A new method of measuring displacement by means of variations of sound frequency and the length of air column in a small pipe (a resonant method using speaker and electric feedback loop)

A new method of measuring displacement by means of variations of sound frequency and length of an air column in a stopped resonant pipe has been investigated. In this study, a speaker was used as the sound-generating source instead of the air jet, which was used for the feedback excitation mechanism on the edge instrument. This paper deals with a new kind of displacement sensor using an electric feedback circuit. A sweep signal is radiated from the speaker through the small hole which is opened on one end of the closed resonant pipe, toward the inside of the air column. The sound pressure takes the largest value at the resonant frequency. The displacement of the plunger which is inserted from the other end of the pipe corresponds to the length of the air column. Thus; the displacement of the plunger corresponds well to the resonant frequency of the air column. The phase-locked loop device is adopted for stability of the resonance and frequency