A Time Constant Meter for Tuning Forks and Other High Q Devices
Charles T. Morrow · The Journal of the Acoustical Society of America · 1952
The time constant meter described operates on the principle that differentiation of an exponentially decaying rectified envelope decreases it in inverse proportion to the time constant and reverses its sign. Hence, it is possible to obtain a null by balancing the differentiated signal against a portion of the original. The circuit is simple, requiring only one amplifier tube and a tuning eye.