When it comes to choosing a microphone for the guitar-related job at hand, there's a few considerations to think about. Not every mic fits every situation'"especially when recording things like acoustics. Let's take a look at a few simple basic guidelines that may help you pick the right tool next time around.
To begin with, most microphones do not record perfectly flat. In other words, when you put an SM57 in front of an acoustic, it's not going to play back through the speakers exactly what that guitar sounds like. In other words, the mic is '˜colored', leaning towards certain frequencies. The trick is to know what mics sound like what, and use them
Speaking of the classic Shure SM57, it's well known to work great on electric guitar amps, both live and in the studio. The SM57 is a dynamic microphone that features a lot more midrange than lows and highs. But that's the beauty of it. When combined with something like the Sennheiser 421, it can make a great duo with the 57 to capture a nasty electric sound. That's because the 421, which is also a dynamic mic, features a different color than the 57. It captures more bass response and is even a bit brighter than the 57.
Turning to another typical mic like the Royer 121, it can compliment the 57 because it's a ribbon mic, which typically features a thicker low end and smoother top end. Not only that, but the Royer features a figure-8 polar pattern, meaning you're going to record in front and behind the mic'"so you'll have some extra room sound.
While any of these mics could work on an amp by themselves, by combining them, you'll be able to choose the best features of each and use it to your advantage. Sometimes I'll mix and match something like a 421 and a ribbon mic, or I'll turn to the 57 and the ribbon. The key is to understand what each mic can do best and use it for that benefit.
The above mentioned mics may not be the right choice for an acoustic guitar, because, as I mentioned they are quite colored. With acoustics, you generally want to capture more of the natural tone of the instrument. That's why I turn to mics such as those from Earthworks and DPA to capture acoustic guitars (and things like mandolins, banjos, percussion and cymbals). Yes, these mics do cost a bit more than your average, but they are'"for the most part'"incredibly flat in response.
The Earthworks mics that I use are slightly older QTC-1s. The reason I turn to them is that they are omnidirectional, meaning they record in a 360-degree pattern around the capsule. They are very natural sounding, so to a large degree, what you hear from your acoustic is what comes back out of the speakers. Place a mic like that anywhere near where the neck and soundhole meet and you're good to go. The same applies for the DPA mics I use. The DPA 2011 is cardioid condenser mic that's incredibly flat in response and captures the source naturally. That also means I need less EQ to make the mics sound good. Often I use no EQ at all when recording acoustic guitars with those mics.
That's not to say great acoustic recording haven't been made with an SM57'"they have. It depends on what your end goal is with the production.You can also use some great large diaphragm mics on acoustics, such as the classic Neumann U-67/87 or anything along those lines. They also sound good on amplifiers, especially when placed back a few feet for some ambience.
Whatever mics you own, you must take the time to learn them. That means record many different sources with them and listen back flat'"with no EQ, compression, reverb or anything else. That way you'll know their strengths and weaknesses and be able to call upon them when needed. It's time well spent and will pay off with a better end result.