Ok, let's mic an electric guitar.
This is gonna be another very long post, so hang in there. I'll try and keep all the techie stuff to a minimum, but there are some concepts that are kinda hard to explain without getting a little technical, so ask questions if you don't understand something - it's probably just due to my poor explanation. Before we get into the miking part, we hafta talk about how speakers radiate sound, so here comes the first drawing:
Figure 1. Imagine a speaker suspended in space - the sound comes off the front of the cone, AND off the back of the cone, more or less equally. The problem with this kind of setup is that the low notes coming off the back side of the speaker cancel the low notes coming off the front of the speaker. Their wavelength is much bigger than the diameter of the speaker and they just go around the frame easily.
Figure 2. Now imagine we've mounted the speaker in the exact center of a huge board 40 feet wide by 40 feet tall. The speaker is still radiating in all directions, but unless a low note is at least 20 feet long, it ain't gonna get around the edge of that board easily. Since we eliminated the possibility of cancellations, the bass comes way up when your standing in front of the speaker, compared to the speaker that was just hanging there on a string. As far as we've concerned, it's now radiating into a hemisphere.
Figure 3. Now put the speaker down low on the board and imagine a floor has been added. What happens? The bass notes double in colume since they're now radiating into one half of a hemisphere. If you put the speaker at the juction of the floor and two walls (a corner), the bass would double again, since all the bass is now radiating into a quarter of a hemisphere. But what does this hafta do with miking an electric guitar? You're about to find out right now.
Figure 4. If we fold the board (shown in Figure 2.) into an open-backed box, we can still prevent a lot of the bass from wrapping around and cancelling out. Starting to get it? Bingo, you're basically looking at a side view of most open backed guitar cabinets, like a Fender Twin. The box prevents some of the low notes coming off the back of the speaker cone from getting around to the front and interferring with the notes coming off the front of the speaker. This arrangement works ok till you get down to around 90 - 120 Hz, and below, right at the nottom end range of a guitar. So how do we get a little more bottom end?
Figure 5. Make the box a little bigger and seal it completely. Now the back notes can't interfere. Recognize the design? A Marshall cabinet? Right!!!
Figure 6. As long as we've come this far, I threw this in. You take the sealed box, cut a hole in it, and then you can tune the air in the cabinet to create a "blowing across a Coke bottle" effect, to add some bottom where the speaker starts to give out.
Keep some of this in mind when I start this next section:
Miking the guitar cabinet.
Guitar amps come in many different configurations, but I'm gonna focus on miking the three most popular speaker designs:
Open back cabinet, single speaker.
Open back cabinet, dual speakers
Closed back cabinet, with 4 speakers.
Here comes another one of those damn drawings:
Figure 1. The two-12" open back speaker combo is one of the most popular units of all time. There are 4 basic mic positions, with several variations:
1. Stick a mic right into the speaker, aimed at the center of the cone. Maximum high end, and least outside noise.
2. Stick a mic right into the speaker, aimed at the edge of the cone. Less high end, and a little more bass.
3. Pull back a bit (12 to 24") and aim a mic right between the speakers. More realistic, but inreased chance of phasing problems and more susceptable to room noise.
4. Use any of the first 3 methods and add a mic aimed at the back of the speaker. Try the phase switch and choose the position that sounds best to you.
What mic to use?
Try
the Shure SM-57, or your kickdrum mic, or any good dynamic for positions 1 and 2. Positions 3 and 4 might use a ribbon or condenser mic to get a little fatter sound. I usually start with position 1 (one mic, pointed into the center of the cone), but I may add something like an AKG D122 on the outside edge of the other speaker to emphesize the bottom end a little.
Figure 2. The single speaker open back speaker cabinet is another popular design. The are same 4 basic mic positions are used:
1. Stick a mic right into the speaker, aimed at the center of the cone. Maximum high end, and least outside noise.
2. Stick a mic right into the speaker, aimed at the edge of the cone. Less high end, and a little more bass.
3. Pull back a bit (12 to 24") and aim a mic at the speaker. More realistic, but inreased chance of phasing problems and more susceptable to room noise.
4. Use any of the first 3 methods and add a mic aimed at the back of the speaker. Try the phase switch and choose the position that sounds best to you.
5. Repeat all 4 mic techniques, but put the amp on a bar stool or chair. Why? Go back to the very first drawing and look at Figure 3. By raising the amp, it now feeds into a hemisphere instead of a half hemisphere, lowering the bottom end a little. Pull the amp away from a wall for less bass, in closer to the wall for more bass. See how the first drawing is starting to fit in?
What mic to use?
Try
the Shure SM-57, or your kickdrum mic, or any good dynamic for positions 1 or 2. Positions 3 and 4 might use a ribbon or condenser mic to get a little fatter sound. I usually start with position 1 (one mic, pointed into the center of the cone), but I may slide it till it's at the outside edge of the speaker to emphesize the bottom end a little.
Figure 3. is simply there to use up some space. I just thought it looked better with 6 drawings instead of 5.
Figure 4. is a standard 4x12 Marshall cabinet. You would use mic positions 1 and 2 for adjusting the high end relative to the bottom end (and remember, you're getting that 1/2 hemisphere bass boost from the floor). To lower some of the bottom end, move the mics to positions 3 and 4 (or try a 57 at position 3 AND a D112 at position 2, then blend them to one track, or record them wide apart to two tracks).
Figure 5. Marshall cabinet with distant miking. Try a ribbon mic, or a big condenser mic to get a fuller sound. Adjust the mic anywhere from about 2 to 10 feet away. If needed, also use one of the mic techniques in Figure 4.
Figure 6. Actually this one is for any cabinet. Scenario: The guitar player isn't happy with any of the mic setups you've tried so far. Have the guitar player play with the controls till he's happy with the sound. Tell him to freeze, right there. Put a mic close to his ear, pointed at the center of the cabinet, and go back and listen. Either an omni, small cardioid (dynamic or condenser), or a large cardioid will usually do fine. The mic is now hearing "exactly" what the guitar player heard in the room when he said he liked the sound. That should end any conflict.
Hey, we're nearing the end of this whole mess - just a few more things to clean up, and then we're done!!!
Oh well, I hope these definitions help anyway.
