THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

17.2 Industrial Applications of Ultrasound 487Figure 17.6. Schematic of a beam-deflection ultrasonic flowmeter.Ultrasonic meters are being used to measure flow rates of rivers, nuclear reactorheat-exchanger fluids, gas-containing emulsion, slurries, corrosive liquids, andwind velocities. These measurement devices carry the advantage that they introducenegligible pressure loss in a system and they are economical to operate andcan handle a wide range of flow rates, pipe diameters, and pressures.Resonance Method of Measuring Sound Propagation SpeedThe resonance method is similar to the interferometer method for measuring velocity,but it can be applied to solids as well as to fluids. This involves the use ofa fixed transducer and a fixed reflector or two transducers spaced a fixed distanceapart. The transducer is driven to sweep through a range of frequencies to determinesuccessive resonances. In a nondispersive medium, the difference betweentwo successive resonant frequencies equals the fundamental resonant frequencyof the medium, i.e.,c = 2 lfwhere l is the distance between the transducer and the reflecting surface and fdenotes the difference between successive resonant frequencies.Motion and Fire SensingOne of the few ultrasonic applications in open air is that of the motion and firesensor, which is restricted to the lower kilohertz range, where attenuation is notvery much. A magnetostrictive transducer placed at some point in a room emitspulses in all directions. The reflected signals from the walls and furniture areeventually picked up by a receiver, from which a constant indication is generated.Any variation in the sound field, caused by an intruder or a change in temperature,