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November 12, 2003
Stuff About Microphones
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.
My guess (as a bass player) is that if you put either of your mics up next to the bass cabinet the amount of extraneous room noise you pick up will be minimal, as the sound from the cab will be so much louder.
It’s more of a problem when you’re trying to record other instruments in the same room as an amplified bass – because lower frequencies tend to be omnidirectional in their proliferation they bleed through to everything else.
Also note that frequency response in directional mics varies with the distance of the sound source from the mic – this usually gets noted as a mid-bass presence boost as you get very close to cardioid mics like your shure – so you’ll hear sounds as much “warmer” when you hold the mic close to your mouth or your instrument.
Interesting tidbit: SM57s are the mics that are used on the president’s podium.
Dude. You should record some bass guitar with the AKG at least once. I’ve gotten very good results recording isolated bass instruments with omnidirectional mics (at a medium distance from the source). In addition to having a different frequency responses at different distances like Oren mentioned (proximity effect), directional microphones like the SM57 have different frequecy responces at different angles of incidence. A sound coming from the side isn’t just uniformly quieter than a sound from the front; it is more quiet at higher frequencies, resulting in a muffled tone for sounds from the sides. So, if you want a more accurate frequency response (and can afford to isolate outside noises in the ‘good-sounding’ room), omnidirectional mics are generally a good bet.
(Excellent post, scott. First-time reader.)
You will always see some sort of “presence rise” in a vocal mic. The bumps you see in both mics at or near the 4-13khz range increase vocal presence and “air,” a desirable colouration to increase the way a vocal sits in a mix, either live or studio. Ask your instructor the true difference between omni and unidirectional. I believe these correlate with the polar pattern. Shure calls the SM57 “unidirectional” and it is a cardiod pattern. AKG also calls their C2000B cardiod with no reference to omni or unidirectional. I think hypercardiod is more unidirectional than cardiod. I believe they are used in a vague sense. In fact, here is a nice link: http://www.record-producer.com/learn/technology_sound_microphones_polar_patterns.cfm. At any rate, certainly use your AKG200b more liberally. Your bass cab is a great place for it. As you said, a condenser is more sensitive. It’s also more accurate and alive and more natural than a dynamic mic. That being said, engineers around the world have used sm57s for everything. Sometimes a more sensitive, accurate mic is a bad thing, revealing other inaccuracies, i.e. poor preamp, poor mic placement,as you mentioned maybe poor room acoustics. There are no rules. I am surprised you are getting poor results with the akg on vox. Try some placement changes, maybe use the -10db pad. I would avoid the bass rolloff, I never understood why the c2000b rolls off at 500hz? Most mics and mixers rolloff at 80hz or 75hz. 500hz is way too high for vox among other sources. That being said, I captured a terrible vocal performance my first time out with sonar because I was holding back. Why? My CAD M177 (condenser vocal mic) was set too sensitively, I couldn’t sing out the way I wanted to. There are no rules. Just trials and results! You’ve got some great mixes, so don’t experiment too much! What a fun art this is! Take care!
One thing you have to keep a careful eye on when miking up a guitar amp/speaker cabinet, or anything else with an SM57 is, the pronounced proximity effect which occurs when you position a 57 right againt the grill, or very close to the source whatever it may be. The published frequency plot, which you have posted here on your site does not take this proximity effect into account. While there is some really good stuff happening, the proximity effect is also boosting some undesirable low-mid mud. 300hz for instance. The closer you are to the speaker, the greater the amount of this boost. As you move the mic further away from the speaker you will hear this effect dissipate more and more. It is often said that microphones, and microphone choice are a form Eq, and this demonstrates that point clearly. Not only is the tone of the microphone a form of Eq, so is how you choose to position it in relation to the source.
This relates in much the same way as boost/cut moves made with an outboard or software equalizer. The proximity effect is a great tool if you spend the required time learning how, and when to use it. Alot of people will tell you to just cram a 57 up against the grill cloth of an amp. While this may very well be a truth, so is the fact that the proximity effect must be controlled through positioning or equalization. In a way telling people to just stick a 57 up on the grill is a bit misleading. This leads to muddy tracks, and the loss of note definition. The SM57 is an outstanding microphone. Just like any instrument though, you have to spend the time to get to know how to fully use it’s strengths, as well as negate it’s weaknesses.