Sound System Quick Fix
Got feedback? Here's the best tool to fix it, and it's absolutely free.
WE'VE ALL WITNESSED IT — usually at a wedding, a meeting, or any event where someone is handed a microphone to address an audience through a portable sound system.
The scenario goes something like this:
A person takes the microphone, holds it at arm's length, and starts talking. It's not loud enough, so the mixer operator turns it up. The system becomes unstable and “ringy,” and maybe even bursts into feedback. Feedback filters are engaged, which notch the first few dominant feedback frequencies. It still isn't loud enough, so the operator increases the level setting again, and the whole process is repeated. The sound is hollow, and feedback results from the slightest move of the microphone. The poor performance is blamed on the microphone, and it's assumed that a new mic and more feedback filters are needed to make the system work.
I have witnessed this scenario many times — both in situations when the users simply didn't know better, and when they should have. This is operator error to the highest degree, and it's completely avoidable without adding or replacing any audio equipment.Acoustic gain
One of the most fundamental tenants of sound reinforcement is the concept of acoustic gain, which is simply a measure of how much louder a talker sounds with the sound system than without it. It's measured in decibels, and the limiting factor is almost always regenerative acoustic feedback. No one should be allowed to operate a sound system without a basic understanding of what affects acoustic gain.
Acoustic feedback has a very simple cause. The sound system is amplifying itself. It has become its own dominant source of input. A live microphone hears everything going on in the room. This includes the source being miked, but unfortunately it also includes the sound from the loudspeaker.
Once the microphone “hears” something coming from the loudspeaker higher in level than it “hears” it in the room, it will start amplifying itself. This is true of desired signals such as talkers and musical instruments and undesired signals such as room noise. So, Step 1 is to make sure that the desired signal is much louder than the room noise at the microphone. Step 2 is to make sure that the level of the talker at the mic is much louder than the level of the talker coming from the loudspeaker at the mic. When these two conditions are met, the sound system will be stable.Some background
The causes and solutions of acoustic feedback are some of the best understood aspects of audio. They follow relatively simple mathematical relationships that all system operators must understand. A complete derivation of the acoustic gain equation is available in several texts, the most notable of which is “Sound System Engineering” by Don and Carolyn Davis. And don't cringe because I used the word “equation.” Equations are just a formal way of expressing something very practical. The “math” will be executed by moving the mic and adjusting the level — no calculator needed.
There are several key distances between sound system components, and these distances ultimately determine how much acoustic gain can be achieved. They include:
The microphone-to-loudspeaker distance — D1
The loudspeaker-to-listener distance — D2
The talker-to-microphone distance — DS
The equivalent acoustic distance — EAD
The number of open microphones — NOM
All of these are self-explanatory except the EAD, which is the distance from the live talker that you want the sound system to bring all listeners to. In other words, if you and I can converse without undue labor at 2 meters in a room (a typical distance), then the goal of the sound system is to bring all listeners to within 2 meters or closer of the live talker. Sound systems are binoculars for the ears. You're free to pick any EAD you like, but the shorter the distance, the more difficult it is to achieve enough stability to realize it. When listening to a sound system you can visualize the EAD by closing your eyes and asking: “If the person I'm hearing is un-amplified, how far am I from them?”
Some of these parameters need to be increased to achieve more acoustic gain, and others need to be decreased. It's that simple. Before you even think about touching a fader to increase the level, the significance of these parameters should first be considered.Balancing the books
Achieving adequate acoustic gain isn't an esoteric mathematical endeavor; it's an exercise in basic accounting. There are debits and credits, and the books must balance. You can't operate a sound system on credit!
The distances that must be maximized to “balance the books” are D1 and EAD. The distances that must be minimized to “balance the books” are D2, DS, and NOM. So, if your sound system has inadequate acoustic gain, you need to:
That's it! There's little else that can be done, and you shouldn't try anything until you have considered these relationships.Is there a quicker solution?
If this is starting to sound confusing, read on. Of all of these variables, only two of them are practical to change “on the fly.” These are DS and the NOM.
Halving the number of open mics only buys you about 3 dB, so let's ignore it as a quick fix for an unstable system.
Of all of the listed distances, the DS is by far the most sensitive. It's so sensitive, in fact, that for now we can just ignore the rest of them.
When a sound system is unstable, the only practical way to get an immediate, tangible improvement in stability and gain is to reduce the mic-to-talker distance.Recipe for stability
Like cooking, sound is a business of ratios. Acoustic gain is no exception. A tangible improvement in acoustic gain would be 6 dB, which requires a doubling or halving of the distances laid out in the acoustic gain equation. This can't be over-emphasized. Read it again. A tangible improvement in the acoustic gain requires a doubling or halving of any distance in the acoustic gain equation. And interestingly enough, it doesn't matter which one you double or halve; 6 dB is 6 dB!
It's perfectly logical to then ask: “Where is the easiest place to get 6 dB?” The answer is by changing DS — the microphone-to-talker distance. Because it's the smallest of all of the distances, it's the easiest to change by a factor of two. It's a physical fact that you can get as much gain improvement from moving the mic from 12 inches to 6 inches from a talker as moving a loudspeaker from 100 feet to 50 feet of a listener!
The acoustic gain equation tells us two important, immutable things: 1) You can't get a lot of gain with large miking distances; and 2) If the system is unstable, reducing the miking distance is the most effective “quick fix.”
Consider the potential improvement in gain if the talker starts with the microphone at 12 inches from his lips, and then proceeds to reduce the distance to achieve more gain:
6 inches — +6 dB
3 inches — +12 dB
1.5 inches — +18 dB
0.75 inches — +24 dB
0.37 inches — +30 dB
Thirty decibels! That's huge! And it's absolutely free of charge.Feedback filters
So what about feedback filters? One of the myths of modern audio is that feedback filters allow violation of the acoustic gain equation. They don't. Because feedback is frequency-dependent, a given frequency may violate the acoustic gain equation more than another. If this frequency is notched out, it's no longer the weak link, and a different frequency becomes the limiting factor in the gain of the system. So, notching a few frequencies can allow the system to be more stable, but you can't get more gain than is predicted by the equation.
Turning up the volume control reduces the stability of a system. Reducing the miking distance doesn't.
The gain is absolutely free. This is because both the talker and the loudspeaker are increasing in level at the microphone. One compensates for the other. In fact, as you reduce DS you can reduce the setting of the level control if all of the extra gain isn't needed. This actually increases the stability of the system.Close proximity isn't an option
Close proximity is mandatory for lots of activities. People accept that it's necessary to speak into the telephone. They accept that it's necessary to hold a newspaper close to read it, and that one must sit close to a computer to type or read the screen. One must sit close to the steering wheel to drive a car.
So why do people insist on not talking into the microphone, and why do sound system operators let them?One more time — the right way
Let's return to the scenario that I described at the beginning of the article. A person grabs a mic and places it at arm's length from his lips. Rather than increase the level control, you hold up a little cardboard sign that says, “Hold the mic closer!” As he moves it closer, the level comes up, and you reduce the trim setting to compensate.
The process is repeated until the system is loud enough. The system sounds good, it's stable, no equalization is needed, and the talker is free to roam with the mic as he pleases. The client is happy, the boss is happy, and you get a bonus for your mastery of the laws of physics.
I suggest that sound system operators everywhere unite and do the following: The next time someone grabs a mic and holds it to his navel, turn it down, not up. When he insists on more level, you insist that he hold the microphone closer.FEEDBACK
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Pat Brown is president of Synergetic Audio Concepts (Syn-Aud-Con) Inc. and Electro-Acoustic Testing Company (ETC) Inc. Syn-Aud-Con conducts training seminars in audio and acoustics worldwide for those who operate, install, and design sound reinforcement systems. ETC Inc. performs precision loudspeaker testing for the audio industry. He can be reached at firstname.lastname@example.org.