Karl Winkler talks about the best mics to use to capture the sound of amplified instruments and how to position them.

Since most of us have amplifiers on stage when performing, getting the best balance in the house often means that we have to mic the amps. This is a very different challenge than miking acoustic instruments, and the challenge grows even greater when some of the instruments are acoustic, but are also amplified. The basic rules for microphone technique still apply, but it's good to consider the specific differences between how amplifiers create sound vs. how instruments and voices create sound.

There is a common myth about using "good" microphones with amplifiers, in that many people are afraid to do so.Background of SoundFirst of all, there is a common myth about using "good" microphones with amplifiers, in that many people are afraid to do so. "Won't my Marshall stack destroy that large diaphragm condenser microphone?" is a question often heard from engineers, mic owners, and artists alike. And the answer is "no." Unless you have a vintage, pre-1960 microphone, you should not be concerned about damaging the mic with a high SPL (Sound pressure level, i.e., loudness). Part of this misunderstanding is that guitar, keyboard, and bass amplifiers sound very loud to our ears. And while they routinely put out SPLs in excess of 100 dB, most good microphones can handle 120, 130, or even up to 150 dB SPL before distortion begins to show in the output signal. But just because you hear distortion in the microphone, this does not mean that you are damaging it!

All instrument amplifiers use a speaker system to create sound from the amplified signal. For some instruments (guitar in particular), a single speaker may be enough. Sometimes, multiple drivers (speakers) of the same type are used in an amplifier, with excellent results (as in a "stack" type setup). For instruments with a wider frequency range, however, "multi-way" systems, consisting of speakers designed to handle different parts of the sound spectrum (highs, mids and lows, for instance) will usually provide better results. Bass and keyboards benefit from wider-range systems due to the nature of the sounds they produce.

Guitar

The guitar produces a relatively narrow spectrum of sound, and therefore most guitar amplifiers and speaker cabinets will have only one type of speaker. Basically, this speaker is designed to reproduce "low-mid" and "mid" frequency sounds. To do this, a speaker with a diameter of between eight and twelve inches is usually optimal. Small practice amps may have a six or eight inch speaker, while most "serious" or pro rigs use twelve-inch speakers. So, if the speaker is twelve inches across, where do you put the microphone? This is the crux of the question, and the answer is fairly simple.

The mic should go where the sound is best. Unfortunately, finding this spot is not always easy. If at all possible, while the guitarist is playing, have headphones hooked up from the microphone output and move the mic around the front of the amp to find where the "character" of the sound is how you want it. If you're the guitarist, have someone else move the mic around until the "best spot" is found. Most often, this spot is about halfway between the center of the speaker and the edge.If you have a situation where the guitar is routed directly to the PA mixer along with the amp being miked, the main concern is "phase." If the sound seems hollow or "swishy," you may need to do one of the following: 1) Move the microphone around until the sound is more "solid" or 2) engage the "phase reverse" or "polarity" button on your mixer or PA head. If neither of these methods seems to help, the best solution is to only use one of the methods for picking up the guitar. It may not have that "magic" sound you are after, but it will be better than the hollow, strange sounding version with two inputs.

Bass

Bass is similar to guitar in many ways except for range. The bass produces a much lower sound, and also some sounds that are fairly high in the frequency spectrum. The result of this is that no single speaker type or size will reproduce the entire range. Often, the solution is to use a multi-way system, combining a fifteen- or eighteen-inch speaker (to reproduce lows and low mids) with a small horn (to reproduce high mids and highs). Bassists that slap, or those that want to have a "brilliant" sound may choose this method of amplification. Now the challenge of placing your microphone becomes much greater, because there are now two sound sources, one from each of the "drivers" in the speaker cabinet. If you can only use one microphone, the best thing to do is back it away from the amplifier a few inches, and look for that "sweet spot" just as you did with the guitar amp. However, if you can use two microphones, you will have the luxury of fine-tuning the sound to a far greater degree, by placing each mic in the best spot for each of the speaker types.

As when using combined sources with the guitar, phase may also be an issue. Use the same method mentioned for guitar to solve this problem.

Keyboards

Keyboard amps are like the more sophisticated bass amps in that they will usually have a multi-way system including a large (usually fifteen-inch) driver for bass and low mids along with a horn for mids and highs. Just like with the multi-way bass setup, you may need to experiment a bit to get the best placement for your mics. And again the phase issue may come up. Use the solution described above. Multiplicity Amplifiers with more than one speaker of the same size may cause confusion, but the solution is easy. You'll only need to mic one of those speakers. For instance, a 4x12 guitar stack has four twelve-inch drivers in it, but you'll only need to place one microphone on one of those drivers to get an excellent sound. The key is to find the best place for that one microphone. Mic Types Generally, good-quality dynamic microphones will give you excellent results from instrument amps.

They are inexpensive, robust, and can sound very good. In fact, dynamic mics can be more "forgiving," making placement will be easier. However, if you are looking for more "sparkle" from the sound or want to capture a truer picture of how the instrument/amp combination sounds, then you may want to consider a condenser microphone. If you have access to both dynamic and condenser mics, then by all means experiment with both to see how they work on your setup. Most often, mics with a cardioid or supercardioid polar response will work best, due to the fact that they block out sound from the rear. This is good especially when you have a drummer or other loud instruments on the stage. Be aware, though, that when you get very close to an amplifier with a cardioid or supercardioid mic, that the bass frequencies will build up. This is known as the "proximity effect." The effect can be useful for boosting weak lows, or getting a really "huge" sound, but too much low end can also sound muddy. Most mixers and PA heads allow you to cut the lows, so use your ears to determine if this is needed. Final MixAll in all, your ears should guide you as to what sounds best. Experiment, and you'll likely find an excellent combination of placement and mic type to give you the best sound. Good luck!