Provided by Live Sound Advice.

 
If terms such as gain structure, impedance matching and headroom are unfamiliar, or worse, give you a headache, don’t worry, you’re not alone. Most church sound techs would rather have their gear work perfectly right out of the box than have to tweak it into compliance.

Nevertheless, when it comes to setting up and operating a sound system, a working knowledge of gain structure (and a few related concepts) will help you get the best possible performance from whatever equipment you use.

In short, gain structuring has to do with setting the relative levels of audio signals going into and out of two or more connected audio circuits.

Audio gear has a range of input and output signal levels within which it sounds good. Going outside of that range results in problems such as hiss, distortion, reduced fidelity (especially when dealing with digital gear) and lowered power output.

It’s vital to have a basic understanding of gain structure in a live sound system and to know some ways to optimize gain structure for each piece of gear in the signal chain. As an aid to understanding, see the diagram of a signal chain in Figure 1. Some sound systems will be simpler than the one depicted in the diagram, while others will be more complex, but the basic principles apply to any configuration. 

Figure 1:  In a typical P.A. setup’s signal chain, one of two input sources is connected to an input channel on a mixer, with a compressor patched in to the channel insert and an external effects unit patched in to the auxiliary loop. The channel signal is routed internally to an equalizer in the console’s master section, and the mixer’s master output bus feeds a power amp’s input.

Maximum Headroom
All audio gear has a peak maximum signal level (above which the signal begins clipping) and what is referred to as its noise floor: the natural noise of the electronics when no input signal is present (Figure 2).

The total difference between the two extremes is called the dynamic range, which is expressed in decibels. For example, if the peak maximum signal level of a device is +24 dBu and the noise floor is -60 dBu, the device has a dynamic range of 84 dB (24 dB + 60 dB = 84 dB).

The difference between the noise floor and the nominal level at which gear operates (+4 dBu on a typical VU meter) is called the signal-to-noise ratio (S/N ratio).

Finally, the difference between the nominal operating level and the maximum peak level is referred to as headroom.

Figure 2: Maximum peak level, nominal level, and noise floor level determine how much headroom you have as well as what your signal-to-noise ratio is.

Why is all that important, and what does it have to do with getting optimal performance from your audio gear?

As a rule, you want to drive the inputs of a piece of gear at as high a level as possible without inducing distortion.

So if the nominal level is +4 dBu and the peak maximum level is +24 dBu, theoretically you have 20 dB of headroom to work with. That means you’ll probably want to set the input level so that the loudest peaks fall a few decibels short of the maximum, say around +16 dBu on a ppm scale (Peak Program Meter).