In this excerpt from “Start to Finish: Vance Powell - Episode 10 - Guitar Overdub Setup” Vance Powell demonstrates his procedure for checking the polarity of mics before beginning recording.

Polarity, as opposed to phase, is purely electrical. Sound waves are a series of compressions (squeezing) and rarefactions (stretching) of the air and they cause microphone diaphragms to move inwards and outwards respectively. A positive (inwards) pressure on a mic diaphragm should result in a positive voltage to be received by the AD converter.

Anything from an incorrectly wired mic lead to a polarity switch on the preamp can cause the polarity to be inverted and the phase to be 180 degrees out relative to the other mics, something which, while inaudible in isolation, will result in serious comb filtering and cancellation when combined with other mics hearing the same audio.

The more complex your signal chain, the more opportunities there are for a polarity flip to have happened so rather than checking every stage, it is quicker to check the whole chain. A device called the Cricket from Galaxy Audio offers an elegant solution for this task. It’s a 2 part device which, amongst other functions, can be used to create a positive pressure pulse and verify the electrical polarity received is also positive.

In the video Vance discovers a polarity issue on one of the mics because of an invert button being left on on one of his external Neve preamps.

If you need to check the polarity of a mic and don’t have a device like the Cricket, checking by toggling the polarity of one of a pair of mics is the time honoured way of checking but this only checks that the mics are in phase with each other. If both mics on a guitar amp were polarity inverted you wouldn’t be able to tell. The added advantage of polarity checkers like the Cricket is that you have a known positive pulse reaching the mic.

Some years ago we featured this ‘no money’ mic check technique which relies on the fact that plosives create such a well defined pressure wave. It’s not as good as a positive-only pulse but it works, just be careful not to blow onto your vintage ribbons!

Watch the free video extract to see the technique in action and read more about it on the PureMix blog.

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