New methods of measuring impulse responses based on carefully designed deterministic signals can further improve the performance offered by classical methods. In fact, these methods are particularly interesting when measuring long impulse responses as the ones analyzed in architectural acoustics. However, the effects 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 different methods of measuring impulse responses: the linear convolution of sweep signals with the inverse filter 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 significant 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 confirmed 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 specific 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.
|Period||08/03/09 → 08/10/09|