Microphones have “polarity,” and it needs to be correct. When a positive air pressure impinges on the microphone, a positive voltage should be present on pin two, relative to pin three on the XLR connector. In short, you want the speaker cone to be moving in the same direction as the microphone diaphragm
Polarity is sometimes incorrectly referred to as phase. Many microphone preamps have a polarity switch that is wrongly labeled “Phase” or might have the Ø (Phase Symbol) next to it. Changing the polarity of a microphone switches the phase 180° at all frequencies, but phase relationships between microphones are dependent on frequency and distance between the microphones.
If you have two microphones right next to each other, and if they have different polarities and you feed them into a mixer with the same gain on each channel, the outputs of the microphones cancel since they are 180° out of phase. But once you separate the microphones, the phase relationship between them becomes frequency-dependent and also dependent on the distance between the microphones.
The standard for microphone polarity is AES26-2001 (r.2006). AES26 states as a “recommended practice (their word for standard)” that pin 2 on the XLR connector shall drive the non-inverting input (or “+”) and pin 3 shall drive the inverting input or “-.” A positive pressure on the front of the microphone should produce a positive-going signal on pin 2 of the microphone.
If you are recording only one microphone, polarity isn’t important, but if you are using two or more microphones in a recording, a microphone that has reversed polarity can cause problems. On drums, it can cause a loss of low-frequencies and cause the drum with the reversed polarity microphone to sound hollow or tinny. If one microphone in a stereo pair has reversed polarity, there will be a loss of low frequencies, and the stereo perspective will be strange. Rather than sounds being positioned across the stereo field, they will sound like they are originating in your head.
Before about 1986, there was no standard for the polarity of microphones; thus, older microphones might have reversed polarity. You should test all balanced cables in your studio for proper polarity using a cable tester or an ohmmeter. If you are using an ohmmeter, verify that pin one on one end is connected to pin one on the other connector, pin two on one end is connected to pin two on the other connector, and pin three on one end is connected to pin three on the other connector.
Once you have verified that all cables in your signal chain are wired correctly, plug the microphone that you wish to test into your DAW and set up to record. If there is a polarity switch on your preamplifier or in your DAW channel, make sure they are disengaged. Engage record and say the letter “p” into the microphone. Expand the waveform of the recorded signal to see the beginning of it and make sure that the initial move is in the positive direction (up). If the waveform initially moves in the negative direction, either the microphone or something in the signal chain has its polarity reversed. You can reverse the polarity by reversing the wires going to pins 2 and 3 of the XLR connector of the microphone.