One of my first jobs in the pro audio business was to make cables, do simple repairs and be a general “tech” at the USC film school audio department. I was one of those destitute students who asked around about “anything I could get” in terms of work. And this was it.
It was a good experience for a number of reasons, foremost of which was that THX guru Tomlinson Holman was one of the main teachers at the film school, and he was often around the department. I had taken a class from him and knew who he was.
One day at the shop, he was hanging around a bit and I decided to ask him about his thoughts on tube vs. solid state amplifiers, figuring that A) he knew a lot about the subject, and B) he would have some interesting insights for a budding engineer like myself. His answer surprised me but gave me something to chew on for many years after that. He said “what you have to think about is the difference between ‘reproduction’ and ‘production’ in terms of what the two different designs accomplish.”
In terms of live sound, I think this same concept is very important to consider. It is fairly common to debate the issue of “reinforcement vs. amplification” and this is close to what I’m getting at. So that these concepts can be more thoroughly examined, I would propose naming three different categories: reproduction, reinforcement, and production.
Reinforcement: The Most Basic Approach
Generally, most music begins with acoustic instruments of one form or another. Even the electric guitar is usually paired with an amplifier which is a very important component of the sound. And thus, the first goal of sound reinforcement is just that: to reinforce the existing acoustic sound so that A) a larger audience can hear the music – i.e. the sound is capable of filling a larger space, or B) that certain instruments can be brought up to the level of other instruments on the stage. This second category is quite common when mixing drums or electric instruments with acoustic sources such as horns or strings. Of course most often, reinforcement is a combination of these two things: some amplification of quieter sources to balance the louder ones, and an overall boost to fill a larger space and project to a larger audience.
An additional thing to consider here is that not all audio frequencies propagate equally. Low frequency sounds generate standing waves and are difficult to absorb, while high frequencies are absorbed easily. Often, along with doing internal balancing between instruments, it is necessary to add or subtract certain frequencies from specific sounds so that the resulting impression is one of “natural-ness”. For instance, you may want to add overhead mics to the drums just to bring out the upper harmonics from the cymbals and hi-hat, even though the drums are plenty loud on their own. In order for this to work, of course you may want to cut everything in the overhead mics below about 400 Hz…