A Method for Measuring the Frequency of Vibration or Rotation
Earle L. Kent · The Journal of the Acoustical Society of America · 1942
This method involves the operation of a synchronous motor whose speed is proportional to the frequency to be measured. The theory might be best given by briefly describing two instruments using this principle. A manufacturer of aircraft instruments desired to measure the r.p.m. of an air-driven gyroscope rotor. An instrument was constructed to pick up the sound created by the air jet striking the buckets on the rotor and the frequency of this sound was divided by the proper number to take care of the fact that there was a multiplicity of buckets on the rotor, and then a synchronous motor was driven with the resulting frequency. A sensitive tachometer was attached to the synchronous motor indicating the r.p.m. of the rotor. This method was used in an instrument for measuring pitch. An electric tachometer was used and the meter covered a range of one semitone. Push-buttons enabled the operator to pick the interval and its octave. An accuracy within 1/10 of 1 percent was maintained. The wave shape and amplitude of the input wave can vary over wide limits without any effect on the accuracy. The accuracy depends entirely upon the type of tachometer used.