Typical wiring scheme for microphones that use one conductor for audio and bias voltage.

Other Mic Issues
How long can the microphone cable be? Because computers use the unbalanced wiring scheme, microphone cables longer than 15 feet will usually pick up electromagnetic interference or cause the sound to become muffled. To preserve sound quality, use the shortest mic cable possible.

Is ‘polarity’ important? If pin 3 of the XLR connector is wired to the Tip of the miniplug instead of pin 2, the polarity of the signal will be inverted. The microphone will sound the same to the human ear, but voice recognition software may not recognize the sound waveform, resulting in a high error rate.

Using Sources With Non-XLR Connectors
If the microphone or other audio source to be used is equipped with something other than a three-pin XLR connector, a little research must be done to find out which portion of the connector carries the audio and which is connected to ground. The audio signal should always be routed to the Tip of the miniplug connector on the computer, and the ground should be connected to the Sleeve of this connector.

No connection should be made to the Ring on stereo mic connectors. Cables for this application are available that terminate in a mono 1/4-in phone plug on one end and a stereo 3.5mm phone plug on the computer end, with no connection to the Ring. A standard audio patch cable combined with an adapter can also suffice.

1/4-in to 1/4-in cable shown with 1/4-in to 1/8-in adapter

Microphones equipped with 1/4-in plugs usually have audio on the Tip and use the Sleeve as the ground. These microphones often have a high-impedance (about 30,000 ohms), which means that only a fraction of their output signal will be transferred to the low impedance (600 to 2,000 ohms) input of the computer.

Special Info for Mac Users
In the case of Apple Macintosh computers, several things are different. Newer Macintosh models, including the G4, iBook, PowerBook G4, and the flat panel iMac, do not have a sound input port. For these models, Apple recommends a USB microphone or a USB audio adapter with a microphone input jack.

Older models do have an audio input and one audio output. These are identified on the back of the computer by a small graphic representation of a microphone (for the audio input) or a speaker (for the audio output.)

Photo of the back of a Macintosh computer where sound output and input are identified