A Study on In-Situ Method of Measuring Acoustic Properties of Materials by using a Parametric Loudspeaker - Reduction of Pseudo Sound due to High Pressure Ultrasound

When measuring surface acoustic impedance and/or absorption coefficient of architectural materials in-situ, interference between the reflection from the target area and the undesired waves, such as reflection from other boundaries and diffraction from sample edge, often causes serious measurement error. The authors have investigated a measurement method of these properties using a parametric loudspeaker, which produces a sharp audible sound beam by the nonlinear-interaction of ultrasound, to minimize such harmful effects. Its strong directivity may reduce the undesired waves by making the sound incident onto the small spatial range. However, the super high sound pressure ultrasound causes "pseudo sound" on the microphone surface, and increases error in the results, especially in lower frequency range. In this study, two methods, phase-cancellation method and application of acoustic filter with phononic crystals, were verified experimentally to eliminate the pseudo sound. As a fundamental investigation, the reduction effect of the two methods against the pseudo sound was verified in a free field measurement. Regarding acoustical property measurement, surface impedance and absorption coefficient of glass-wool boards were measured applying the two methods in an ordinary room. The results showed good agreement with those measured by the impedance tube method for frequencies higher than 1 kHz.