Any mic can favor sound from a particular direction. Some pick up sound mainly from in front and back while others pick up sound equally from any direction.
An illustration that shows the directionality of a microphone is known as a polar response chart or polar pattern, and can help us understand how we can use each mic to it’s best advantage. The most simple microphone polar pattern is known as omnidirectional. This means the microphone captures sound with equal strength from any direction. Here’s the polar pattern of an omnidirectional mic:
This chart is a graph and the mic is at the center of the circle. When a sound is at “zero degrees” it is directly in front of the microphone and is referred to as being “on axis.” When sound enters a mic from anywhere else except directly in front we say it is “off-axis”. A sound coming from directly behind the mic is 180 degrees off-axis. The bold circle in figure 1 shows the directions from which the mic is picking up sound. A perfectly omnidirectional microphone would have a polar response that is a perfect circle.
Notice that there are additional traces (dotted and dashed) that are not quite round. This shows the pickup at high frequencies, 5 kHz and 8 kHz. Even when using the best omnidirectional microphones there is some loss of high frequency sound when that sound comes from directly behind the mic.
All polar patterns are actually three dimensional — difficult to show on paper. So the omni pattern is really more like a globe or sphere than a flat circle as shown below.
Depending on what you are trying to record, an omnidirectional mic can be a big help or a royal pain in the neck. For example, if you are trying to capture the audience noise, then an omni is a good thing because it lets you capture a wide area of the crowd. This avoids picking up the voices of only three or four people.
However if you are singing in a Band, an omnidirectional mic may pick up more than just your voice. What you need is some kind of directional mic.
A directional mic is one which favors sound coming from a particular direction. It might favor the front…or the front and sides…or possibly even pick up the front and rear but reject the sides. Let’s take a look at a cardioid pickup pattern (the name comes from the heart shape of the pattern).
Obviously, this pattern is not completely round. At 0 degrees this microphone will pick up sound just fine. But as the sound source moves off to the sides, the microphone rejects it until — when the sound is directly behind the mic — the microphone does not even hear the sound! This makes a cardioid mic good at rejecting noise from audience members. Sometimes cardioid is called “unidirectional” but this really is not accurate since “uni” means one and there is no mic in the Expanded Universe which picks up sound from only one direction. However, these pickup patterns are theoretical models that do not necessarily hold true in practical applications.
Also, microphones that share the same pickup pattern typically do not share the same rejection abilities. In other words, one brand/model of cardioid microphone may be better at rejecting sound from the rear than another brand/model of cardioid microphone.
A microphone with a cardioid pickup pattern can be very helpful in a variety of situations. When performing on stage you might have several monitors and it’s critical that sound from the monitor doesn’t reach the microphone or you’ll get feedback. Instances of feedback can be reduced (though rarely eliminated) by using a cardioid microphone and placing the stage monitor in a spot where the mic does not really hear sound. Specifically the monitor should be placed directly in front of the performer. Since the performer is directly in front of the mic, the monitor is now directly behind the mic and this is the area in which a cardioid mic rejects sound.
The last of the “big three” microphone pickup patterns is called the bidirectional or figure eight pickup pattern. This type of microphone picks up sound from in front of and behind the mic, but will not pick up sound from the sides (Figure 4). The pickup pattern looks like two globes, one in front and the other in back of the microphone. Sound is picked up at 0 degrees and 180 degrees but is rejected at 90 degrees and 270 degrees.
On most bidirectional mics there will be some kind of marking on the mic’s body to show you where the front is.
A bidirectional mic can be very useful in certain situations. A great application for a bidirectional mic is when two singers or interview subjects need to share a single microphone. Unwanted noise from the sides will be rejected so I’d be able to keep Joh Yowz’s voice separated from the girls’ mic.
There are also two specialized variations of the cardioid pickup pattern: supercardioid and hypercardioid. Although they look similar there are a few subtle differences.
The hypercardioid pattern.
The supercardioid pattern.
Both have a narrower angle of acceptance in front than the cardioid pattern and both have a small lobe of pickup in the rear. Upon close examination, you can see that the hypercardioid is slightly tighter (even narrower) in front.
The areas of maximum rejection for the two patterns are roughly the same, but not identical: approximately 130 and 230 degrees off-axis for hypercardioid, and roughly 120 and 240 (shown as 120 degrees in the opposite direction) degrees off-axis for supercardioid.
As a result, hypercardioid rejects sound from the sides a bit better than supercardioid but — depending on frequency — may be a little more sensitive to sound from directly behind.
The stage monitors must be placed differently when using supercardioid or hypercardioid mics (rather than a cardioid mic) on stage for vocals. A monitor is placed behind the mic (as one would do with a microphone with a cardioid pattern) it will most like result in feedback.For the most feedback resistance, the monitors should be placed in front and slightly to the sides of the performer in the “nulls” of the pickup pattern.
It is very important to realize that the physical shape of a microphone has almost nothing to do with the mic’s pickup pattern. There are mics with a round heads that are cardioid or hypercardioid and there are square mics which are omnidirectional. The only way to be sure is to either look on the mic for a pattern drawing which may or may not be present, or check the specs from the user’s guide, the company’s website or other resource. Another thing to keep in mind is that directionality decreases as frequency gets lower.
A cardioid mic may reject a voice from the back of the mic,
but it might not reject a low frequency sound (like from a bass guitar) from the same direction. This is because low frequency sounds bend around objects very easily. Some condenser mics such as the Neumann U87 or AKG C414 have switches which change the polar pattern of the mic, making them extremely versatile. But, this comes with a price tag; there aren’t many quality microphones under about $400 with switched patterns.
Some mics have interchangeable heads or capsules with different pickup patterns, allowing you to buy one microphone body and then buy only the capsules with the pickup patterns you need — usually at a more modest cost. “Modest” means about $300 for the body and another $300 for each of the heads. Hey, no one said quality audio was cheap!! – Darth