How instruments radiate sound

Instruments do not radiate sound equally in all directions, and the pattern changes with frequency. For microphone placement this is more useful than any single recipe.

Strings do not radiate sound

The string of a violin or piano is thin and cannot move enough air by itself. It drives the body, and the body radiates. A violin radiates mostly from the top plate and the f-holes; a piano radiates from its soundboard. A microphone placed close to the strings hears the attack and the string's high partials, but the full sound arrives from the body.

Violin and bowed strings

• At low frequencies, the violin radiates in nearly all directions.
• Above about 1 kHz, the radiation becomes directional, with strong components up and forwards, and the pattern is changing from note to note.
• Body resonances, typically near 270 Hz (the air resonance) and 450 to 550 Hz, shape the sound across the notes.

A microphone above and in front of the violin typically gives a rounder sound than one at the f-hole.

Brass

Sound leaves through the bell.

• At low frequencies, the bell is small relative to the wavelength and radiation is nearly omnidirectional.
• At high frequencies, the sound forms a narrow beam along the bell axis. A trumpet on axis sounds much brighter than 90° off axis.

So a microphone in front of the bell gets bright, brassy sound; one at the side gets a mellower sound. The difference can exceed 10 dB in the high frequencies.

Flute

The sound comes from the embouchure hole and from the open finger holes, or the open end of the tube. A flute sounds fuller in the room than on axis, and microphones close to the embouchure pick up breath noise as well as tone. The low notes radiate in almost all directions, and the high notes more in the direction of the player's line of sight.

Clarinet and oboe

The sound comes from the first open tone holes and the bell. Which one dominates depends on the fingering: only for the lowest notes, with all holes closed, does the bell do most of the work. For most notes the open holes radiate much of the sound, and the bell adds the higher partials, beamed along its axis. A close microphone at the bell therefore does not simply sound dark; it can sound thin or edgy on some notes and hollow on others, while the balance changes from note to note. A microphone about 0.5–1 m away, aimed at the middle of the instrument, averages these sources into a more even picture.

Voice

The voice is directional in the same way: mostly forward at high frequencies, wide and almost omni at low frequencies. Singers who turn their head change the high-frequency balance.

Consequences for placement

1. Start with a distance where the sound has combined, usually a metre or more.
2. Check the instrument from several positions, and listen to a microphone in each of them.
3. Avoid the bell for brass if you want a rounder sound.
4. Remember that proximity effect changes the bass of close directional microphones, see Proximity effect.

Draft: written by an AI assistant, awaiting technical review.