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Microphone selection and placement has nearly infinite possibilities but the "best" solution is whatever works to produce the "sound" you had imagined. Even the basics are acoustically complex so it's best to start with the more common techniques, understand why they are useful and then adapt them to the real world.

Default Placements
Even if the room acoustics are "ideal" the positioning of a mic relative to the sound source usually has a profound impact on the sound that is recorded. For example:
  • Near-field: This is the "acoustic zone" defined as any position up to a distance equal to the length of the longest resonant part of the source. For an acoustic guitar, it is the length of the body plus neck. Within the near-field, the sound will change drastically as position changes. Keep in mind, there is NOT a boundary between near and far-field -- this is a diffuse spatial variation with no hard dividing lines.
  • Far-field: Everything else. For an acoustic guitar, since the body resonance provides most of the sound projection, the longest body dimension is generally a more practical measure of the near-far-field point. Also note, that as the mic distance reaches the far-field point, and especially beyond, the room acoustics will often provide a substantial part of the sound -- good or bad.
  • 3:1 Rule: A "rule-of-thumb" for positioning multiple mono mics with multiple sources. For isolation, position each mic away from any other mic by AT LEAST 3X the distance from mic to source. This keeps the "bleed" from one source from being too noticeable a sound level into another source's mic.
  • Panning: This refers to mixing after recording the track(s); but it has direct reliance to the mic type and mic positioning. When using a pair of near-field, SCMs in X-Y orientation for stereo definition, pan them hard left-right.
  • Depth: This refers to the recorded sound image while mixing, but mic selection and position are important in creating this "depth". Tracking both near-field AND far-field allows adding selective reverb which "smears" the sound in a controlled way. For example, adding just a touch of reverb to close-mic'd, centered vocals keeps them "up front" (so "near") and more reverb to far-field LCM instruments moves them "away".
  • Stereo X-Y: (Coincident): Positioning two mics (same type to keep the phase vs frequency the same) with capsules close together and the mics angled at 90-135 deg apart will give a good stereo field. If an acoustic guitar (and musician) is good and the room acoustics are good as well, then far-field recording at about 2-ft with X-Y mics will allow much less positional sensitivity. Using a 110 degree angle, instead of 90, will give a little extra stereo separation.
  • Wide Stereo X-Y: (Near-Coincident): The stereo separation can be increased by keeping the 90 deg angle but moving the capsules left and right (still crossed) away from each other. The rule of thumb is to position the mics at a distance from the source where each mic (cardioid) points toward the edge of the source's sound field. Again, pan the two tracks hard left and right. Separation increases still more when recording large sources like orchestra a choirs.
  • EQ: Again, this is a reference to the mixing stage, but it relates directly to mic selection and positioning -- get that right and there will be less futzing with repairs during the mix. If you need more than 4 dB adjustment with equalization, the mic, placement or source isn't right so go back and fix it.
  • 12th Fret: A good starting position for acoustic guitar is 4-8 inches from the 12th fret. This avoids the "boominess" of the sound hole but an X-Y Stereo pair of SCMs will deliver clarity and be less responsive to room acoustics. If the guitar is not the primary instrument in the mix or does not need a lot of clarity, focus more on isolation of the sound than on getting sereo separation or even stereo for that matter; a single LCM can be a better option.
  • 4-Source Acoustic: Another option for tracking a sola acoustic guitar is recording 4 channels simultaneously: 1) direct from the bridge pickup, 2) 3) stereo X-Y at the 12th fret and 4) from an "edge of near-field" cardioid LCM. This assumes, of course, that the room acoustic are good (minimal bass reflections and not totally dead). This allows more options in the mix.
  • Voice-1: While positioning an LCM in close (eg. 4 inches) takes advantage of the proximity effect, there's never a need to let the singer "swallow" the mic. Pop filters may not be necessary if the vocalist faces straight ahead and level with the mic placed at nose height since the breath tends to angle down a bit.
  • Voice-2: There is a huge variation of tonal effects possible by using proximity and off-axis positioning. The proximity effect start at about 400 Hz which is also about the lower end of the human voice.
  • Voice-3: Fixing a vocal track with EQ in the mix is extremely hard. Get the sound right before tracking.
  • Voice-4: Use distance to adjust the low-end response (closer for more breathy bass -- the proximity effect) and use angle/position (higher: more nasal, lower: more resonant) to adjust the high-frequency effect.
  • Voice-5: Tinker till it works but never record vocals in stereo -- the phasing issues cannot be repaired in the mix. Generally, vocals are centered in the mix so record them in a dead room (no reverb).
  • Other Instruments:
    • Horns: LCM, cardioid, 18-in from bell minimum, bell level or above
    • Harp: SMCs, X-Y stereo, 4-ft away, at top or above harp and angled down
    • Flute: LCM, cardioid, 12 inches and from below to avoid breath sounds
    • Small percussion: Small omni or SMC cardioid, about 2-ft above the instrument
    • Harmonica: Small omni or SMC cardioid, about 6-in in front and 1-ft above the instrument
    • Acoustic piano: Assuming an open grand, use SMCs, X-Y stereo, at 6-8 ft. The sound mostly comes from the soundboard. For uprights, angle them away from the wall to reduce reflections. Close mic positioning is problematic. Open grand lids reflect sound from the room as well as the strings. Try SCMs under a grand piano -- less problematic than under the lid.
    • Electric guitar amp: Mic the speaker(s) up close:
      • Center of cone: Maximizes the high-end and minimizes outside noise
      • Edge of cone: Less high-end and more bass
      • 1-2 ft out front: More room pickup, phasing between speakers is difficult
      • Use dynamic mics in close, LCMs for far-field
      • Elevating the amp off the floor can help flatten the bottom end
    • Drums: These occupy a very large sonic space (as well as physical space) and are heavilly influenced by room acoustics. Both near-field (literally inside the drums) and far-field mic placement is used. Obviously, low self noise and high sensitivity are not generally required whereas high max SPL can be important. Lots of experimentation is required.