Did you know that one can often tell just by looking at a mic if it's an omnidirectional mic, or something more directional? All mics are open at the front of the mic, what's known as zero degrees on-axis. A directional mic (e.g., cardioid, supercardioid, hypercardioid) achieves its pickup pattern by allowing the sound energy to get to the back of the diaphragm as well as to the front. Sound energy arriving at the back of the diaphragm travels through the rear entry ports to what has been called an acoustical phase shift network. In other words, it takes longer for that sound wave arriving from the front of the mic to get to the back of the diaphgram.

When that condition exists, the sound energy pressing on the front of the diaphragm and the sound energy arriving at the back of the diaphragm actually serve to reinforce one another. However, when the sound arrives from some point off-axis, like from the back of the microphone, the sound energy arriving at the front and the back of the diaphragm work to cancel each other. When that condition exists, the mic will put out less output from sounds arriving off-axis than from sounds arriving on-axis.

So it should come as no surprise that one can see those rear entry ports on the housing of the microphone, and here are a couple of photos to illustrate. The photo on the left is the CK62-ULS omnidirectional capsule from AKG. Notice that there is no way for the sound energy to reach the back of the diaphragm. All sound picked up by the mic has to hit the diaphragm from the front. Note the small circle silkscreened onto the side of the mic, indicating that it has an omnidirectional pattern.

By comparison, the photo on the right is the AKG CK61-ULS, the cardioid version of the capsule. The rear entry points are obvious, and to underscore the point the manufacturer has the cardioid pattern silkscreened onto the side. Both capsules are interchangeable heads intended to attach to one of the mic preamps from the ULS series of microphones, which is an absolutely great sounding series of microphones.

OMNI CAPSULE	CARDIOID CAPSULE

So take a look at the microphones you are currently using. You probably already know what kind of polar pattern the mic is made with, but now you can know with a greater degree of certainty if the mic you're holding is an omni or a cardioid just by looking at it. Well, actually, you really can't tell what type of directional pattern it has (cardioid, supercardioid, hypercardioid) without looking closer for the silkscreened pattern or digging out the manual. But this little party trick still helps.

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