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Mono vs. Stereo Sound

by Craig Tollis

January 29, 2007—Something that's come up on several recent edit jobs, and that I've been asked about, has been the use of mono versus stereo sound sources.

Most modern non-linear editing systems, including Final Cut Pro and Avid, now tend to assume that you are dealing with sound in stereo pairs. That is, your source will have two channels of sound that are designed to be sent to left and right speakers during playback. For many consumer and music sources, this is a pretty good assumption. For professional sound, however, it can get you into trouble.

For example, your modern camcorder is likely to have a pair of microphones set into the front of it, and makes a stereo recording of the subject you're shooting. Music from CDs, mp3s or other common sources is likely to be a stereo mix created in the recording studio.

On the professional side, however, a boom mic or other single capsule mic is inherently mono. Lavalier (tie clip) mics are usually mono too. Sound effects and "natural sound" recorded on location are probably mono, particularly if they were recorded with a boom mic.

So, even when there are two tracks of audio on your capture source, they may not be a stereo pair, but two completely different sound sources. How this is set up will depend on the decisions made by your sound mixer during shooting. In an interview, for example, there may be one track used for the interviewer and one for the interviewee. There might be one track from a lav mic worn by the interviewee and a second safety track from a boom mic or reference track from a camera mic. For a dramatic piece, there might be multiple booms, or a combination of booms and lavs.

In these situations, it's important to listen to the sound from each track separately and pick which one is best for your application. (Note that depending on the action this may change during the take!) Most edit systems will let you temporarily mute tracks. There will generally be one track that will be either louder, clearer, cleaner or otherwise preferable. You can discard the poorer quality audio clip that you won't be using. Mixing a poorer quality track with a better quality track typically degrades the overall quality and doesn't gain you anything. You'll probably need to pan your single track to center too.

The general rule is to consider your sound source. Most natural sound sources are inherently mono - a voice, a closing door, a bell, a hammer striking a nail - the sound comes from one specific place and is best recorded by a single well positioned microphone. The editor or sound engineer can then mix it, equalize it, pan it, and add reverb or echo to give it a spatial position within the final mix that corresponds to how it appears in the picture.

A recording that is intended to pick up a number of sources, ambient, atmospheric or environmental sound that has spatial quality, may be in stereo. For example, the sound of machines in a factory, traffic in a street, or people working in an office.

In this case, make sure you pan stereo sound left and right, if your edit system has not already done this by default. It is particularly valuable for music, where the sound engineer has probably gone to some effort to create space and depth in a stereo mix, which will add impact to your production.

If in doubt, stop and take a listen. Primary sound should be clear and easy to understand. Natural sound or ambient sound should say something about the location and set a mood. Sound effects should be reasonable and motivated. As with all production elements, ask yourself - what do I want from this, what does it add?

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