Sound Capture - Multiple-microphone recording

Learning Outcomes

  • Explain why engineers use more than one microphone to capture a single instrument, ensemble or drum kit.
  • Distinguish between coincident and spaced stereo microphone techniques.
  • Describe how phase cancellation, polarity problems and comb filtering affect recorded sound.
  • Identify exam listening clues for stereo width, room ambience, close miking and microphone bleed.
  • Evaluate suitable microphone choices and polar patterns for common multi-mic recording situations.
  • Apply practical rules such as the 3:1 rule to reduce phase problems in multi-microphone setups.

Edexcel A-Level Music Technology (9MT0) Syllabus

For Component 3: Listening and Analysing, you need to recognise, describe and evaluate music technology techniques by ear and from technical information. Multiple-microphone recording belongs to sound capture: it affects stereo image, tone, room sound, separation, phase and the sense of space in a recording.

  • Recognise the sound of close microphones, ambient microphones and stereo microphone pairs.
  • Explain the purpose of common stereo techniques such as coincident pair and spaced pair.
  • Discuss how microphone placement affects balance, tone, width and reverb capture.
  • Identify phase-related issues that can occur when two or more microphones capture the same source.
  • Evaluate the suitability of cardioid, figure-of-8 and other polar patterns in multi-mic situations.
  • Relate microphone technique to typical sources such as drum kit, piano, acoustic guitar, vocal groups and ensembles.
  • Use accurate technical vocabulary in written answers, especially for phase, stereo image, mono compatibility and bleed.

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

  1. What is the main difference between a coincident pair and a spaced pair?
  2. Why can two microphones on the same sound source cause phase cancellation?
  3. What does the 3:1 rule try to reduce in multi-microphone recording?
  4. Why might a condenser microphone be useful as a drum overhead or room microphone?
  5. What listening clues suggest that an instrument has been recorded with both close and ambient microphones?

Introduction

Multiple-microphone recording means using two or more microphones as part of one capture setup. This might be a stereo pair over a piano, several microphones on a drum kit, a close microphone plus a room microphone on a guitar amplifier, or a set of microphones placed around an ensemble. It is a powerful recording method because it gives the engineer more control over balance, tone, stereo width and ambience.

It also creates technical risks. When several microphones capture the same sound at slightly different times, their waveforms may combine badly. The result can be a thin, hollow or filtered tone, especially when the recording is played in mono. In the exam, you may need to identify the likely use of multiple microphones from what you hear, or evaluate whether a given setup is suitable for the task.

Key Term: multiple-microphone recording
A recording method using two or more microphones to capture one source, several sources, or a whole performance space.

Why engineers use more than one microphone

A single microphone can capture a clear and usable sound, but it often gives only one perspective. Multiple microphones allow different aspects of the sound to be recorded separately and then balanced in the mix.

For example, a grand piano may be recorded with two condenser microphones inside or above the open lid. One microphone captures more of the lower strings and the other captures more of the higher strings. Panned left and right, they create a stereo image that reflects the width of the instrument. An upright piano might be recorded using a pair of condensers behind the piano facing the soundboard, or from the keyboard side near the player. Each placement changes the amount of hammer attack, resonance, room reflection and stereo spread.

A drum kit is another clear example. A kit is one instrument, but it is made of several sound sources: kick drum, snare, toms, hi-hat, cymbals and room sound. Engineers often use close microphones on individual drums, plus overhead microphones for cymbals and the overall kit image. They may also add room microphones to capture natural ambience. This gives control, but every extra microphone increases the chance of bleed and phase issues.

Key Term: close miking
Placing a microphone near a sound source to capture a direct, present sound with less room ambience.

Key Term: ambient miking
Placing a microphone further from the sound source to capture more of the room sound and natural reverberation.

Condenser microphones are often chosen for stereo pairs, overheads and ambient positions because they are sensitive and can capture detail, high frequencies and transients well. They are suited to vocals, acoustic guitar, piano, orchestral instruments, drum overheads, small percussion and room capture. However, because condensers are sensitive, they may need careful placement, phantom power and sometimes a pad switch when used near loud sources.

Dynamic microphones are often used for loud close-miked sources such as snare drum, guitar amplifiers and kick drum because they are generally less sensitive and can handle high sound pressure levels well. In a multi-mic setup, it is common to combine both microphone types: for instance, a dynamic microphone close to a guitar amp cone and a condenser microphone further back to capture room tone.

Test Tip: In a listening answer, link the microphone technique to the sound. For example: “The piano has a wide stereo image, suggesting a pair of microphones rather than a single mono microphone” is stronger than simply writing “stereo microphones.”

Stereo pairs: coincident, spaced and related techniques

A major use of multiple microphones is stereo recording. A stereo microphone technique uses two microphones to create a left-right image. This can make a recording feel more realistic, especially for piano, acoustic guitar, percussion, choirs, strings and whole ensembles.

A coincident pair places two microphones so that their capsules are very close together, often almost touching. A common version is the X-Y pair, usually using two cardioid microphones angled apart, often between about 90° and 135°. Because the capsules are in nearly the same position, the sound reaches both microphones at almost the same time. This means the technique has good phase coherence and usually strong mono compatibility. The stereo effect is mainly created by level differences: a sound from the left is picked up more strongly by the microphone pointing left.

Key Term: coincident pair
A stereo microphone setup in which two microphone capsules are placed very close together, with the stereo image created mainly by level differences.

A spaced pair uses two microphones placed some distance apart. This is sometimes called A-B stereo. The stereo image is created by both level differences and time-of-arrival differences. A sound from the left reaches the left microphone slightly earlier than the right microphone, which can produce a wider and more open stereo image. However, because the microphones are in different positions, phase problems are more likely, especially when the signal is summed to mono.

Key Term: spaced pair
A stereo microphone setup in which two microphones are placed apart, creating width through level and time differences.

Spaced pairs are often used for piano, drum overheads, choirs and ensembles. They can sound spacious and impressive, but the engineer must listen carefully for weak bass, hollow midrange or unstable imaging. Moving the microphones slightly can make a large difference.

Other stereo methods are useful to know, even if an exam question does not name them directly. ORTF uses two cardioid microphones angled apart and spaced roughly like human ears, giving a balance of level and timing differences. Blumlein uses two figure-of-8 microphones at 90°, capturing a natural stereo field with room information from front and back. Mid-side uses a forward-facing microphone and a side-facing figure-of-8 microphone; it allows stereo width to be adjusted after recording, but needs decoding to produce normal left-right stereo.

Key Term: stereo image
The perceived left-to-right placement and width of sounds in a stereo recording.

Microphone polar pattern is central to stereo recording. A cardioid pattern picks up mainly from the front, with reduced pickup at the sides and strong rejection at the rear. This makes it useful when the engineer wants focus and less room sound. A figure-of-8 pattern picks up from the front and back while rejecting the sides. This can be useful in Blumlein or mid-side techniques, but it may capture more room sound than expected.

Exam Warning: Do not assume that “stereo” automatically means “better.” A stereo pair may add width, but it can also introduce phase problems, weak mono compatibility or too much room sound.

Phase, polarity and mono compatibility

The biggest technical issue in multiple-microphone recording is phase. Sound travels through air, so it reaches microphones at different times if they are at different distances from the source. When those signals are combined, some frequencies may reinforce each other and others may cancel. This can change the tone dramatically.

Key Term: phase cancellation
A reduction in level at certain frequencies caused when similar waveforms combine out of phase.

A simple example is snare drum recording. The top microphone captures the stick attack and drum head. A second microphone under the snare captures the rattle of the wires. Because the two microphones face opposite sides of the drum, one signal may have opposite polarity relative to the other. If the engineer combines them without checking, the snare can lose body and sound thin. Flipping polarity on one channel may restore the weight of the drum.

Polarity and phase are related, but not identical. Polarity inversion turns the waveform upside down: positive pressure becomes negative pressure. Phase differences are usually caused by time delay between similar signals. In exam answers, “phase cancellation” is usually the safer term when discussing multiple microphones at different distances. “Polarity inversion” is more specific and should be used when one signal is electrically or directionally inverted.

Key Term: polarity inversion
Reversing the positive and negative parts of an audio waveform, often using a polarity switch on a channel or preamp.

A common result of phase interaction is comb filtering. This occurs when a sound is combined with a slightly delayed version of itself. The frequency response develops a series of peaks and dips, producing a hollow, nasal or coloured tone. It can happen when two microphones capture the same source at different distances, or when a direct signal is mixed with a delayed reflection.

Key Term: comb filtering
A pattern of frequency peaks and dips caused when a signal is mixed with a delayed version of itself.

Mono compatibility matters because many listening systems do not reproduce a perfect stereo image. Club systems, phone speakers, radio broadcasts and some public address systems may sum stereo signals partly or fully to mono. A recording that sounds wide in stereo may lose important elements in mono if the microphones are out of phase. Coincident pairs usually perform better in mono than widely spaced pairs because the capsules are close together.

The 3:1 rule is a practical guide for reducing phase problems when using multiple microphones. If one microphone is a certain distance from a sound source, another microphone should be at least three times that distance from the first microphone’s source, where possible. For example, if a microphone is 20 cm from a guitar amp, a second microphone on another source should ideally be at least 60 cm away from that amp sound. This does not solve every problem, but it helps reduce the level of spill reaching the other microphone.

Key Term: 3:1 rule
A placement guideline stating that the distance between microphones should be at least three times the distance from each microphone to its intended source.

Test Tip: If an exam question asks you to evaluate a microphone setup, mention both the benefit and the risk. Example: “A spaced pair may give the piano a wide stereo image, but the engineer should check for phase cancellation and mono compatibility.”

Bleed, separation and room sound in real recordings

In multi-mic recording, microphones rarely capture only the intended source. A vocal microphone may pick up guitar spill. A snare microphone may pick up hi-hat. Drum overheads capture cymbals, but also snare, toms and room reflections. This unwanted pickup is called bleed or spill.

Key Term: bleed
Sound from an unintended source being captured by a microphone.

Bleed is not always bad. In a live band recording, bleed can help the performance sound natural because the microphones share some of the same acoustic space. Classic recordings often have audible spill because the musicians performed together in the same room. However, too much bleed can make mixing harder. If the hi-hat is very loud in the snare microphone, boosting the snare’s high frequencies may also make the hi-hat harsh. If guitar amp spill is loud in the vocal mic, editing or tuning the vocal may reveal unwanted guitar sound.

Engineers control bleed using microphone choice, polar pattern, placement and screens. A cardioid microphone can reject sound from the rear, so pointing the rear of the microphone toward an unwanted source can help. Close miking increases the level of the intended source compared with background spill. Gobos or acoustic screens can reduce spill between instruments. However, placing too many screens in a room may make the recording sound dry or boxy.

Room sound is another key feature. Close microphones provide detail and presence, while ambient microphones capture natural reverberation. The further the microphone is from the source, the higher the proportion of reflected sound compared with direct sound. This can add depth, especially for classical ensembles, choirs, drum kits and acoustic instruments. It can also make a recording muddy if the room has poor acoustics.

In exam listening, a close-miked sound often has strong detail, little natural reverb, clear transients and an “up-front” position in the mix. Ambient or room microphones produce more distance, more reflections and a sense of the recording space. A drum kit recorded with room microphones may have larger cymbal wash, longer decay and a more live feel. A piano recorded in stereo with room included may sound wide and natural, but less sharply defined than a close mono capture.

Exam Warning: Avoid vague comments such as “the microphones sound good.” Describe what you hear: wide stereo spread, central mono image, close attack, room ambience, spill, phasey tone, or natural acoustic reverb.

Typical multi-mic setups and exam-ready evaluation

For vocals, multiple microphones are less common than single-mic recording, but they can still appear. A lead vocal might be captured with a close large-diaphragm condenser and a more distant room microphone for natural ambience. This can add depth, but it risks phase issues and unwanted room colour. A pop shield and shock mount are often used with the close microphone to reduce plosives and stand vibration.

For acoustic guitar, a pair of condenser microphones may be used to capture body and string detail. One microphone might point near the 12th fret while another captures the body or bridge area. This gives tonal control but can cause phase cancellation if both mics pick up the same frequencies at different times. A coincident pair reduces phase problems and gives a stable stereo image; a spaced pair may sound wider but needs more care.

For electric guitar amplifier recording, a close dynamic microphone near the speaker cone gives punch and definition. Moving the microphone toward the centre of the cone tends to increase brightness; moving it toward the edge tends to soften the tone. Adding a condenser microphone further back can capture room sound and speaker “air”. The engineer must balance the two signals carefully and check phase.

For piano, stereo capture is common because the instrument covers a wide pitch range and has a large physical soundboard. A grand piano may be recorded with two microphones under or above the open lid, spaced to capture low and high strings. An upright piano may be recorded from the rear soundboard or from the keyboard side. Condenser microphones are often suitable because they capture detail, transients and high frequencies. The room position matters because pianos produce complex reflections.

For drums, multiple microphones are almost standard in many pop, rock and metal productions. A minimal setup might use a kick mic, snare mic and one or two overheads. A larger setup may add tom mics, hi-hat mic, ride mic, stereo overheads and stereo room mics. The overheads often define the overall kit image, not just the cymbals. Snare and kick close mics add impact. Room mics add size. The main risks are bleed, phase cancellation between close mics and overheads, and excessive cymbal harshness.

For ensembles and choirs, the aim may be natural balance rather than individual control. A stereo pair placed in front of the group can capture a realistic image. Spot microphones may be added for quiet soloists or sections, but they must be blended carefully so the recording does not sound unnatural. In this type of recording, the acoustic quality of the room is especially significant.

When writing exam answers, always connect the technology to musical impact. A wide stereo piano may support a ballad by filling space around the vocal. Close drum mics may suit rock because they give power and separation. Room mics may suit jazz or classical recording because they preserve ensemble blend and performance space. Poor microphone placement may cause a thin tone, blurred transients or unstable imaging.

Key Point Checklist

This article has covered the following key knowledge points:

  • Multiple-microphone recording uses two or more microphones to capture a source, instrument group or acoustic space.
  • Stereo pairs create width and can make instruments such as piano, drums, choirs and ensembles sound more natural.
  • Coincident pairs place capsules close together and usually have good mono compatibility.
  • Spaced pairs can produce a wider image but are more likely to suffer phase problems.
  • Condenser microphones are often useful for overheads, ambience, piano and acoustic instruments because they capture detail and transients.
  • Dynamic microphones are often useful for loud close sources such as drums and guitar amps.
  • Phase cancellation occurs when similar signals combine out of phase, causing a thinner or weaker sound.
  • Comb filtering creates a hollow or coloured tone when a signal combines with a delayed version of itself.
  • The 3:1 rule helps reduce spill and phase interaction between microphones.
  • Bleed can add natural cohesion but can also make mixing and processing harder.
  • Close miking gives presence and separation, while ambient miking captures more room sound.
  • Strong exam answers describe the audible effect of microphone technique, not just the equipment used.

Key Terms and Concepts

  • multiple-microphone recording
  • close miking
  • ambient miking
  • coincident pair
  • spaced pair
  • stereo image
  • phase cancellation
  • polarity inversion
  • comb filtering
  • 3:1 rule
  • bleed