Choose Your Tool Wisely: Mics, Directions and Reading the Poles
Date: February 7, 2014
I attended a concert recently from a large regional choir. As often happens with large ensembles and performance halls, a single solo mic was brought down front for one piece. When it came time for the soloist, though, it was not one singer that approached the mic but a trio. Even before the trio sung their first notes I knew what the result would be: predictably, all the microphone picked up was the singer in the middle. It's a common mistake repeated throughout the choral world: a misunderstanding of how mics work and how best to use them. Thankfully, it's an easy issue to address. To understand the issue fully, let's take a brief refresher on microphone science.
The Cone of Silence?
I often hear two misconceptions from musicians and conductors about microphones: either that mics "work in a straight line, and whatever is in front of them is what they'll pick up, or that mics "have a cone" (this is often explained with the person making a pie wedge shape with their hands), and that this is what the mics will receive. The theory goes that if the musicians are within "a certain range" of the mic, they'll all be picked up equally. The cone theory usually coincidentally matches the physical dimensions of the mic in question, as if mics were some kind of soundproof scoop which collected anything with their arc. The truth is only slightly more complicated, but much more useful. All mics have something called a polar chart which explains their behavior.
In this article I'm primarily referring to the Shure SM58, since it's one of the most common vocal mics in the world, and either it or a competitor/knockoff are the biggest offenders in improper mic usage in my experience. This is not a good choral microphone-- it is meant for pop lead vocals. Their website says it all: "Applications-Kareoke, Harmonica, Live Vocals." Unless you have a stage/jazz choir where every member is miked individually, do not go buy this mic for your choir.
(SM58 Polar Pattern, from Shure.com)
This chart is telling us two useful things: first, the dotted lines are showing us how the mic treats different frequencies (in Hz). Second, the mic is showing us how sensitive it is at different locations relative to the microphone head. Imagine that the "face" of the microphone is in the center of the chart, and a musician standing directly in front of the mic is at "0." Someone directly behind the microphone is at "180." Remember as you read this that the concentric circles are not distance from the mic (this is a common mistake when one starts visualizing microphone arrangements), but that they refer to the strength of the sound in dB. Let's look at an example: analyzing the frequencies present in the human voice is a whole different series of posts, but let's choose a generic frequency of 500 Hz on the left-hand graph. Someone standing in front of the mic registers at a certain sound level, while another musician standing next to the mic producing the same amount of sound and the same distance away will generate -5 dB comparatively.
-5 dB Doesn't Sound Like Much
Second common mistake when talking about microphones: Many people think of dB as some kind of volume scale. Without turning it into an advanced Math and Physics lecture, dB is a logarithmic scale of relative sound levels. That means that it scales very quickly. Cutting the sound level in half roughly translates to -3 dB. 10 dB translates to multiplying a sound level by factor of 10. So in other words, cutting a sound level by -5 dB represents well over a 50% reduction in the energy picked up by the mic in this scenario. Even putting a second musician at 60 degrees off of center on the mic eyeballs to roughly -3 dB on that chart, or a 50% reduction.
Solo Mics Are Not Group Mics
Microphones like the SM58 are simply not designed to pick up sound from multiple sources. The fact that all frequencies start curving in on the chart almost as soon as you leave 0 indicates that anyone not standing directly in front of the mic will suffer greatly in sound. This is actually an advantage of the SM58 in the venues for which it was designed: Live pop vocals. A microphone which drops off sharply from 0 will pick up less ambient sound from drums and other instruments on a small stage. Great application, but the wrong application for us.
Polar patterns are easy to find for every kind of microphone. In fact, some mics even have a representation of the polar pattern printed on the mic itself! Look at the mics in your sound system or auditorium. If you have a bucket full of mics with patterns similar to the SM58, you should at least plan on having one set up for every member of a small breakout ensemble rather than one or two "solo" mics for a trio or quartet. Thankfully, there are many mics which are much more forgiving about space. If you are programming pieces with breakout ensembles and you know that they will need to be miked, you should plan ahead to find, borrow or purchase a microphone with a wider polar pattern. Talk to your local sound shop or in the forums here to get recommendations on good mics for your application.