered impulse responses of different room locations before the back-integration, reducing decay curve oscillations or curving due to lack of diffusion, mainly at the lower frequencies. Some experimental results are presented for a six-microphone system in a reverberation room for sound power measurements. The measurement chain is described, including the sound source and the devices used. Establishing a multi-channel system for reverberation time measurements is easily feasible nowadays, with many options of high quality AD/DA converters. Such a system dramatically reduces the measurement time and could increase the precision of various acoustic test types that require spatial average reverberation times.