Sensors employing elastic waves have been devised for detecting a wide range of non-acoustical measurands. Examples include: biological substances (antigens in a liquid); chemicals, such as styrene vapor and hydrogen sulfide; electrical and magnetic fields; temperature; and a host of mechanical measurands, such as position, force and pressure, acceleration, and material properties such as viscosity. The reason for using an acoustical sensor varies with the application. High sensitivity is attractive in the case of chemical, acceleration and temperature sensing. Near-linear response and wide dynamic range are also important considerations. Low cost and simplicity characterize several acoustical position sensing systems for use with robots and computer displays. In many of these applications, the output variable is a frequency, which is a convenient input variable for digital storage and processing circuitry.Most of the newer acoustic sensing devices employ resonators or delay-line bulk-or surface-wave oscillators whose frequency is affected by the measurand. Resonant elements that are used include conventional piezoelectric crystal resonators, such as plates and tuning forks, SAW resonators formed on crystalline piezoelectric substrates or in sputtered piezoelectric thin films, and tiny micromechanical beams and bridges formed by integrated-circuit fabrication techniques. Other sensors of current interest, particularly for position sensing and materials characterization, use measurement of timing and reflection, transmission, or attenuation made on ultrasonic waves propagating in air or in a solid.Device principles, operating characteristics, and possible limitations of a number of these acoustic sensors will be described.
