Setting sound system level controls: The most expensive system set up wrong never performs as well as an inexpensive system set up correctly.
Jul 1, 1997 12:00 PM, Dennis Bohn
Correctly setting a sound system's gain structure is one of the most impor-tant contributors to cre-ating a sound system that sounds good. Conversely, an improperly set gain structure is one of the leading contributors to systems that sound bad. The cost of the system is secondary to proper setup. The most expensive system set up 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 an often misunderstood topic.The key to setting level controls lies in the simple understand ing of what you are trying to do. A few minutes spent in mastering this concept makes most setups 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.
Mastering the lingo Audio-speak is full of jargon, but none so pervasive as the decibel. Those unfamiliar 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 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: It cannot swing more voltage than is available. The minimum output signal is determined by the noise floor of the unit: 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. The minimum level is determined by the overall background noise. It is significant that the audio equipment noise is usually swamped by the HVAC plus audience noise. Typical minimum noise levels are 35 dB to 45 dB-SPL, with typical loudest sounds being in the 100 dB to 105 dB-SPL area. (Sounds louder than this start being very uncomfortable.) This yields a typical usable system dynamic range on the order 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. The lower limit is set by the HVAC and audience noise; the upper end is determined by the comfort level of the audience. This usable 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.
Because you cannot do anything about the system's 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 to 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) because the peak value equals 1.414 times the rms value.
Music's wide crest factor of 4 to 10 translates into 12 dB to 20 dB. This means 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 of headroom in each unit to avoid clipping.
Preset all controls 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 and level controls. The audio signal will ensure that all wiring has been done correctly, that no cables are bad and that no audible hum or buzz is picked up by improperly grounded interconnections. Once you are sure the system is operating 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 signal level through the system without making yourself and others stark raving mad.)
Position all gain and level controls to their off or minimum settings.
Defeat all dynamic controllers, such as compressor-limiters, gate-expanders and enhancers, by setting the ratio controls to 1:1 or turning the threshold controls way up (or down for gate-expanders).
Leave all equalization until after you have correctly set the gain structure.
Gain settings A detailed discussion of how to run a mixing console lies outside the range of this article, 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 signal-to-noise (S/N) 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; let's leave that to another day.) Therefore each stage works to degrade the overall S/N 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, in general, the bigger the input signal, the better the output S/N ratio.
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, 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 or 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.
Once you've set all the input gains and then created the overall desired mix (involving all sorts of art and science I'm not going to get into), then you must set the output level controls in a similar manner: Advance the output control until the output OL light begins to flicker. This is the maximum output level.
(Note that a simple single microphone pre-amp is set up in the same manner as a whole mixing console.)
Setting outboard gear I/O level controls All outboard unit level controls (except active crossovers) exist primarily for two reasons:
They provide the flexibility to operate with all signal sizes. If the input signal is too small, a gain control brings it up to the desired average level; if the signal is too large, an attenuator reduces it back to the desired average.
They act as level controls for equalizers, providing make-up gain in the case where significant cutting of the signal makes it too small, or the opposite case, where a lot of boosting makes the overall signal too large, requiring attenuation.
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 pre-amp'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 ratio through each piece of equipment, thereby maximizing the S/N ratio of the whole system. And that means setting limits such that your maximum system signal goes straight through every box without clipping. Choose between one of the three methods: OL light, oscilloscope or AC voltmeter. With the console or pre-amp set up as described, you now need a convenient sound source. Use an oscillator (built-in or external) and feed in a tone around 1 kHz. (Personally, I hate the sound of 1 kHz, so I prefer something lower, say, around 400 Hz.) Or you can substitute pink noise for the OL light or oscilloscope methods, but pink noise will not work for the AC voltmeter method because the ACVM will not respond fast enough to catch the peaks.
OL light method. Set the level of the oscillator (NOT the console's or pre-amp's output level; it has already been set!) by turning up its own control, if existing, or by using a spare channel on the console. Turn it up such that the output OL indicator just begins to light. It's a large signal, on the order of +20 dBu, and would be very loud if you had not already turned off the amps. What you have now is the maximum expected signal level running through the system. From here on, everything will be set so it does not clip with this signal. Once this is done, operators can run the system as loud as they want without fear of feedback or distortion.
Oscilloscope method. Using the OL light is a fast and convenient way to set this level. However, a better alternative is to use an oscilloscope and actually measure the output to see where excessive clipping really begins. This method gets around the many different ways that OL points are detected and displayed by manufacturers. There is no standard for OL detection. If you want the absolute largest signal possible before real clipping, you must use an oscilloscope. And, of course, if the unit or console does not have an OL indicator, then an oscilloscope is mandatory to establish the actual clipping point. For a really clever alternative to an oscilloscope, see "Piezo Magic" in the Spring 1996 issue of The Syn-Aud-Con Newsletter.
AC voltmeter method: If an oscilloscope is out of the question, another alternative is to use an AC voltmeter (preferably with a "dB" scale). Instead of relying on the OL indicator to tell you when you have a maximum signal, you choose a very large output level, say, +20 dBu (7.75 Vrms) and define that as your maximum level. Now set everything so nothing clips at this level. This is a reasonable and accurate way to do it, but is it an appropriate maximum? Well, you already know that you need 12 dB to 20 dB of headroom above your average signal. It is normal pro-audio practice to set your average level at +4 dBu (which, incidentally, registers as 0 dB on a true VU meter). Because all high-quality pro-audio equipment can handle +20 dBu in and out, then this value becomes a safe maximum level for setting gains, giving you 16 dB of headroom - plenty for most systems.
Outboard gear falls into three categories regarding gain and level controls: no controls; one control, either input or output; or both input and output 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 oscilloscope, or by setting an output level of +20 dBu as shown on your AC voltmeter.
With two controls it is important to set the input control first by turning up the output control just enough to observe the signal. Set the input control so that it barely lights the OL indicator, then back it down a hair, or set it just below clipping using your oscilloscope. Now set the output control also to just light the OL indicator, or just at clipping using the scope. No good way exists for optimally setting an input control on a unit with two level controls using only an AC voltmeter.
For Rane digital audio products, such as the RPM 26 DSP multiprocessor, in which input A-to-D metering is provided with the RW 232 software, setting the input level gain is particularly easy and extremely important: Using the maximum system signal as the input, open up the input trim box and simply slide the control until the 0 dBFS indicator begins lighting. This indicates the onset of digital clipping and is definitely something you want to avoid, so this is the maximum gain point.
Setting power amplifiers 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. 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 such that whatever input level your system may require causes full power.
They do not change the available output power. They only change the required input level to produce full output power. OK, I feel better.
Clearly understanding that point makes setting these controls elementary. You want the maximum system signal to cause full power; therefore, set the amplifier controls to give full power with your maximum input signal using the following procedure:
Turn the sensitivity controls all the way down (least sensitive; fully CCW; off).
Make sure the device driving the amp is delivering max (unclipped) signal.
Warn everyone you are about to make a lot of noise!
Put on hearing protectors and turn on the first power amplifier.
Slowly rotate the control until clipping just begins. Stop! This is the maximum possible power output using the maximum system input signal. In general, if there is never a bigger input signal, this setting guarantees the amplifier cannot clip. (If this much power causes the loudspeaker to bottom out or distort in any manner, then you have a mismatch between your amplifier and your loudspeaker. Matching loudspeakers and amplifiers is another subject beyond this article.)
Repeat this process for each power amplifier.
Turn the test signal off.
Active crossover output level controls Setting the output attenuators on active crossovers differs from other outboard gear in that they serve a different purpose. These attenuators allow you to set 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 1 m when driven with 1 W. Ergo, you want to set these controls for equal maximum loudness in each driver section. Try this approach:
Turn down all the crossover outputs except for the lowest frequency band, typically labeled "low-out." (Set one channel at a time for stereo systems.)
If available, use pink noise as a source for these settings; otherwise use a frequency tone that falls mid-band for each section. Turn up the source until you verify the console is putting out the maximum system signal level, which should be somewhere around the console clipping point. Using an SPL meter (turn off all weighting filters; the SPL meter must have a flat response mode) turn down this one output level control until the maximum desired loudness level is reached, typically around 100 dB to 105 dB-SPL. Very loud, but not harmful. (Note that one to two hours is the permissible noise exposure allowed by the U.S. Dept. of Labor Noise Regulations for 100 to 105 dB-SPL, A-weighted levels.)
OK. 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:
Mute this output section. Do not turn down the level control; you just set it! If a mute button is not provided on your crossover, then disconnect the cable going to the power amplifier.
Turn up the next output section: either "high-out" for two-way systems or "mid-out" for three-way systems, until the same maximum loudness level is reached. Stop and mute this output.
Continue this procedure until all output level controls are set.
Un-mute all sections, and turn off the test source.
Congratulations! You have finished correctly setting the gain structure for your system.
Now you are ready to adjust equalization and set all dynamic controllers. After equalizing, remember that you must 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 is straightforward: take all necessary gain in the console or pre-amp, set power amplifier sensitivity controls for a level appropriate to pass the maximum system signal without excessive clipping, and set active crossover output controls to correct for loudspeaker efficiency differences. Once you understand what the terms mean and how those knobs and lights function, you're on the road to a sound system that sounds great.
decibel Abbr. dB. Equal to one-tenth of a bel. (After Alexander Graham Bell.) The preferred method and term for representing the ratio of different audio levels. It is a mathematical shorthand that uses logarithms (a shortcut using the powers of 10 to represent the actual number) to reduce the size of the number. For example, instead of saying the dynamic range is 32,000 to 1, we say it is 90 dB. (The answer in dB equals 20 log x/y, where x and y are the different signal levels.) Because the decibel is a ratio, it has no unit. Everything is relative, therefore it must be relative to some 0 dB reference point. To distinguish between reference points, a suffix letter is added as follows:
0 dBu A voltage reference point equal to 0.775 Vrms (u = unterminated, i.e., the impedance is irrelevant).
+4 dBu Standard pro audio voltage reference level equal to 1.23 Vrms.
0 dBV A voltage reference point equal to 1.0 Vrms.
-10 dBV Standard voltage reference level for consumer and some pro audio use (e.g. TASCAM), equal to 0.316 Vrms. (Tip: RCA connectors are a good indicator of units operating at -10 dBV levels.)
0 dBm A power reference point equal to 1 mW. To convert into an equivalent voltage level, the impedance must be specified. For example, 0 dBm into 600 V gives an equivalent voltage level of 0.775 V, or 0 dBu; however, 0 dBm into 50 V, for instance, yields an equivalent voltage of 0.224 V--something quite different. Because modern audio engineering is concerned with voltage levels, as opposed to power levels of yore, the convention of using a reference level of 0 dBm is obsolete. The reference levels of 0 dBu or -10 dBV are the preferred units.
0 dBr An arbitrary reference level (r = re, or reference) that must be specified. For example, a signal-to-noise graph may be calibrated in dBr, where 0 dBr is specified to be equal to 1.23 Vrms (+4 dBu); commonly stated as "dB re +4," that is, "0 dBr is defined to be equal to +4 dBu."
0 dBFS A digital audio reference level equal to full scale. Used in specifying A-to-D and D-to-A audio data converters. Full scale refers to the maximum peak voltage level possible before digital clipping or digital overload ("overs") of the data converter. The full scale value is fixed by the internal data converter design and varies from model to model.
Continue the discussion on “Crosstalk” the Millimeter Forum.