INTEGRATED-IMPULSE METHOD MEASURING SOUND DECAY WITHOUT USING IMPULSES

A method of measuring linear-system responses (such as room responses) is described using “maximum-length” pseudorandom noise as the test signal. In this manner, high signal-to-noise ratios can be achieved, even for measurements in noisy environments and for low-power test signals. Pseudo random noise has also been used successfully as the test signal in the “integrated-impulse” method of measuring sound decay and reverberation time. Thus, the need to radiate a short pulse of high peak energy for impulse type measurements is completely avoided. Improvements in signal-to-noise ratios are equal to the period length of the pseudorandom noise, typically 40 dB in room acoustical applications. The necessary digital processing to realize these gains in signal-to-noise ratio and accuracy of response can be performed on available minicomputers. Apart from maximum-length sequences, another type of periodic binary sequency, called the “Legendre sequence, ” can be used as a test signal. Like maximum-length sequences, Legendre sequences have flat power spectra, but their discrete Fourier components have only two phase angles (~±90°), thus simplifying their digital representation (for storage and transmission). In fact, the discrete Fourier transform of a Legendre sequence is equal (within a constant factor) to the sequence itselfl Legendre sequences exist for all period lengths equal to a prime number of the form 4k 1, where k is an integer. Thus, there are many more period lengths to choose from than for maximum-length sequences. © 1979, American Association of Physics Teachers. All rights reserved.