Method of Measuring Audiofrequencies
Don Lewis, Paul E. Griffith · The Journal of the Acoustical Society of America · 1941
We have recently developed a method of measuring audiofrequencies which has proved to be quite accurate as well as fairly rapid. The apparatus consists of a vacuum-tube oscillator which controls a multivibrator whose fundamental frequency is 100 c.p.s.; a beat-frequency oscillator equipped with an incremental-frequency control calibrated in cycles per second over a 100-cycle range: a G.R. Type 636-A wave analyzer; and a cathode-ray oscilloscope. The measurement of frequency is made in the following manner: (1) With the incremental-frequency condenser set at zero, the beat-frequency oscillator is adjusted to the unknown frequency through the use of Lissajous figures on the oscilloscope. (2) The wave analyzer is then set to the multiple of 100 cycles which is nearest to the frequency being measured, as shown by the main dial of the oscillator, and adjusted exactly by tuning it to one of the harmonics of the multi-vibrator. (3) The analyzer is then switched to the oscillator and the incremental-frequency dial (which reads from −50 to +50 cycles) is adjusted until a maximum deflection is obtained. (4) The unknown frequency is the frequency at which the analyzer is set modified by the reading on the incremental-frequency dial. The accuracy of the method depends primarily upon the constancy of the multi-vibrator and the accuracy with which it is calibrated. Checks which have been made show that the standard error of measurement is less than 0.25 percent.