Correctly setting a sound system's gain structure is one of the most important contributors to creating an excellent sounding system. Conversely, an improperly set gain structure is one of the leading contributors to bad sounding systems. The cost of the system is secondary to proper setup. The most expensive system set wrong never performs up to the level of a correctly set inexpensive system. Setting all the various level controls is not difficult; however, it remains a very misunderstood topic.
The key to setting level controls lies in the simple understanding of what you are trying to do. A few minutes spent in mastering this concept makes most set-ups intuitive. A little common sense goes a long way in gain setting.
A dozen possible procedures exist for correctly setting the gain structure of any system. What follows is but one of these, and is meant to demonstrate the principles involved. Once you master the fundamental principles, you will know what to do when confronted with different system configurations.
Audio-speak is full of jargon, but none so pervasive as the decibel. Mastering gain, or level control settings also requires an understanding of dynamic range and headroom.
Dynamic range is the ratio of the loudest (undistorted) signal to that of the quietest (discernible) signal in a piece of equipment or a complete system, expressed in decibels (dB). For signal processing equipment, the maximum output signal is ultimately restricted by the size of the power supplies, i.e., it cannot swing more voltage than is available. While the minimum output signal is determined by the noise floor of the unit, i.e., it cannot put out a discernible signal smaller than the noise (generally speaking). Professional-grade analog signal processing equipment can output maximum levels of +26 dBu, with the best noise floors being down around -94 dBu. This gives a maximum unit dynamic range of 120 dB -- a pretty impressive number coinciding nicely with the 120 dB dynamic range of normal human hearing (from just audible to painfully loud).
For sound systems, the maximum loudness level is what is achievable before acoustic feedback, or system squeal begins. While the minimum level is determined by the overall background noise. It is significant that the audio equipment noise is usually swamped by the HVAC (heating, ventilating & air conditioning) plus audience noise. Typical minimum noise levels are 35-45 dB SPL (sound pressure level), with typical loudest sounds being in the 100-105 dB SPL area. (Sounds louder than this start being very uncomfortable, causing audience complaints.) This yields a typical useable system dynamic range on the order of only 55-70 dB -- quite different than unit dynamic ranges.
Note that the dynamic range of the system is largely out of your hands. The lower limit is set by the HVAC and audience noise, while the upper end is determined by the comfort level of the audience. As seen above, this useable dynamic range only averages about 65 dB. Anything more doesn't hurt, but it doesn't help either.
Headroom is the ratio of the largest undistorted signal possible through a unit or system, to that of the average signal level. For example, if the average level is +4 dBu and the largest level is +26 dBu, then there is 22 dB of headroom.
Since you cannot do anything about the system dynamic range, your job actually becomes easier. All you need worry about is maximizing unit headroom. Fine. But, how much is enough?
An examination of all audio signals reveals music as being the most dynamic (big surprise) with a crest factor of 4-10. Crest factor is the term used to represent the ratio of the peak (crest) value to the rms (root mean square -- think average) value of a waveform. For example, a sine wave has a crest factor of 1.4 (or 3 dB), since the peak value equals 1.414 times the rms value.
Music's wide crest factor of 4-10 translates into 12-20 dB. This means that musical peaks occur 12-20 dB higher than the "average" value. This is why headroom is so important. You need 12-20 dB of headroom in each unit to avoid clipping.
After all equipment is hooked-up, verify system operation by sending an audio signal through it. Do this first before trying to set any gain/level controls. This is to make sure all wiring has been done correctly, that there are no bad cables, and that there is no audible hum or buzz being picked up by improperly grounded interconnections (See Sound System Interconnection). Once you are sure the system is operating quietly and correctly, then you are ready to proceed.
A detailed discussion of how to run a mixing console lies outside the range of this Note, but a few observations are relevant. Think about the typical mixer signal path. At its most basic, each input channel consists of a mic stage, some EQ, routing assign switches and level controls, along with a channel master fader. All of these input channels are then mixed together to form various outputs, each with its own level control or fader. To set the proper mixer gain structure, you want to maximize the overall S/N (signal-to-noise) ratio. Now think about that a little: because of the physics behind analog electronics, each stage contributes noise as the signal travels through it. (Digital is a bit different and is left to another Note and another day.) Therefore each stage works to degrade the overall signal-to-noise ratio. Here's the important part: The amount of noise contributed by each stage is (relatively) independent of the signal level passing through it. So, the bigger the input signal, the better the output S/N ratio (in general).
The rule here is to take as much gain as necessary to bring the signal up to the desired average level, say, +4 dBu, as soon as possible. If you need 60 dB of gain to bring up a mic input, you don't want to do it with 20 dB here, and 20 dB there, and 20 dB some other place. You want to do it all at once at the input mic stage. For most applications, the entire system S/N (more or less) gets fixed at the mic stage. Therefore set it for as much gain as possible without excessive clipping. Note the wording excessive clipping. A little clipping is not audible in the overall scheme of things. Test the source for its expected maximum input level. This means, one at a time, having the singers sing, and the players play, as loud as they expect to sing/play during the performance. Or, if the source is recorded, or off-the-air, turn it up as loud as ever expected. Set the input mic gain trim so the mic OL (overload) light just occasionally flickers. This is as much gain as can be taken with this stage. Any more and it will clip all the time; any less and you are hurting your best possible S/N.
(Note that a simple single mic preamp is set up in the same manner as a whole mixing console.)
All outboard unit level controls (except active crossovers: see below) exist primarily for two reasons:
Many outboard units operate at "unity gain," and do not have any level controls -- what comes in (magnitude-wise) is what comes out. For a perfect system, all outboard gear would operate in a unity gain fashion. It is the main console's (or preamp's) job to add whatever gain is required to all input signals. After that, all outboard compressors, limiters, equalizers, enhancers, effects, or what-have-you need not provide gain beyond that required to offset the amplification or attenuation the box provides.
With that said, you can now move ahead with setting whatever level controls do exist in the system.
Whether the system contains one piece of outboard gear, or a dozen, gains are all set the same way. Again, the rule is to maximize the S/N through each piece of equipment, thereby maximizing the S/N of the whole system. And that means setting things such that your maximum system signal goes straight through every box without clipping.
The RaneGain test set is a handy tool kit based on techniques first developed by Pat Brown of Syn-Aud-Con for use in quickly setting sound system gain controls. It consists of two pieces: a self-contained, phantom-powered 400 Hz generator and a separate audio Transducer housed in an XLR connector. The RG Generator plugs into any mic input on a mixing console (or separate mic preamp) having phantom power in the range of 12-48 VDC, providing a convenient sound source. The RG Transducer plugs into the output of each unit and sounds a warning whenever the output level is clipped.
First, a sound source is connected to the mixing console (or separate mic preamp) to provide the maximum system signal output, then this signal is used to set the outboard units.
The most convenient sound source is one built into the mixer or preamp. If a built-in generator is available, use that; if not, use an external oscillator, such as the RaneGain generator or other test equipment. Connect the generator to an unused channel in the mixing console or to the input of the mic preamp. Carefully set the generator level and the channel input fader so the mic stage does not overload. Next, adjust the master output fader (or preamp output level control) for the largest level possible without clipping the output stage. Determine this maximum level using any of the four methods: RaneGain Test Set, OL Light, Oscilloscope, or AC Voltmeter described below.
Outboard gear falls into three categories regarding gain/level controls:
Obviously, the first category is not a problem!
If there is only one level control, regardless of its location, set it to give you the maximum output level either by observing the OL light, or the transducer, or the oscilloscope, or by setting an output level of +20 dBu as shown on your AC voltmeter.
With two controls it is very important to set the Input control first. Do this by turning up the Output control just enough to observe the signal. Set the Input control to barely light the OL indicator, then back it down a hair, or set it just below clipping using your oscilloscope, or until the RG transducer buzzes. Now set the Output control also to just light the OL indicator, or just at clipping using the scope, or just buzzes. (Note: there is no good way to optimally set an input control on a unit with two level controls, using only an AC voltmeter.)
If your system uses active crossovers, for the moment, set all the crossover output level controls to maximum.
Much confusion surrounds power amplifier controls.
First, let's establish that power amplifier "level/volume/gain" controls are input sensitivity controls. (no matter how they are calibrated.) They are not power controls. They have absolutely nothing to do with output power. They are sensitivity controls, i.e., these controls determine exactly what input level will cause the amplifier to produce full power. Or, if you prefer, they determine just how sensitive the amplifier is. For example, they might be set such that an input level of +4 dBu causes full power, or such that an input level of +20 dBu causes full power, or whatever-input-level-your-system-may-require, causes full power.
Amplifier input sensitivity controls do not change the available output power. They only change the input level required to produce full output power. Clearly understanding the above, makes setting these controls elementary. You want the maximum system signal to cause full power at the amplifier.
Many loudspeakers are not designed to reproduce full power sine waves or pink noise test signals for extended, or sometimes even short periods of time. This is especially true of multi-way loudspeaker systems which are designed to handle extended periods of loud music or speech signals which are considerably less demanding than sine or pink noise signals. Paging systems that use 70/100 volt distribution transformers are more likely to be OK when driven with full power sine and pink noise signals for a little while.
To set the amplifier controls to achieve full power with your maximum input signal use the following procedure. If your system uses active crossovers, for the moment, set all the crossover output level controls to maximum.
Setting the output attenuators on active crossovers differs from other outboard gear in that they serve a different purpose. These attenuators allow setting different output levels to each driver to correct for efficiency differences. This means that the same voltage applied to different drivers results in different loudness levels. This is the loudspeaker sensitivity specification, usually stated as so many dB SPL at a distance of one meter, when driven with one watt. Ergo, you want to set these controls for equal maximum loudness in each driver section. Try this approach:
Okay. You have established that with this maximum system signal this driver will not exceed your desired maximum loudness level (at the location picked for measurement). Now, do the same for the other output sections as follows:
Congratulations! You have finished correctly setting the gain structure for your system.
Now you are ready to adjust equalization and set all dynamic controllers. Remember, after EQ-ing to always reset the EQ level controls for unity gain as required. Use the Bypass (or Engage) pushbuttons to "A/B" between equalized and un-equalized sound, adjusting the overall level controls as required for equal loudness in both positions.
Optimum performance requires correctly setting the gain structure of sound systems. It makes the difference between excellent sounding systems and mediocre ones. The proper method begins by taking all necessary gain in the console, or preamp. All outboard units operate with unity gain, and are set to pass the maximum system signal without clipping. The power amplifier sensitivity controls are set for a level appropriate to pass the maximum system signal without excessive clipping. Lastly, active crossover output controls are set to correct for loudspeaker efficiency differences.
"Setting Sound System Level Controls" Download this note in PDF.