Methods of Measuring Impulse Responses in Architectural Acoustics

Projecte final de carrera realitzat en col.laboració amb la Technical University of DenmarkNew methods of measuring impulse responses based on carefully designed deterministic signals can further improve the performance o ered by classical methods. In fact, these methods are particularly interesting when measuring long impulse responses as the ones analyzed in architectural acoustics. However, the e ects of background and impulsive noise, distortion and time-variance are known rather qualitatively. For this reason, the ISO 18233 encourages to develop a deeper understanding of the theoretical bases of these techniques. In this sense, this project presents an in depth analysis of two di erent methods of measuring impulse responses: the linear convolution of sweep signals with the inverse lter and the circular crosscorrelation of maximum length sequences (MLS) and inverse repeated sequences (IRS). The results of this work reveal that the sweep technique can provide signi cant reduction of distortion compared to MLS/IRS technique but, unlike what is explained in the literature, sweep signals cannot reject all distortion artifacts from the causal part of the impulse response. Besides, it is proved that IRS sequences are immune to distortion of even order. On the other hand, it is con rmed that synchronous averaging procedure improves the SNR at the microphone position by 3 dB per doubling the number of averages. Alternatively, it is also proved that the noise contaminating the measured impulse response is reduced by 3 dB every time that the length of the excitation signal is doubled. In terms of impulsive noise, the sweep technique only contaminates speci c frequency bands of the system's impulse response, whereas the MLS/IRS technique uniformly distributes all impulsive noise artifacts over the entire measured impulse response. Finally, it is also shown that MLS/IRS measurements are more vulnerable to time-varying systems than sweep measurements