Sound intensity measurement in the air and on the surface

Sound intensity measurements require detection of both sound-pressure and sound particle velocity. The sound pressure is easily measured with a condenser microphone. The sound particle velocity can be detected indirectly by the conventional pressure gradient method using two closely spaced microphones, the spacing of which is a function of frequency. Alternatively, particle velocity can be detected directly by a new microphone that uses the interaction of the audible sound field and an ultrasonic wave. This new principle allows the construction of a sound intensity probe with a very broad frequency range (20 Hz-5 kHz) without readjustments to the probe. The particle velocity detection principle may also be used for noncontact measurement of surface velocity. As a result, surfaces of any materials may be explored with the probe without any direct contact, eliminating the limitations (such as surface loading, poor mounting methods, and sensitivity problems) posed by conventional accelerometry techniques.