Microphone Polarity Patterns
As a home recordist considering microphone choices, remember sound quality is dependent not only on microphone construction but also on polarity pattern. Microphones pick up sounds in different ways, which are known as polarity patterns. Here’s how the various patterns work:
Omnidirectional mics can capture sounds all around them.
Cardioid (or directional) mics pick up sounds just in front of them.
Figure-8 (or bidirectional) mics pick up sounds from both the front and back.
The polarity patterns on microphones are represented on a chart that often comes with the microphone (or is part of its spec sheet). This chart is often called a polar graph, and the graph shows how well the microphone picks up various frequencies in front of or behind it.
The omnidirectional mic can pick up sounds coming from anywhere around it. Omnidirectional mics are useful for situations where you want to capture not only the source sound, but also the sound of the room that the source is coming from. You can find omnidirectional mics used in stereo pairs for drum overheads and groups of acoustic instruments, such as orchestras.
Omnidirectional mics are not generally used for close miking — when you place the mic less than a foot from the sound source — because they tend to catch too much background noise. You can see the pickup pattern of an omnidirectional mic in the following illustration. The round pattern shows that the mic picks up sound from all directions.
Cardioid microphones pick up the sound in front of them and reject sounds that come from behind. Cardioid mics are the most common types for live bands because you can control the sound that they pick up. If you have a cardioid mic on the tom-tom of a drum set, the mic picks up only the sound of that drum and not the sound from the other instruments around it.
Cardioid mics all produce more bass when they are close to the sound source. This is called the proximity effect. Essentially, the closer the mic is to its source, the more bass the mic picks up. You don’t find the proximity effect in omnidirectional or figure-8 mics. Many cardioid condenser mics have a bass roll-off switch that allows you to eliminate added bass that may occur from having the mic close to the source.
Figure-8 mics (also called bidirectional mics) pick up sound from both the front and back, but not all the way around. If you look at the following graph, you can see that sound is not effectively picked up from areas on the sides of the microphone.
Figure-8 mics are often used to record two instruments simultaneously. For example, you can place the microphone between two horn players with the side of the mic perpendicular to the players. This allows you to capture both instruments while eliminating any sound in front of the musicians.
Most figure-8 condenser mics have the same frequency response for both the front and back sides, but some ribbon mics produce very different responses, depending on whether the sound is coming from the front or the back. For instance, a Royer r121 mic picks up more high frequencies from the back side of the mic than the front.
You can use this to your advantage when recording an instrument. If the sound has too many low frequencies, just turn the mic around a little or a lot, depending on how many high frequencies you want to add.
Some condenser microphones can switch among various pickup patterns. These are generally large-diaphragm mics. These mics have a switch that allows you to choose from cardioid, omnidirectional, or figure-8.
These mics can do this because they generally contain two sets of diaphragms and backplates, which are positioned back to back. You may want to have at least one of these types of microphones around to give you more variety in microphone positions.