Polar Pattern Power

Dec 1, 2009 12:00 PM, By Brian Heller

           
 

LEARN A TECHNIQUE THAT LETS YOU ADJUST A MIC'S DIRECTIONAL RESPONSE—EVEN AFTER THE SESSION IS OVER

The directivity of a microphone is a key variable in the art of capturing any sound. Because mic placement is among the hardest things to undo or fix in the mix, choosing the best directional pattern and placing the mic the right distance from the source for the particular situation is crucial. Although there is no substitute for making good mic choice and placement decisions, the two-mic technique outlined in this article gives you a lot more flexibility than a single mic would. By combining the signals from the two mics, you'll be able to change the polar pattern and vary it continuously, even after you've finished recording.

Under Pressure

FIG. 1: A bidirectional mic is fully pressure-gradient. A sound wave of high pressure at the front pushes the diaphragm, creating a positive voltage (a). A sound wave of high pressure at the back pushes the diaphragm the other direction, creating a negative voltage (b).

Before getting into the details, a little background is helpful. When describing the design of a microphone, there are a few different characteristics to consider. The most commonly mentioned are condenser and dynamic, which are descriptions of the way a mic gets its job of transducing air pressure to voltage done. But these transducer types don't describe the directionality of the mic. For that, I'll use the terms pressure and pressure-gradient.

Pressure mic describes how the omnidirectional polar pattern does its job. It basically functions like a very fast, very sensitive barometer, detecting the minute changes in air pressure that represent the sound around it. This happens by creating a sealed capsule with a flexible diaphragm on one end that is moved by the force of air-pressure changes. Think about it as if the mic capsule were an empty coffee can with a balloon stretched over one end. When the air pressure around the can changes, the balloon moves in or out in response to it. This movement is converted into changes in voltage. When a point of high pressure hits the diaphragm and pushes it in, a positive voltage is produced. When the pressure outside the sealed can is lower than that inside, the diaphragm pushes out and a negative voltage results. Just like any mono channel, a pressure mic doesn't know anything about which direction a sound is coming from, only that a pressure change happened somewhere.

A pressure-gradient design is quite different because the diaphragm is open at both ends, allowing sound waves to hit it from each side. It works not unlike the coffee can if it had both ends punched out and the balloon stretched in the middle of the container. If the diaphragm is pushed in from the front by arriving high pressure, the mic outputs a positive voltage. Once the low-pressure area of that arriving wave gets to the diaphragm, it gets pulled out, causing a negative voltage. If sound hits the back of the diaphragm, the reverse happens — high pressure on the back pushes it in towards the front and produces a negative voltage, and low pressure pulls it and produces a positive voltage (see Figs. 1a and 1b). If sounds are arriving at both the front and back in some combination (as usually happens in the real world), the diaphragm is moved more toward the direction of whichever one is stronger. So a pressure-gradient mic doesn't know which direction a sound came from, but it does know the difference between the level of sound at the front and back. If you didn't guess already, a pressure-gradient mic produces the bidirectional (figure-8) pattern you're probably familiar with.

These are the two basic directional designs used in most conventional microphones. It turns out that they can be combined in different ways to create the different polar patterns we know and love (although mic manufacturers typically use other techniques for that purpose). For example, the cardioid pattern is created by taking a pressure design and turning half of it into a pressure-gradient one by opening the back to incoming sounds.

Mix and Match

All this is background to help you better understand the recording technique I'll tell you about now. This method can be used to make some changes in the characteristics of a mic's sound after recording is complete. The basic idea is that if you use both a pressure mic and a pressure-gradient mic to record the same source to separate tracks, you can change the characteristics of the recording by combining the two mics' signals to different degrees. How can this possibly work? By using the simple and powerful phase relationships between the mics' polar responses.