Sean M. Connelly
Linda P. Franzoni
North Carolina State Univ., Ctr. for Sound and Vib., Dept. of Mech. and Aerosp. Eng., P. O. Box 7910, Raleigh, NC 27695-7910
There is a need for nonintrusive measurement devices. When the wavelength of the sound wave is physically the same scale as that of the microphone, its sound pressure cannot be measured accurately. Acousticians have worked around this difficulty by using smaller microphones and, when necessary, using probe tubes with even smaller diameters attached to small microphones. This need for very small diameter devices is especially critical at high frequency. The use of probe tubes introduces another level of complexity to acoustic measurements. The resonant dynamics of the tube itself must be taken into account. The tube supports standing waves that cause the microphone frequency response to be very irregular. Typically, damping material, such as foam or steel wool is introduced into the probe tube to attenuate standing waves. However, even when damping is successfully applied, the response remains irregular and the damping material results in a significant reduction of signal-to-noise ratio. The new concept is designed to eliminate standing waves without introducing damping in the signal path between the microphone and the end of the tube. Therefore, the signal-to-noise ratio will not suffer significant degradation, and behavior will be consistent from probe to probe.