Microphones aren’t perfect. They’re crude electro-mechanical devices based on hundred year-old technology. The inherent inefficiencies of the technology result in different mic designs altering the sound in different ways, much like the different colors of a stained glass window alter the light passing through it. Once this sonic “tint” has been captured by the recording medium, it can’t be undone.
Audio engineers learn to match the qualities of different mics to different sources: a hard source get’s a soft mic, a soft source gets a hard mic. If one uses a hard mic on a hard source, the result can be a very unpleasant sound. Imagine fingernails on a chalkboard. Likewise, using a soft mic on a soft source can result in a mushy, lifeless sound.
One would think a perfectly accurate mic would be ideal. These mics exist, but they are seldom used in the recording studio, other than for calibrating equipment. This is because we’re not looking for a perfectly accurate reproduction of audio, we’re looking to sculpt the sound, creating a sonic landscape that lures the listener into our musical world.
Generally speaking, we use mics with tubes or transformers in them when we want to draw attention to the sound, (vocals, lead instruments, important percussion elements.) Tubes and transformers add 3rd harmonic distortion, which can impart a sort of larger-than-life quality. For supporting sounds we use neutral mics. Or, if we want to surround the mix with a sheen, we might use bright small diaphragm condenser mics on the acoustic guitars and pan them hard left and hard right, and use a less bright mic on the vocal in the center of the mix so the presence of the vocal is different than the presence of the music that’s framing it. The options are endless.
Studio mics come in three basic designs: Condenser mics, dynamic mics, and ribbon mics. Large diaphragm condenser mics are less detailed than small diaphragm condensor mics, but more detailed than dynamic mics. Ribbon mics are generally associated with a smoother, less bright sound. They all have their role in capturing an enticing sonic landscape. Here are a few examples from a typical recording scenario:
Lead vocal: Large diaphragm condenser (LDC) tube or transformer mic, or clean LDC mic into a tube or transformer preamp for added mojo.
Harmony vocals: clean LDC through a clean preamp.
Bright acoustic guitar: neutral LDC or small diaphragm condensor (SDC) mic .
Dull acoustic guitar: bright SDC mic to add life to the sound.
Percussion/drums: dynamic mic (like a Shure sm57) which tends to soften the transients (spikes in the sound) creating a more “mixable” track.
Solo instrument: whatever flatters that instrument and gives it that larger-than-life quality. On a violin or other stringed instrument, it could be a LDC going through a preamp with transformers, or a tube mic, or a ribbon mic - or it could be a SDC if the performance is detailed with a lot of small, fast notes. Electric guitar could be the venerable sm57 on the amp, perhaps with a room mic added for more atmosphere.
Mic choice is also influenced by the production elements. A guitar/vocal could sound great using a pair of full-bodied mics, the same guitar and vocal in a busy mix could require mics with less low end information and more presence. That’s because, in a busy mix, there’s more competition for sonic real estate. We’re forced to “shrink” the sounds of the individual elements so they don’t all run into each other and create an impenetrable sonic sludge.
Another factor is pickup pattern. Cardioid pattern is standard. It “hears” what’s in front of it, with some rejection to sound coming from the sides and behind. Supercardioid and hypercardioid shrink the focus a bit, giving more rejection at the sides, but opening up an area directly behind the mic that does pick up sound. Figure 8 pattern (also called bi-directional) catches what’s directly in front and directly behind the mic equally, with almost total rejection of what’s at the sides.
Cardioid mics are used for pretty much everything, especially if rejection of other nearby sounds is not an issue. We might use figure-8 mics on a live guitar/vocal, aiming the vocal mic up so the null of the pattern rejects the sound of the guitar, and aiming the guitar mic down so the null of the pattern rejects the voice. This way, in the mix, we have more control over the balance between guitar and vocal.
Omni pattern is 360 degrees. It hears everything equally. We might use an omni mic to pick up the room sound, or for a more natural response of a close-mic’d acoustic instrument. (Close-micing increase the proximity affect in cardioid mics - the booming bass of the announcer’s voice, for example.)