A novel measurement tool is developed that is capable to measure the reflection coefficient of acoustic materials in situ and thus in real live situations such as a car.
The measurement tool is a combination of two novel methods, the surface impedance method [1], [2], and the mirror source method [3].
The surface impedance method measures the acoustic impedance close to the surface of an acoustic absorbing material. The method is very sensitive for highly reflective surfaces [1], [2], [6].
The mirror source method uses a miniature monopole sound source that is placed close by the acoustic reflecting material. A particle velocity microphone (a Microflown [4], [5]) 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. From the ratio with a reference measurement on a fully reflecting plate the reflection coefficient can be determined.
In this paper some results of both methods are shown and it also shown that there is a minor effect of reflecting surfaces in the environment on the measured absorption curves. This observation paves the way for application of the described method in cars.