Why do some mixing consoles have a 'matrix' section?
In recording, we mix to stereo or 5.1-channel surround sound. In concert PA, we mix to mono or stereo. These methods of working are simple and consistent. In theater however, things are rather different.
A theater show could demand as many channels of outputs as the sound designer, in consultation with the director, sees fit. So there could be a center cluster for vocals in musical theater supported by a stereo PA for the band.
In drama there could be a car pulling up outside the window of a house set. So the best source of sound would be a loudspeaker exactly where the car is supposed to be.
A kitchen scene could involve a frying pan on a cooker. A loudspeaker inside the prop cooker would be the best place to play the frying sound effect.
A ghostly apparition might be required to fly around the theater. So loudspeakers distributed among the audience would be best way of giving sound to this vision. The possibilities are endless.
To make this possible a theater console needs many outputs. Some would say as many outputs as inputs. Multitrack recording consoles have many outputs, but that doesn't make them suitable for theater work.
The problem with the normal system of routing channels to group outputs is that you can route a channel to as many groups as you like. But it will be at the same level in every group. The group levels can be controlled, but you couldn't mix two signals so that one was louder in Group 1 and quieter in Group 2, and the other signal was quieter in Group 1 and louder in Group 2. No matter what you do with the faders, this just cannot happen.
So a theater console has a matrix section . At its simplest, a matrix can have two inputs and two outputs. Each of the two inputs leads to a level control, and each of the two outputs has a level control too, so you can vary the proportions of the two input signals in the two outputs. There are consoles with a matrix as small as 2 x 2, and even such a small matrix can be useful. But practically an 8 x 8 matrix is much more versatile. Outside of theater, a matrix can also be used to create multiple foldback mixes, live and in the studio.
A matrix output isn't something you would use on the spur of the moment. It takes insight into the possibilities and careful planning to maximize its benefits.