Sound Absorption Measurement of Acoustical Material and Structure Using the Echo-Pulse Method

A new approach is proposed for measuring the sound absorption coefficient of material at normal incidence, that bases on the echo-pulse method in a circular tube. Two types of acoustical material and structure were employed: foam and pipe exit. Two different waveforms called the Butterworth and the damped sine pulse are successfully generated and used as incident pulses in the experiment of sound absorption measurement in a circular pipe. The absorption coefficient of the acoustical material measured by the echo-pulse method is consistent with that of B&K 4206 impedance tube measurement. Discrepancy between them is totally less than 10% in broadband frequency range from 200 Hz to 5k Hz, mainly due to the different internal tube diameters of these two tubes and the fitting of the sample, i.e. the difference between the sample diameter and the internal tube diameter. Since the incident sound pulses are all with the features of regular waveform, short duration and broadband frequency spectrum, sound absorption measurement results conducted by the echo-pulse method are more reliable than that of B&K 4206 impedance tube method especially in low-frequency range. Further validation of this new method is also given through the absorption coefficient measurement of pipe exit. The echo-pulse method is relatively simple as only single microphone is used for measurements. It is believing that this method could also be used for in-situ measurement of sound absorption coefficient in outer space.