Acoustically isolated paired air transducers for 50-, 100, 200-, or 500-kHz applications

Solid piezoelectric disks, sealed within a metal case, have a relatively high acoustic impedance Z and exhibit undesirable ringing. Nevertheless, they can be used in a variety of air-coupled ultrasound applications, including industrial applications where robust construction is important. To overcome the drawback of high Z, quarter-wave impedance matchers are necessary. Ringing usually prevents one from using high-Z matched transducers in pulse-echo mode. But if the transducer case is isolated acoustically from the surrounding structure, pairs can be installed side-by-side or in other configurations. It has been found that numerous potential applications become practical if the case contains a circumferential flange that can be sandwiched between a pair of soft O-rings. Applications include measurement of the distance to a reflective target; measurement of flow in the axial direction in a pipe; measurement of secondary flow components such as swirl or crossflow; measurements related to sound speed in a gas, namely, average molecular weight, or temperature; and air-coupled measurements of thickness and elastic properties in solid or porous boards. Designs are presented, showing how one, two, four, or eight transducers can be installed in standard 1-, 2-, or 3-inch pipe plugs or 2-inch flanges. A 2-inch pipe plug accommodates two transducers, while a 2-inch flange is big enough to contain eight transducers, e.g., pairs operating at 50, 100, 200, and 500 kHz. Combining transducers of different center frequencies partly overcomes the narrow bandwidth of any individual transducer.