One mic to hear them all… if only that were possible. Area miking is an art, but also a curse in sound reinforcement circles. A choir, an orchestra and the occasional bluegrass group who saw it on O Brother, Where Art Thou? Not a good idea. But moving to the topic at hand, here are some things to think about when miking a group.
Your Weapon of Choice
The polar patterns we all know are: omnidirectional, cardioid, supercardioid, hypercardioid and bidirectional. Starting with cardioids, the SM58 still remains one of the best go-to mics when the program director walks up to you and says “By the way, we have the elementary chorus doing the Pledge of Allegiance and we need to make sure that the parents can hear.” So throw four of those on stands and away you go.
Supercardioid mics, either in the form of a shotgun mic for video or as a boundary floor microphone — such as a PZM — are great in the right circumstances. A PZM can be ideal if you cannot place a solo mic on a stand out in front due to space restrictions or choreography. PZMs placed on the floor and spaced about every 15 feet can be astounding if you can have the soloist stand relatively in front of it. Once choreography is involved, foot noise can be a problem, but just roll the high-pass filters up as much as possible without your bass sounding like a tenor, and you are good to go.
Working with a string quartet, a boundary mic can be perfect to set on the floor in the center as they “half-circle” around and can control their dynamics — that is, if you have only one microphone available and aren’t doing a live recording. Alternatively, choose an omni mic — placed on a low solid-based stand in the middle — and let them go to town on that Stravinsky piece.
Hypercardioid mics, such as a pencil condensers, may have less 180° rear rejection, but have a greater side null in their pickup pattern, which can be great for overhead miking of orchestras from 12 feet in the air. This can let you pick up the soloist in the first chair while having the ability to back off the fader and blend it back into the overall natural room ambience.
In the end, you’re choosing a mic to achieve a relatively coherent and balanced sound. Your distance-miking will either require you to choose a sensitive mic for your low input signal or simply a model that has the largest frequency range to capture every nuance of the material being performed.
There are three common techniques for miking a group. Multi-miking can be tough, because you have to deal with balancing and creating a natural sound. You can also try the “X-Y” method, where you have two directional mics with the capsules close together and angled them out at the same degree from the center line. This is great for capturing a stereo image for recording, but when doing live reinforcement, placing pairs or single cardioids above every instrument duo or section (such as in a big band) will reproduce a nice, even coverage.
The 3-to-1 Rule
This little axiom states that, when using multiple microphones, the distance that you have placed between your microphones should be three times greater than the distance between your microphone and the source. This is to help minimize phase cancellation from sounds arriving at the microphones at slightly different times when they’re placed different distances from the same source. You follow the 3:1 rule so that you can decrease the phase cancellation to an audible difference of around 1dB; otherwise, you definitely can notice what is missing from your source signal when reproduced. Note that the 3-to-1 rule applies equally whether mics are stand-mounted or hung overhead. Fig. 1 shows a correct application of the 3-to-1 rule on a chorus — you can see how the distance between the two mics is approximately three times the distance of the mics to the source.
Inverse-Square Law
Sound reinforcement is all about converting the acoustical energy we receive and transferring it into electrical output and then back to acoustical energy. Unfortunately, the farther the mic is from a source, the lower the incoming signal we receive at the console, and the less we are able to reproduce. The Inverse-Square law says the same things, but states that when the distance from a source doubles, the signal decreases by 6 dB.
So when you’re asked to place the microphones farther from the choir or up higher from the cello section in the orchestra because they will get in the shot of tomorrow’s press release, it’s up to you to explain that to the photographer. Good luck.
Sometimes, You Can’t Win
So you’ve picked the mics that are perfect for the type of material you are trying to reproduce, you have placed them accordingly following the 3-to-1 rule and kept the Inverse Square Law in mind. You even positioned your speakers so your mics are out of the line of fire to reduce feedback. Good for you! The show starts, the group begins to perform and… Huh? What are they saying? Why am I having to ride my fader at +12 dBu and why is it so thin? I hate to say it, but even if you follow all of the rules, you will still run into situations where it just does not matter one bit.
As an example, let’s say the annual production of the local Men’s Chorus is upon you. Bless these guys for trying, but “professional performers” is just not the phrase for them. But you do what you can to make them sound like rock stars. Then, during rehearsal, 46 guys step on stage, the piano begins with its accompanying piece and… Squeek squeek squeek, baaaaah wooosh, chirp chirp. If you have been there yourself, then you know I’m being kind. The chorus may be plentiful as a unit, but the support for dynamic range and energy is still waiting to get on the train and run you over. Your head drops in a woeful sigh and you place one microphone per vocal section in a stand in front of the risers — a total of four mics on this chorus, and then you hope for the best.
The piano begins again, the chorus goes for the gusto and begin to build and build and then… the bass player and drummer come in to round out the jazz trio of backline musicians. Wait for it, wait for it. The director turns to you and, in a mild mannered voice, says “The choir cannot hear themselves, so will you please place monitors in front of the risers.” AAAAbbbbbsssoooolutely! Just fake a smile and begin the “give and take” implementation. Now you have to place the monitors close enough to the risers so that the chorus can hear, but not too close so that the entire section or sections can hear without pulling out four separate monitors. The microphone must now be moved back and placed behind the monitors so that they’re not pointing right to the tenor’s forehead, yet will still pickup the group.
Now the distance of the microphones breaks the 3-to-1 rule, and you have gaps in your reinforcement, so you must place another mic or two on stands to cover the sections, but you are not beginning to hear what comb filtering with microphones is really like. God help you if they actually happen to have a band shell behind the risers, because now you have monitors that are cranked because “we still can’t hear ourselves” is coming every 30 seconds, and you are already dealing with a ridiculously low input level. Oh yeah, the drums are overpowering the Yamaha ES8 keyboard that is coming out of the Roland amplifier, so now you have to throw a DI in line of the cab and put a little bit into the wedges. That signal is so much more than what you are getting from the choir, so you have to “crack” it into the monitors.
Next thing you know, you are the composer of the entire show, because you ended up having to also put the bass and snare into the monitors so that the choir can keep a beat. Which also resulted in placing more boundary microphones in front of the choir and brings your total up to eight microphones on stands for the choir, one DI on the keys, one on the bass and a snare mic. What happened to natural acoustic experience of a choir concert? So basically you now mix to taste and have a beer when it’s all over and give yourself a victory pat because you managed to work against laws of physics for two and a half hours. Being on your toes every second of every breath, literally.
Evan Hooton is Blog Master for www.ProAudioSpace.com
