A method for measuring oblique incidence absorption coefficient of absorptive panels by stretched pulse technique

This paper proposes a pulse method for estimating the acoustical performance of absorptive panels to reduce noise reflections when applied to road structures such as parallel walls and ceilings of double deck viaducts. The acoustical performance is specified on the basis of oblique incidence absorption coefficient. It is determined from the ratio of the reflected sound energy from the surface of a test panel to that reflected from a rigid floor. Considering its high power, flat spectrum and stability, an optimum Aoshimaʹs time-stretched pulse (OATSP) is applied as a short duration test signal. The measurements were made on glass fiber with thickness of 50 and 100 mm. The absorption coefficients at oblique angles from 0 to 45° show good agreement with theoretical values based on the empirical model of Delany and Bazley. Ceramics and other materials with non-flat surfaces were also tested. For application, the averaged oblique incidence absorption coefficient, which is averaged over incident angles and weighted on the representative spectrum of A-weighted road traffic noise, is proposed as a single number index for the evaluation of noise control design at highways.