Measuring sound absorption using local field assumptions

This paper describes a novel three-dimensional sound intensity probe consisting of 8 MEMS (micro-electromechanical systems) microphones and a novel free-field calibration method. The use of MEMS-microphones allows for a very compact design compared to probes that employ 1/2-inch condenser microphones. A novel free-field calibration method, based on the principle of microphone interchange, is developed to determine the relative amplitudeand phase errors. An advantage of this method is that is does not require knowledge of the radiation characteristics of the sound source. By repeating the calibration at different positions from the source, a relative amplitude accuracy of less than 2% and relative phase accuracy of less than 0.2◦ are obtained between 1300 and 3000 Hz. Using the calibration results, the measured cross-spectral density matrix can straightforwardly be corrected. Such a correction is useful up to 3000 Hz, as above this frequency, probe-induced diffraction becomes noticeable. 1Reproduced from: E.R. Kuipers, Y.H. Wijnant, and A. de Boer, Development and calibration of a MEMS-microphone based 3D sound intensity probe. Submitted to Applied Acoustics 18 Chapter 2. Development and calibration of a sound intensity probe