# How to measure microphone output impedance

Is there a method to measure the output impedance of a condenser microphone?

## 1 Answer

Of course. First you must have a repeatable amount of sound. Measure the output amplitude without any load (a high-impedance load, like a o-scope probe, is OK). Next load the output with a resistor equal to your best guess of the output impedance, let's say 1k. Measure the output amplitude again. If it is half the original value your guess was correct. Otherwise solve the equation

B * 1k = A * ( 1k + X )


where

A = open amplitude
B = loaded amplitude
1k = the load resistor you used
X = output impedance


Note that, apart from the requirement for a repeatable sound level, this is the same procedure you would use to find the impedance of any voltage source.

• That is mostly correct. But keep in mind that the output impedance of a mic will change with frequency. Ideally you would do this test with a sine wave at several frequencies (a different test for each frequency). Doing the test with something other than a sine wave will produce inconsistent results that are hard to interpret. But +1 anyway. – user3624 Oct 5 '12 at 19:00
• Thanks. I've worked with speakers where we have measured the impedance using a B&K machine producing a sweep. I wasn't sure if microphones were so variable across frequency, so I left the question very vague. Thanks again! – learnvst Oct 5 '12 at 19:08
• @David You are probably more experienced in this than I, but lreanvst mentioned a condensor microphone, which AFAIK typically has a FET amplifier built in. Wouldn't the FET produce a more-or-less constant output impedance? – Wouter van Ooijen Oct 5 '12 at 21:15
• While a condenser mic usually has a FET amp built in, there is also often a capacitor on the output used as a sort of DC blocking cap. The quality, type, and size of this cap will often make the output impedance vary with freq. – user3624 Oct 5 '12 at 21:29