Study and development of an in situ acoustic absorption measurement method

Sound absorbing materials are used in many applications to attenuate unwanted noise. However, existing measurement methods can only be used on a limited number of material packages and under restricted circumstances. Since PU probes are relatively new, the possibilities they offer for (absorption) measurement methods are still largely unexplored. This thesis concerns the study and development of such methods. A practical measurement device has been developed, with which measurement applications have become feasible that were previously impossible or impractical. The device contains a loudspeaker and a probe, and because they can be positioned close to the sample, the influence of reflections and background noise on the measurements is low, relative to other in situ methods. In addition, characterization can be done with millimetre resolution and small samples can be used. The calibration and the sample measurement can conveniently be performed with the same device. Measurements have been performed under idealized conditions, but also inside cars and concert halls under non-anechoic conditions, on road asphalt whilst driving, and on non-homogeneous samples. To obtain the plane wave absorption coefficient corrections are applied for the spherical sound field as present above the sample during the measurement. Deviations are found with existing sound field models because they disregard spherical waves inside the sample. Therefore, a new approach is proposed that that takes these effects into account and which is based on the extrapolation of multiple measurements at different loudspeaker heights. The results of this approach have been verified with other measurement methods and simulations, in some cases even revealing flaws of the latter.