1.1 What Frequency?
The frequency you choose depends on your application. Every transducer will resonate at multiple frequencies depending on its piezoelectric material composition and its geometry. A deep discussion of this topic is not within the scope of this document. As such, this application note only discusses transducers of the disc variety working in axial or thickness mode.
- The frequency of your transducer and the transmit pulse length can determine axial (depth) resolution. The higher the frequency, the tighter the resolution [1]. In general a 1 MHz transducer yields better than 1 mm resolution in most liquid level sensing applications. Proximity sensing applications on the other hand use 40 kHz transducers, while gas flow meters use 200 kHz transducers. In general, propagating an ultrasonic wave through liquids allows system designers to use transducers in the MHz range while applications where the ultrasonic wave will propagate through a gas require transducers anywhere from 40 kHz to 500 kHz depending on the distance the wave needs to travel and the accuracy requirement of the measurement.
Equation 1. Axial resolution = (λ * # cycles)/2
- The higher the transducer frequency, the tighter the beam width [2,3].
- The smaller the transducer diameter, the wider the beam width [2,3].
Equation 2. Sine(?) = 1.2 * V/(DF)
where:
? = Beam divergence angle
V = Sound Velocity in the material (m/sec)
D = diameter of transducer (m)
F = Frequency of transducer (Hz)