What is a mix minus system? How and why is it used? Let’s tackle the second question first. In conference rooms, collaboration spaces, and government chambers and other similar venues, we often need to have several microphones open at the same time. For anyone accustomed to the world of audio, lots of microphones all turned on at the same time equals a high chance for feedback, rendering the system unusable.
The Limits of Traditional Design
When we reach the limits of physical audio system design (microphone and speaker selection and placement), we have to reach inside our electronic bag of tricks. There are several tactics that we can employ, such as gating, compression, and AGC (automatic gain compensation), but one of the most effective is the tried and true mix minus system.
Mix Minus Explained
Picture two people at opposite ends of a 50’ dining room table. They each have a microphone in front of them and a speaker mounted above in the ceiling. The first says “Pass the salt”. If his voice went to both ceiling speakers, you would possibly end up with feedback.
On the other hand, if I set it up so that his voice only came through the speaker on the far end 50’ away, his microphone wouldn’t “hear” this sound to re-circulate it through the system, avoiding the “ feedback loop”. In the same way, the person on the other end of the table could say “Ok. Can you pass the pepper”, and in the same way it only comes out of the speaker at the opposite end of the table.
This same concept can be applied with lots of speakers and microphones. Your AV designer just needs to set up the system so that the microphones directly below a particular speakers are never being fed to that ceiling speaker, avoiding the “loop”.
The best mix-minus support systems will include one microphone and one speaker for every single position. The speaker could be either overhead, or mounted in front of the person (mounted in a dais, for instance). This allows the most precise tailoring and the most natural possible sound. Obviously, this entails an aesthetic impact, due to the number of devices. Sometimes this ratio will not be feasible for budget reasons. Here is a good rule of thumb: The higher the expectation for mission-critical operation and the more reflective the room surfaces (resulting in poorer acoustics), the more important it is to move towards this 1:1 ratio.
If you have a client that has a very large meeting space that must be covered in glass and marble, start working early on the concept that you will need to have one microphone and one speaker for every person at the table. Otherwise, the collaboration space will be beautiful and ornate, occupied with an unhappy client. There is no magic bullet. Only understanding and managing the rules of the game. Get as much absorption into the room as you and your client can live with. If this still results in mostly reflective surfaces, technology will need to come to the rescue. In most cases, this technology will come in the form of lots of microphones and speakers.
Increasingly, clients expect magic sound. We want the audio to reach our ears with a perfect spectral balance at every seat, with the audio perfectly in sync with the picture (regardless of the size of the room). This is not accurate sound. It’s magic sound. You see lightning first and then you hear it because the speed of sound is much slower than the speed of light. The opera singer will sound different at the front of the hall than the rear due to the high frequency absorption of the air and the greater involvement of progressively lower frequencies in the reverberant field.
When clients have come to expect not natural, but magical sound, mix minus systems are one of our most effective and flexible tools to deliver these desires. Frequently, these issues could be solved more directly with proper acoustical treatment of the space. However, this would compromise aesthetics, so we’re back to our electronic bag of tricks. A magician’s work is never done.