MEASUREMENT OF THE SOUND-ABSORPTION COEFFICIENT INSITU - THE REFLECTION METHOD USING PERIODIC PSEUDORANDOM SEQUENCES OF MAXIMUM LENGTH

The problem of measuring the sound-absorption coefficient in situ is approached in a systematic way accounting for parasitic reflections and background noise. The basic reflection method is improved by using pseudo-random binary sequences of maximum length as the test signal. The measurement procedure is orientated towards digital processing techniques and uses the advantages of modern instruments. The inherent cross-correlation process greatly improves the noise immunity. The relationship between the loudspeaker characteristics and the sample size is clarified. The use of a non-rectangular analysis window is suggested. The proposed method was tested in a normally furnished room using portable instruments; the normal-incidence results are in good agreement with values measured using the conventional Kundt's tube method.