In Situ, Broad Band Method to Determine the Normal and Oblique Reflection Coefficient of Acoustic Materials

It has been shown that the combination of a particle velocity (u) sensor and a sound pressure (p) microphone can successfully be used to determine the reflection coefficient of acoustic materials [1, 2, 3 and 4].

In this paper, a practical measurement technique and a so called pu-probe impedance meter are described that require no Kundt's tube or anechoic room. The procedure allows fast (less than 60 seconds) sound absorption measurements under both normal and oblique angles of incidence on small test samples (less than 30*30cm). The total size of the pu-probe impedance meter is 60 centimeters and weights less than 1kg. The method can be used in combination with a sound card based software package.

At first, the pu-probe is calibrated in approximately 20 seconds. The frequency response of the impedance sensor is transformed to an impulse response. This response is time windowed to cancel out the room reflections.

After the calibration, the reflection coefficient (and the real and imaginary part of the impedance) of the acoustic materials are determined by putting the pu-probe in close proximity of the acoustic sample.