Tag Archives: Gain

Two Speed Limits

The amount of level that an audio rig can deliver is often less than the theoretical maximum.

Please Remember:

The opinions expressed are mine only. These opinions do not necessarily reflect anybody else’s opinions. I do not own, operate, manage, or represent any band, venue, or company that I talk about, unless explicitly noted.

I don’t think that very many people have trouble understanding the concept of “not enough PA.” For most folks, it just stands to reason that an FOH (Front Of House) or monitor system has a finite amount of level available.

Notice that I said, “most folks.” There are some people out there who do seem to believe that all PA systems are capable of infinite loudness. These people frighten me. They also wreck lots of gear when they’re allowed to drive an audio rig. Audio humans of this particular tribe are genetically related to the tribe of audio humans who must use every ounce of available system power, regardless of whether it’s a good idea or not. “It just doesn’t sound right until I see the clip lights, you know?” *Sigh.*

Anyway.

The idea that audio gear has finite performance limits is pretty intuitive, especially when we’re talking about equipment involved in output transduction (converting electricity to sound pressure waves). What’s not so intuitive is that a system’s maximum practical output is almost always lower than its true maximum.

The True Maximum

The absolute, full-tilt boogie, no-holds-barred maximum SPL that an audio system can produce is most heavily determined by the following:

  • How many drivers you’ve got. This is often simplified into “how many boxes didja bring?” because, most of the time, audio humans deploy drivers that have been conveniently bolted into different kinds of enclosures.
  • How much power you can put into the drivers you have. This can be made more tricky if you don’t actually have enough mains power to drive the amps at full-throttle.
  • The space that the drivers are firing into. Enclosed, highly reflective spaces are “loud” because acoustical energy is reflected back into the room, combining with the “direct” output from the audio system. Outdoors, or in highly absorptive rooms, a much greater proportion of acoustical energy is captured or radiated away from the listeners, never to add to the direct sound.

When you put this all together, you can get yourself some numbers regarding the maximum SPL (sound pressure level) achievable, assuming that no other factor stands in the way. Like I said, though, other factors usually DO stand in the way. One factor is essentially social – that is, how much auditory input people will accept before they perceive the sensation to be unpleasant. This is an important concept, but it’s not really within the scope of this article. However, other major, technically-based limiting factors do apply.

Distortion

A loudspeaker system might be capable of producing x-amount of SPL, but that doesn’t mean that it can do so in a pleasing fashion. It’s entirely possible that you’ll encounter unacceptably high distortion before you hit the absolute maximum level that a system can produce.

Even though I think most techs unconsciously account for this phenomenon when estimating or empirically determining the maximum output available from a rig, I also think it’s important to mention.

A, *ahem*, “dirty” secret that isn’t necessarily intuitive to folks outside of pro audio is that amplifiers are quite capable of producing more than their “rated” power. Rated power is a number that corresponds to what the amplifier can do, based on a certain amount of distortion that the manufacturer finds “acceptable.” Better manufacturers are less tolerant of distortion. One reputable manufacturer is willing to claim that a certain amplifier can put about 397 watts into two, 8 Ohm loads at less than 0.02% distortion. (You have to do a bit of math, because the actual rated power is 500 watts, and the distortion number is given at 1 dB BELOW rated power, for some reason.)

Distortion that low is pretty hard to hear. According to the calculator at Sengpiel Audio, 0.02% THD is about 73 dB down from the original signal. I rigged up a test with two tones (1 kHz and 2 kHz) in my DAW, and I couldn’t hear the 2 kHz tone (at all) against the 1 kHz signal when the 2k channel was pulled down 73 dB.

Anyway, here’s the deal.

That same amplifier will display greater continuous output if greater distortion is allowed. That 397 watts is a continuous rating, based on an RMS (Root Mean Square) voltage – a bit more than 56 volts. To get that RMS voltage, you need a peak voltage of just a bit less than 79.7 volts. Into an 8 Ohm load, 79.7 volts produces an instantaneous power of almost 800 watts. Take note of that “instantaneous,” though. That 800 watts isn’t applied to the loudspeaker for very long, and so, when everything gets averaged out, the loudspeaker only experiences about 400 watts. (It’s actually more complicated than this, especially with actual music, but this will do for an illustration.)

As you push an amplifier harder and harder, its peak voltage output will remain the same, but the RMS voltage will increase. This is because the amplifier spends more and more time producing output voltages that are closer and closer to the peak voltage.

This power isn’t “free” though. The more you push the amp, the more distortion you get. Some manufacturers will allow for much higher distortion at “rated” power, so as to be able to publish a bigger number on the spec sheet.

Bottom line?

The absolute maximum SPL that you can achieve with a rig under a given set of acoustical conditions may actually require that the system be driven into audible, unpleasant distortion. This distortion isn’t just limited to the amplifiers, either. You could be driving any part of the signal chain too hard, even the loudspeaker drivers themselves. The effective “speed limit” on the rig may be brought down (and brought down a lot) by just how far the system can go before it sounds like a pile of garbage.

Of course, for some folks, this (frighteningly) doesn’t matter very much, leading to the classic pro-audio line of “Well, it sounds like !@#$, but it’s REALLY !@#$%^& loud!”

Gain Before Feedback

Then again, you might never even get to the point of audible distortion. GBF (gain before feedback) issues can lower your effective speed limit even more. The underlying reason for this isn’t necessarily obvious: A given amount of gain only guarantees a repeatable output level if the input signal’s overall level remains unchanged. If the input signal’s overall level changes significantly, any fixed gain is correlated to, but not absolutely matched with a particular system output situation.

In other words, you may have a huge amount of gain applied to something, but if the input signal is very small, the final output can still be very quiet. Now, add in the fact that live audio is almost always a “partially closed loop,” and *WOOOOOOOOoooooossssssquuuuuuEEEEEAALLLL!*

Feedback.

“Feedback” is when the output of a system returns to an input of the system. This can be used for some very cool things, but it can also cause serious problems. In the partially-closed-loop situation of live audio, some fraction of the output of the rig finds its way into a microphone or instrument pickup, and is then re-added to the system output. The parts of the signal that are in phase sum constructively, causing the system output to rise, which means that more signal finds its way back into the system, which means that the system output rises still further at those frequencies, which –

*SQQQQUUUEEEEEEEeeeaaaaaaaoooooooooOOO!*

You get the point.

GBF is just a shorthand for “how much can we turn this thing up before it starts to ring.” With some sources, it simply isn’t possible to stay out of feedback while simultaneously applying sufficient gain to drive a rig at full-tilt. There just isn’t enough input.

Where you see this “in the wild” is with the quiet singer (or the singer who’s moderately loud but wants to stand 5 feet from the mic), and also with the quiet player of the [acoustic instrument that you can’t get a good mic placement on]. The same gain structure with a louder source would drive the system all the way to the limiters, while also clipping the console’s input circuitry. However, for these folks, you’re barely making 90 dBC. Maybe not even that.

And yes, you’ve gotten out the EQ and notched the major problem areas. The issue is that any more EQ will either completely wreck the sound of the source, or just reduce the overall gain by the same amount you tried to add.

When you arrive at this kind of situation, you’re at the effective speed limit. You can’t get any more output, because you can’t add any more gain. Turning things up will just result in feedback, terrible sound, or no net gain at all.

As with a lot of things in life, just having the basic capacity to do something doesn’t mean it can actually be achieved. The circumstances have to be right.