Wiring Schematic diagram

The post explains an electronic circuit method for measuring the velocity of sound accurately. Unlike many experimental methods for determining the velocity of sound in a medium, this electronic method can readily be applied with no more knowledge of physics than the fact that velocity is equal to displacement divided by time. The method involves the measurement of time taken for a sound front to pass between two microphones (Ml and M2) placed at known distances. The time is measured directly with a scaler timer operated by sound-activated switches (Fig. 1). The microphones may be crystal type. The micro-phone separation must be in excess of 5 metres to give a record of time delay of about 15 milliseconds, The unit is  sufficiently sensitive to be activated even by clapping of hands.

The circuits are built as shown in Fig. 2, to operate the start and stop functions of the timer respectively. Transistor (Tl) amplifies the microphone pulse by an amount determined by the setting of the potentiometer VR. This pulse is then passed to the gate of  SCRl, which is biased with just insufficient gate current to cause triggering. Thus the applied pulse turns SCRI on, and since diode Dl is back-biased,T2 is turned off.

This initiates the timer circuit which then commences timing. A few milliseconds later the sound pulse reaches the stop microphone (M2) and the second circuit operates similarly to cease the timing. The reset button Sl turns SCRl off and is a double-pole switch connected to operate both the circuits simultaneously. T1 and T2 may be BC 107, and D1 a lN4l48, though any general-purpose silicon npn transistor and diode should give satisfactory performance. VRl is used to adjust sensitivity to avoid incorrect triggering from background noise. Since SCRI is only activated by a sharp trigger pulse, the unit may be operated in quite high background noise level. The initial setting of VRI should be such that just a gentle tapping of the micro-phone causes operation 0f the timer. SCRI is a 2N1770 or equivalent. Resistor R1 may need slight adjustment to ensure quick discharge of C2.

The following circuit explains how to measure velocity of sound: