New Displacement Measuring Method Using Variations in Sound Frequency of Air Columu in a small Pipe : Electric Acoustic Noncontact Displacement Sensor

A new displacement measuring method using variations in sound frequency and length of an air column in a closed resonant pipe has been investigated. This noncontact displacement sensor is oscillated by the feedback loop, which is composed of a speaker, microphone and phase-locked loop. The sound pressure takes the largest value at the resonant frequency. The displacement of the measuring object corresponds to the impedance of one end of the air column. Thus the displacement of the measuring object has good correspondence to the resonant frequency of the acoustic system. The original resonant point is measured by transfer function from the FFT analyzer. An electrical analogy is adopted for theoretical analysis of the acoustic system. The results of the calculation and experiments coincide significantly