Two complementary Microflown based methods to determine the reflection coefficient in situ

Two complementary methods have been developed that are capable to determine the reflection coefficient of acoustic absorbing materials in situ. The methods are compared with a measurement in the Kundt's tube. The first method is based on the measurement of the surface impedance [1], [2], [3]. Here a prototype device based on this method is presented that for calibration and impedance determination uses of a well defined sound source [9]. The surface impedance method proves to work best for materials with a not too high reflection coefficient [1]. A second (novel) method is developed for materials with a high reflection coefficient. It uses of a monopole sound source that is placed close by the acoustic reflecting material. A Microflown [8] is placed close to the monopole source in such way that its sensitive direction is aiming at the acoustic reflecting material and its non sensitive direction is aiming at the source. This way it is only measuring the 'mirror source': the reflected image of the monopole sound source. It is therefore called 'the mirror source method'. From the transfer function between the reference velocity sensor inside the monopole source and the Microflown outside the source, the reflection coefficient can be determined. First results of this method are presented. The results of both methods are compared to a Kundt's tube measurement. Traditionally this method works with two pressure transducers but it is also possible to use a combinational pressure and velocity transducer (the PU probe). This PU method is already reported in [1], [5], [6], [7]. The calibration should be very accurate to be able to measure highly reflective materials in a reliable way [5], [9]. In this paper an improved calibration procedure for this method is presented. © (2006) by the Katholieke Universiteit Leuven Department of Mechanical Engineering All rights reserved.