Evaluation of Electro-acoustic Techniques for In-Situ Measurement of Acoustic Absorption Coefficient of Grass and Artificial Turf Surfaces

The classical methods of measuring acoustic absorption coefficient using an impedance tube and a reverberation chamber are well established [1, 2]. However, these methods are not suitable for in-situ applications. The two in-situ methods; single channel microphone (P- probe) and dual channel acoustic pressure and particle velocity (Pu-probe) methods based on measurement of impulse response functions of the material surface under test, provide considerable advantage in data acquisition, signal processing, ease and mobility of measurement setup. This paper evaluates the measurement techniques of these two in-situ methods and provides results of acoustic absorption coefficient of a commercial artificial Astroturf, a Dow quash material, and a grass surface. The single channel microphone method uses impulse response calculations from a Maximum Length Sequence (MLS) signal excitation of an electro-acoustic loudspeaker and a Fast Hadamard Transformation (FHT) based cross-correlation algorithm by the de-convolution of recorded single channel microphone signal and the input MLS signal. The dual channel Pu-probe method is based on calculation of the complex impedance of the material surface under test from the frequency response function between the sound pressure and particle velocity time domain signals measured simultaneously at the same position on a material. The complex reflection coefficient calculated from both these methods further provides the acoustic absorption coefficient of the material under test.