Microphones are the first tools to use to capture sound and send it through the sound system.
They’re amazing, really.
But, as with many things in life, it is possible to have too much of a good thing.
We don’t like to hear about that, but in this particular case using too many microphones could cause some serious sound system problems.
Increased Chance of Feedback
There’s an interesting thing that happens when more microphones are turned on in a live sound system. The risk of feedback increases.
You want to pay attention to the “number of open mics” in your mix.
There is some fancy jargon that we sound geeks like to throw around when we talk about this, but I’ll try to avoid getting too deep here.
The important concept is that when you double the number of microphones in use, you will actually decrease the amount of overall volume potential you can get from those mics without feedback.
If you have a stage full of microphones, you’ll only be able to turn up the sound system to a certain point before there is feedback. By comparison, you would be able to turn up the sound system much louder if you only have one microphone turned on.
Bad Sound Quality
More mics and more sound do not necessarily mean better sound.
Multiple Microphones on One Source
The primary issue with too many mics on one source is that the sound will arrive at one microphone at a different time than the other microphone. This causes an effect called “comb filtering” which sounds like a subtle warping or phasing as certain frequencies are canceled out.
This problem often happens when miking choirs or when using overhead instrument mics.
Fortunately, there is a convenient rule to follow when using multiple mics on one source. It’s called the 3:1 rule.
Consider the distance of the source (a choir for instance) to the first microphone.
The second microphone should be placed three times the distance from the first mic as the first mic is placed from the source.
As an example, Mic #1 is placed three feet from the choir. Mic #2 should be placed nine feet away from Mic #1.
Considering this example, you can see how there are often too many microphones used on choirs. I know I’ve made that mistake before.
Another place this can happen is with drum overhead mics and lectern mics. Again, it’s all about the distance from the source to each mic and how far apart the mics should be spaced.
Reducing the number of mics on your choir or other sources can actually clear up your sound and give you more volume potential (see the first tip above).
Wrong Microphone on the Source
It is possible to isolate more sound from the source and still use multiple microphones on that source. The coverage pattern (or polar pattern) of the microphone can help you capture the sound you want and reject the background noise you don’t want.
You still need to follow the 3:1 rule, but selecting the right coverage pattern can give you more control of the sound you want to hear and better rejection of the noise you don’t want to reinforce.
- Omni-directional mics pick up sound 360 degrees around the mic. This typically isn’t good for live sound, though these mics can be used in some scenarios and applications (like theater or very close mic placement of headset mics).
- Cardioid mics pick up sound in a heart-shaped pattern in front and around the sides of the mic. Most handheld vocal mics and many instrument mics use this type of pattern.
- Super-cardioid and hyper-cardioid mics pick up sound in a much narrower beam in front of the mic, with greater rejection of sound from the sides. Some engineers use these mics for choirs, instruments, and even handheld vocals.
It can be helpful to select a wide coverage pattern mic for single-mic applications, and perhaps use a more narrow coverage pattern when using multiple mics or distance miking applications.
The guidelines above are listed in order of importance, so go through the setup and correct any obvious violations of these principles.
Sound quality will dramatically improve if the right mic is put in the right spot and the number of open mic channels is kept to a minimum.
And for specific tips for miking choirs, check out this post on the Great Church Sound blog.