Theory and application of a new method for the in-situ measurement of sound absorption

In many applications of noise control engineering, knowledge about the sound-absorbing properties of acoustically reacting surfaces is essential. Being able to measure sound absorption of surfaces in-situ, i.e. at the site but also for the actual sound source, is very useful and would eliminate the need for laboratory measurements. This now seems possible using a new quantity called the total intensity which can be determined if we know the sound pressure and particle velocity in a certain direction. For plane waves, it can be proven that this quantity is the sum of the incident- and reflected sound intensity in that direction. As the active intensity is also known, the incident- and reflected intensity can be calculated. Thus, one obtains the spatially averaged sound absorption coe cient of a surface (in direction n) by measuring the sound pressure and the particle velocity (in direction n) near that surface and by determining the active- and the incident intensity in a spatially averaged sense. In the paper the theory behind the method will be described, preliminary experimental results and some simulation results will be shown. The paper will conclude with an outlook of future research.