Microphone Impedance

Author: N.H. Crowhurst

In most electrical circuits, impedance, or resistance, refers to the relationship between voltage and current flowing in some component. This is easy to measure because the voltage source is external and the current through the components can be measured.

Measuring an electrical impedance

In a microphone, the voltage source is internal or inside the component, so that we cannot measure it separately. But we can measure the open-circuit voltage and then measure current when we short-circuit the microphone, so that there is no voltage. The relationship between this voltage and current is the impedance of the microphone.

Going back to the theoretical case of the previous page: without the matching transformer, the open-circuit voltage was 1 microvolt, while the short-circuit current, determined by the resistance of .05 ohm, is 20 microamperes.

In the transformer secondary, the open-circuit voltage is 500 microvolts, while the short-circuit current is .04 microampere. Therefore the effective impedance on the transformer secondary is 500/.04 or 12,500 ohms. This is the impedance the microphone presents on the transformer secondary.

Measuring microphone impedance

Thus the real impedance of .05 ohm has been multiplied by 12,500/.05, or 250,000. The impedance matching ratio 250,000:1 is the square of the turns ratio (500 X 500 = 250,000).