Recording Real Instruments: Microphone Physics, Placement & Room Reality

Studio Guide 04 · Cloud Atelier · Updated April 2026 · ~13 min read

Recording an acoustic instrument is recording a three-dimensional source in a three-dimensional room using a transducer that has a three-dimensional pickup pattern. Position changes the result more than any plugin you can apply later. This guide treats microphone placement as the geometric problem it actually is.

1. Polar patterns: cardioid, omni, figure-8, hyper

The polar pattern describes how a microphone’s sensitivity varies with the angle of arriving sound. Patterns are not abstractions — they are direct consequences of how the diaphragm is mechanically loaded inside the capsule. Understanding the four standard patterns is the prerequisite to predicting what a microphone will pick up before you press record.

Two implications matter in practice. First, the null point of a polar pattern (the angle of zero sensitivity) is a placement tool: aim the null at a sound you want to reject. A figure-8 microphone’s nulls at 90° let you reject a drum kit while capturing the singer at 0° and the room behind at 180°. Second, polar patterns vary with frequency. Almost every cardioid becomes more omnidirectional at low frequencies (below 200 Hz) and more directional at high frequencies (above 4 kHz). Off-axis colouration is unavoidable; minimising it is a placement skill.

2. Proximity effect and the inverse-square law

Pressure-gradient microphones (cardioid, hypercardioid, figure-8) emphasise low frequencies as the source moves closer. The boost can exceed +10 dB at 100 Hz when the source is 5 cm from the diaphragm. This is the proximity effect. It is not a defect — it is the consequence of pressure difference between front and rear of the diaphragm becoming non-linear at close range. Use it when you want warmth (broadcast voice). Avoid it when the natural balance of the instrument matters (acoustic guitar, classical violin).

Distance also obeys the inverse-square law: sound intensity falls as 1/r², or roughly 6 dB per doubling of distance. Move from 30 cm to 60 cm and the direct level drops 6 dB while room reflections drop only 1–2 dB. The direct-to-reverberant ratio collapses fast. Close-mic for isolation; distance-mic for room character.

3. The 3:1 rule and phase cancellation

When two microphones capture the same source from different distances and their signals are summed (any time you mix two mic tracks), comb filtering occurs unless one signal is at least 9–10 dB lower than the other. The 3:1 rule is the geometric shortcut: place mic 2 at least three times the distance from mic 1 as mic 1 is from the source. The 6-dB-per-doubling level drop ensures the far microphone’s contribution to the sum is too low to cause audible interference.

The rule applies to multi-singer recording, drum overheads vs close mics, and any situation where two microphones may be mixed together. Where the 3:1 rule cannot be honoured (a guitarist standing next to a singer), accept that the rejection of each microphone’s polar pattern must do the isolation work instead, and aim the rejection nulls accordingly.

4. Stereo techniques: XY, ORTF, Decca Tree, M/S, A/B

Five techniques cover almost every two-microphone stereo decision. Each trades off image width, mono compatibility, and natural ambience differently.

For home recording, XY and ORTF cover almost everything. XY is foolproof and mono-compatible; ORTF sounds wider and more natural at the cost of slightly more critical placement. M/S deserves attention for one-microphone-pair voiceover or acoustic recordings: the M (mid) signal alone is mono-compatible, and you decide stereo width in the mix by varying the level of the decoded sides.

5. Guitar amps: SM57 placement, the cone-edge tone curve

The Shure SM57 has been the world standard guitar-amp microphone for forty years. It survives because its tight cardioid pattern rejects bleed, its high SPL handling tolerates close placement to a 100-watt cabinet, and its 5–6 kHz presence boost flatters distorted guitar tone. The art is placement relative to the speaker cone.

A typical 12-inch guitar speaker reproduces different frequency content at different points across its radiating surface. The centre dust cap emits the most high-frequency energy: bright, edgy, sometimes harsh. The cone edge at the basket emits darker, mellower midrange. The halfway point between centre and edge is the canonical placement: balanced, with enough top end to cut through a mix and enough body to feel solid.

Two refinements. Tilt the SM57 slightly off-axis (10–20°) to tame harshness; on-axis is brightest, off-axis is darker. Add a ribbon (Royer R-121) or large-diaphragm dynamic (Sennheiser MD 421) one foot back, on-axis for a fuller second source. The two mics blended together — honour the 3:1 rule or check phase invert — produce the balance heard on most rock records.

6. Acoustic guitar: 12th-fret vs sound-hole vs bridge

Acoustic guitar is one of the trickiest sources to record well at home, because the entire body of the instrument radiates sound and the mic captures whatever portion of that radiating field it points at. Three placement zones produce predictable results.

• 12th fret, 15–25 cm out: the canonical position. Balanced spectrum, picks up the strings ringing over the fingerboard plus body resonance from the upper bout. Use a small-diaphragm condenser (Rode NT5, KM 184).
• Sound hole, 20–30 cm out (slightly off-axis): bassy and full but boomy if pointed straight in. Use as a second mic, blended with the 12th-fret mic.
• Bridge, 15–25 cm out: brighter, percussive, captures attack. Useful for fingerpicking detail or strummed rhythm guitar.

For a single-mic recording, the 12th-fret position is correct 90 percent of the time. For two mics, a 12th-fret + bridge spaced pair (mind the 3:1 rule) gives stereo width without the boom of a sound-hole mic.

7. Piano: spaced pair vs ORTF vs over-the-shoulder

Piano is a wide-radiating source. The instrument is mechanically and acoustically larger than any other common studio instrument and captures from at least 20 cm of stereo separation. Three setups dominate.

• Spaced pair over the strings: two small-diaphragm condensers 30 cm above the strings, one over the bass strings (low end) and one over the treble side (high end), 60–90 cm apart. Wide, intimate, modern pop/jazz sound.
• ORTF over the music stand: two cardioids in ORTF configuration over the player’s shoulder. More distant, balanced, classical sound with natural ambience.
• One mic for upright piano: a single small-diaphragm condenser positioned where the upper soundboard meets the back panel, 25–40 cm out. Surprisingly usable for solo writing.

8. Drums in a small room: the four-mic minimum

A full drum recording uses 8–16 microphones in a professional studio. In a home room with limited treatment and few channels, a four-mic setup captures most of what matters.

• Kick (Shure Beta 52 or AKG D112): 5–15 cm inside the resonant head, pointing at the beater impact area. SPL handling and low-frequency response are required.
• Snare top (Shure SM57): 2–5 cm above the rim, pointing across the head toward the centre. Watch for hi-hat bleed by aiming the null at the hat.
• Overheads (matched pair, XY or spaced): centre of the kit, 80–120 cm above. Captures cymbals plus a stereo image of the whole kit. Phase-align with the snare top.

The single largest mistake in home drum recording is treating the overheads as cymbal mics. They are not — in a four-mic setup, the overheads carry the toms, much of the snare, and the cymbals simultaneously. EQ them as kit mics, not cymbal-only.

9. Room modes for low-frequency instruments

Bass guitar, kick drum, baritone sax, double bass — any instrument with significant energy below 200 Hz interacts strongly with the room’s axial modes (covered in Studio Guide 01). The most useful countermeasure is placement: do not record bass instruments in the corner of a room. A corner reinforces all three axial modes simultaneously and produces a 5–15 dB low-frequency bump that no microphone choice can correct.

Position the source at 1/3 of the room length, 1/3 of the room width, 1/3 of the room height from one corner whenever practical. This puts the instrument off the pressure maxima of axial modes and averages the response. The same principle is why subwoofer placement guides recommend the Sumiko or crawl method — you are searching for the location that minimally excites the room’s low-end problems.