are advanced temperature sensing systems that exploit this shift in propagation speed versus temperature. These systems measure temperature by using precise timing of the reflected ultrasound pulse reflection over a known distance. They then do a “reverse correction” to determine what temperature would have caused that change in propagation speed. Transducer parameters start the process After determining the application requirements, designers must then select a suitable audio driver and associated receiver that
can operate at the appropriate frequency, typically at a relatively high 40 kilohertz (kHz) for position sensing/detection, and several hundred kilohertz for fluid flow sensing. The benefits of high- frequency transducers include increased resolution and focused directivity (forward-facing beam pattern), but the disadvantage is increased signal path attenuation. The rate at which the ultrasonic energy scatters and is absorbed while propagating through the medium of air increases with frequency. This results in a decrease in the maximum detectable distance if other factors are held constant. The 40 kHz frequency is a compromise
between factors such as efficiency, attenuation, resolution, and physical size, all of which are related to wavelength. To begin the selection process, it’s helpful to know that transducers used for ultrasonic sensing are characterized by several top-tier parameters. Among these are: ■ Operating frequency, tolerance, and bandwidth: As noted, 40 kHz is common for many basic applications, with a typical tolerance and bandwidth of several kilohertz. ■ Drive voltage level: This specifies the voltage level for which the transducer provides optimal performance. It can range from a few tens of volts to 100 volts, or more.
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