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. Pseudorandom 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 itself! 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.