Correctly setting a sound system’s gain structure is one of the mostimpor-tant contributors to cre-ating a sound system that sounds good.Conversely, an improperly set gain structure is one of the leadingcontributors to systems that sound bad
Correctly setting a sound system’s gain structure is one of the mostimpor-tant contributors to cre-ating a sound system that sounds good.Conversely, an improperly set gain structure is one of the leadingcontributors to systems that sound bad. The cost of the system is secondaryto proper setup. The most expensive system set up wrong never performs upto the level of a correctly set, inexpensive system. Setting all thevarious level controls is not difficult; however, it remains an often 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 thisconcept makes most setups intuitive. A little common sense goes a long wayin gain setting.
A dozen possible procedures exist for correctly setting the gain structureof any system. What follows is but one of these, and is meant todemonstrate the principles involved. Once you master the fundamentalprinciples, you will know what to do when confronted with different systemconfigurations.
Mastering the lingoAudio-speak is full of jargon, but none so pervasive as the decibel. Thoseunfamiliar or rusty with decibel jargon are directed to the sidebar,”Decibel Notation and Its Many Reference Levels.”
Mastering gain or level control settings also requires an understanding ofdynamic range and headroom. Dynamic range is the ratio of the loudestundistorted signal to that of the quietest discernible signal in a piece ofequipment or a complete system, expressed in decibels (dB). For signalprocessing equipment, the maximum output signal is ultimately restricted bythe size of the power supplies: It cannot swing more voltage than isavailable. The minimum output signal is determined by the noise floor ofthe unit: It cannot put out a discernible signal smaller than the noise(generally speaking). Professional-grade analog signal processing equipmentcan output maximum levels of +26 dBu, with the best noise floors being downaround -94 dBu. This gives a maximum unit dynamic range of 120 dB – apretty impressive number coinciding nicely with the 120 dB dynamic range ofnormal human hearing (from just audible to painfully loud).
For sound systems, the maximum loudness level is what is achievable beforeacoustic feedback or system squeal begins. The minimum level is determinedby the overall background noise. It is significant that the audio equipmentnoise is usually swamped by the HVAC plus audience noise. Typical minimumnoise levels are 35 dB to 45 dB-SPL, with typical loudest sounds being inthe 100 dB to 105 dB-SPL area. (Sounds louder than this start being veryuncomfortable.) This yields a typical usable system dynamic range on theorder of only 55 dB to 70 dB – quite different than unit dynamic ranges.
Note that the dynamic range of the system is largely out of your hands. Thelower limit is set by the HVAC and audience noise; the upper end isdetermined by the comfort level of the audience. This usable dynamic rangeonly averages about 65 dB. Anything more doesn’t hurt, but it doesn’t helpeither.
Headroom is the ratio of the largest undistorted signal possible through aunit or system to that of the average signal level. For example, if theaverage level is +4 dBu and the largest level is +26 dBu, then there is 22dB of headroom.
Because you cannot do anything about the system’s dynamic range, your jobactually becomes easier. All you need worry about is maximizing unitheadroom. 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 to 10. Crest factor is the termused to represent the ratio of the peak (crest) value to the rms (root meansquare – think average) value of a waveform. For example, a sine wave has acrest factor of 1.4 (or 3 dB) because the peak value equals 1.414 times therms value.
Music’s wide crest factor of 4 to 10 translates into 12 dB to 20 dB. Thismeans that musical peaks occur 12 dB to 20 dB higher than the “average”value. This is why headroom is so important. You need 12 dB to 20 dB ofheadroom in each unit to avoid clipping.
Preset all controlsAfter all equipment is hooked up, verify system operation by sending anaudio signal through it. Do this first before trying to set any gain andlevel controls. The audio signal will ensure that all wiring has been donecorrectly, that no cables are bad and that no audible hum or buzz is pickedup by improperly grounded interconnections. Once you are sure the system isoperating quietly and correctly, then you are ready to proceed.
Turn down all power amplifier level and sensitivity controls.
Turn off all power amplifiers. (This allows you to set the maximum signallevel through the system without making yourself and others stark ravingmad.)
Position all gain and level controls to their off or minimum settings.
Defeat all dynamic controllers, such as compressor-limiters, gate-expandersand enhancers, by setting the ratio controls to 1:1 or turning thethreshold contr