I own exactly two microphones: a Shure SM57 and an AKG C2000B. They’re both considered to be good, inexpensive mics, but I never knew much about them until recently. Below are some basic microphone notes for the beginner, cribbed from my recording class. I suppose if I had stayed awake during Physics class in high school, I’d know all this already.

Direction

Most microphones come in two standard flavors: unidirectional and omnidirectional. A unidirectional mic is designed to only hear what it’s pointed at. Omnidirectional mics are designed to pick up sound in all directions, and can be used to capture the more ambient qualities of a sound. If you’re miking a drum kit and want to record only the snare on a single track, you’d use a unidirectional mic like the Shure. If you want to capture the natural reverb of a room or a gang vocal, you’d use an omnidirectional mic like the AKG.

If you want to get into the gritty details of polar patterns, look for words like cardioid, hyper-cardioid, and figure eight. These describe (usually with a diagram) where the mic “hears” best. A cardioid (heart-shaped) pattern indicates that the mic is most sensitive in front, but tapers off to nothing at the opposite side (where the notch of the heart pattern is). A figure eight patterned mic hears equally well on two opposite sides, but not the adjacent sides. You get the idea.

Dynamic vs. Condenser

Microphones are also either dynamic or condenser type by design. The difference is mostly due to boring electronics stuff having to do with gold plates vs. copper coils, but here are the things to remember:

Condenser mics are extremely sensitive and good for capturing quieter instruments, like a softly plucked guitar or a breathy vocal. They also do a good job of capturing room acoustics, like the echo you hear bouncing off the ceramic tiles when you sing in the shower. They are also quite fragile and require a phantom power source. My AKG is a condenser mic.

Dynamic mics like my SM57, while not as sensitive, are rugged and better designed for loud sources, like a Marshall stack.

Frequency Response

And now a bit about how microphones “hear” stuff.

In general, the human ear can only detect frequencies between 20 Hz and 20,000 Hz (or 20 KHz). People who can easily hear the natural tones produced by this range of frequencies are considered to have perfect hearing. Thus, if you want to capture the natural sound of an instrument or vocal, you should choose a microphone that “hears” at least as well as the human ear does.

How well a microphone captures this range of frequencies is its frequency response. Ideally, the frequency response of a microphone should be “flat” — meaning that the microphone “hears” all frequencies equally well.

Armed with this knowlege, I went home, dug through my file cabinet and found the tech spec sheets for the two microphones I own. Each sheet includes a graph that plots the frequency response for that mic. Frequencies are shown along the horizontal axis, from the 20 Hz low end to the 20 KHz high end. The amount of “response” is shown along the vertical axis, in decibles.

Here’s the graph for the Shure SM57:

Now that I know how to interpret these graphs, I was surprised to see how the SM57 responds to the higher frequencies. Right around 3 KHz, the volume severely increases, peaking out at 6.5 KHz at around +6 decibles. Also interesting is the way the response “rolls off” at lower frequencies starting right around 200 Hz, dropping to -10 dB by the time we reach 50 Hz.

Based on this graph, I now know that the Shure SM57 responds more acutely to high frequencies while dropping the lower ones. This does not mean that the SM57 is a bad mic; it just means that I can expect to hear more treble and midrange tones in a sound recorded with it.

Now look at the graph for the AKG:

The AKG’s response is nice and flat for the most part. There’s still a boost up in the high frequencies, but unlike the Shure, the curve is gentle and starts around 6 KHz. Also notice how the AKG hears a lot more of the bass end than the Shure, with the dropoff occuring around 90 Hz. Even at 50 Hz, the response is only at -6 dB, compared to the Shure’s -10 dB response. (The AKG also has a “low cut” switch; the dotted line shows the frequency response with the switch activated.)

Even though the AKG has a great low-end response, I wouldn’t use it to record electric bass. Why? Because the AKG is omnidirectional — it would pick up more ambient room noise and speaker hiss than the Shure. You can buy good unidirectional condenser mics.

The lesson to be learned here is that anything that captures or reproduces sound — that is, microphones and speakers — has a frequency response which can alter the color and shape of the sound as it is being recorded or listened to. That’s why it’s important to know what kind of frequency response your mics are bringing to the table.

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