How do you choose which microphone to use in a given situation? Can you look at the specs for a given mic and deduce the best way to use it? The short answer is no, you can’t. It would be great to look at the spec sheet for a new microphone and be able to tell from the specs how to best use that mic, but it doesn’t really work that way, and here’s why.
There are four main pickup patterns for microphones, but you can also look at these pickup patterns as a continuum that is omnidirectional at one end and hypercardioid at the other. Omnidirectional microphones pick up sound from all around them, in a 360-degree radius. A figure-8 (or bidirectional) pattern is equally sensitive directly in front of the mic and directly behind the mic, and there’s a complete null on the sides when you are off-axis, 90 degrees away from the front of the mic. A cardioid pattern means that the mic is less sensitive to sounds coming in off-axis from the diaphragm of the mic.
Looking at the representation of a cardioid pattern, you’ll see that there is a small area of greater sensitivity 180 degrees off-axis, directly behind the diaphragm. A hypercardioid microphone is more directional than a cardioid, and the area of greater sensitivity 180 degrees off-axis is larger. It’s a continuum, though, and dependent on the specific mic; some manufacturers even describe multi-pattern microphones as having five patterns rather than three: omnidirectional, wide cardioid, cardioid, narrow cardioid, and figure-8. Understanding the basic concepts is the key.
Most microphones have a published frequency response chart, but these aren’t as helpful as you might think when choosing a microphone for a specific task. While a frequency response graph that shows a broad boost above 10K or a dip around 400Hz will give you a general idea of what the mic might sound like, once you get to specific applications (a small-body fingerpicked acoustic versus a strummed dreadnought, or one soprano versus another), the broad overview of a frequency response chart is superseded by the details of a specific application.
Though directional microphones (cardioid, hypercardioid, and figure-8) are less sensitive to off-axis sounds than omni microphones, they do not completely reject sounds coming from the sides. In addition, the frequency response of the mic will change as you move off-axis, since some frequencies are attenuated more than others. While polar response graphs will show the microphone’s off-axis frequency response at selected frequencies, the actual effect of the off-axis coloration will depend on the room the mic is in. This off-axis coloration can have a dramatic effect on the overall sound of the microphone.
Here’s a hypothetical example. Assume that you have a cardioid mic with an on-axis frequency response that is ruler flat. At 45 degrees off-axis, we’ll assume that this mic is 6dB less sensitive overall, but that the frequency response is no longer flat; instead, it has a 3dB boost at 1kHz. What this means in the overall scheme of things is that off-axis, there is a pretty nasty high-mid peak. You can even carry this further — what happens if at 90 degrees off-axis, the mic is most sensitive to 500Hz? This would add another coloration.
No directional mic is free of off-axis coloration, though large-diaphragm mics seem to exhibit more of this phenomenon than small-diaphragm mics. It should also be noted that not all off-axis coloration is detrimental to the overall sound and usefulness of the mic; in fact, some coloration can be quite euphonic. You should, however, be aware that it exists, and that it is one of the many reasons you can’t choose a microphone for a particular application based solely on published specifications.
In addition to off-axis coloration, directional microphones also exhibit a proximity effect. As the mic is moved closer to the source, the bass response of the mic increases at a rate greater than the high-frequency response. To illustrate this point, take any directional mic, and talk into it from two feet away. After you decrease the gain so that the overall level is the same, talk into it from two inches. The difference should be noticeable.
While some manufacturers’ frequency response charts will have a second frequency response graph showing the low-frequency bump caused by the proximity effect, the amount of the increase is distance dependent. The proximity effect is often a very valuable thing, but you have to know about it and be able to compensate for it if needed, either by moving the microphone farther from the source or by using a highpass filter.
Summing it up
As you can see, a number of variables — from mic design and measurement to specification display — means that if you want to know how a microphone will work in a specific application, you really need to plug the mic in and try it yourself.