With mix-minus, the most common challenge is executing it well. The concept itself is simple: You have several microphones and several loudspeakers in the same room. When you talk into a microphone, sound comes out of the speakers. As you turn up the volume in the room, you’ll eventually hit a point where a microphone picks up too much sound from a loudspeaker and puts the system into feedback. Mix-minus is the process of selectively attenuating (or eliminating) a microphone’s contribution to nearby loudspeakers. Then, when you talk into the microphone, the sound comes out at full volume in distant loudspeakers, but it comes out at a lower volume in nearby loudspeakers to reduce the chance of going into feedback.
The hard part is figuring out how much to attenuate each microphone in each speaker zone, and there are several different ways to approach it. If the room is perfect, with no acoustic problems, and microphones and loudspeakers are in the ideal locations, it wouldn’t be too difficult to determine a simple attenuation pattern for each of the microphones. For example, -10dB in the nearest zone, -6dB in the next closest zone(s), and then full volume to all other zones would work. However, real-world scenarios are rarely so neat and tidy. Rooms have fancy stained concrete conference room tables, walls made entirely of glass, and mic/speaker locations determined by aesthetics rather than sound quality.
In these types of scenarios, you can discover interactions between mics and speakers that seem absurd. For instance, you’d normally expect a microphone to pick up the most sound from the closest loudspeaker. However, in real-world scenarios with reflective surfaces, it’s not unusual to find a speaker that splashes sound onto a nearby reflective wall, which then bounces off a window, and then into a relatively distant microphone at just the right angle. Somehow you find that you need to attenuate the mic more in a distant zone than you do in a nearby zone. Sound can be weird and counterintuitive.
It’s a good idea to prepare yourself for a mix-minus system by doing all of your Potential Acoustic Gain / Needed Acoustic Gain (PAG/NAG) calculations beforehand, and there are plenty of online resources that can help with those calculations. That’s the science of mix-minus. When you get into a real-world, non-ideal room, mix-minus also becomes an art, requiring you to use your ears, play around with the system, and learn from trial and error to achieve the best results.