Determination of Acoustical Impedance of Absorbing Surfaces by Two-Microphone Transfer Function Techniques: Measurement In-Situ

In the fourth and final of a series of papers on the measurement of acoustical impedance of absorbing surfaces using the two-microphone transfer function technique, the application of the technique to the measurement of a suspended ceiling and to soft grassland is described. Real materials often exhibit extended reaction, and detailed experimental data is currently difficult to obtain for such samples. But for precise sound field predictions, accurate acoustical data is required. The suspended ceiling tiles investigated are extended reaction panels and measurements are performed in-situ. Consequently, no restrictions on the sample area, edge conditions, or homogeneity are applied. Comparisons with theoretical models are presented. These investigations indicate that the panel absorber can be described by its surface impedance, and that the two-microphone transfer function technique can be used to successfully measure this characteristic in-situ. The surface impedance of soft grassland is also successfully measured using both normal and oblique incident sound. These measurements are compared with theoretical models, which are then used to obtain information concerning the structure of the ground, without invasive sampling.