Stereo microphone techniques

Today’s goals

Predict how angle and spacing affect the stereo image.
Choose an array for a source and room, and explain the tradeoff.
Decode M/S and check a stereo recording in mono.

Microphone arrays

Two microphones can capture a stereo scene.
Matched pairs help XY, Blumlein, ORTF and AB stay balanced.
Measure capsule spacing and the angle between microphones.
M/S uses complementary patterns. Decca Tree uses three mics.

Recall: stereo cues

What changes between channels when we separate the capsules?

Review table of directional hearing cues

XY coincident pair

XY microphone array and reproduced stereo image

Capsules sit nearly together, minimizing arrival-time differences.
Angled directional patterns create level differences between channels.

Distinguish interchannel differences at the microphones from differences between a listener’s ears. Ask students to predict the mono result from the capsule placement.

XY Coincident Pair Techniques involve the use of two cardioid or hyper-cardioid microphones.
The microphones are positioned closely together, with their capsules as close as possible.
They are angled between 90° to 130° apart, with their pick-up patterns crossing in front of the sound source.
The entire microphone array is aimed at the center of the sound source.
Each microphone is positioned 45° to 65° off-axis from pointing directly forward.
Sound source 1 originates from the center of the source and is equidistant and equally off-axis from both microphones.
This results in both microphones picking up identical sound simultaneously.
When the microphone signals are panned hard left and hard right on a stereo loudspeaker system, both loudspeakers reproduce the same sound simultaneously.
Identical direct sound and inter-aural crosstalk reach each of the listener’s ears, creating a phantom center image precisely between the loudspeakers.

XY: width and mono

Capsule angle and polar pattern shape the image width.
XY often gives a compact image and reliable mono playback.
Off-axis tone depends on the microphones and source position.

What would you change if the image were too narrow?

Listen: XY

Where do the drums sit? Which image is wider?

Example 7.1

Example 7.2

Mic It! companion audio, Chapter 7

Blumlein pair

Two figure-eight mics, coincident and crossed at 90°.

Listen: Blumlein

How does the room sound compare with XY?

Example 7.3

Mic It! companion audio, Chapter 7

Near-coincident pairs

Spacing adds arrival-time differences to level differences.

Near-coincident microphone array and stereo image

ORTF, NOS and DIN

ORTF: 17 cm / 110° · NOS: 30 cm / 90° · DIN: 20 cm / 90°

ORTF Technique:

Developed by the Office de Radiodiffusion-Télévision Français (ORTF).
Utilizes two cardioid microphones.
Microphones are set at an angle of 110°.
Capsules are spaced 17 cm (6.7 in) apart, which is similar to the spacing of human ears.
Produces a satisfying, transparent stereo image.
Offers relatively good mono compatibility.

NOS Technique:

Developed by Netherlands Radio (NOS).
Employs two cardioid microphones.
Microphones are set at a 90° angle to each other.
Capsules are spaced 30 cm (12 in) apart, wider than ORTF.
Mono compatibility may not be as assured as with ORTF due to wider spacing.
Provides a slightly bigger and more immersive stereo image compared to ORTF.
Narrower angle of incidence between the microphones reduces off-axis pickup.

DIN Technique:

Standardized by the German Deutsches Institut Für Normung (DIN).
Uses two cardioid microphones.
Microphones are positioned 20 cm (7.8 in) apart.
Set at an angle of 90°.
Microphones are not as off-axis as in an ORTF array but are spaced further apart than ORTF.
Provides a good balance of time arrival and amplitude differences.
Particularly effective at shorter distances.

Listen: ORTF, NOS and DIN

Which has the clearest center? What changes in width?

Example 7.4

Example 7.5

Example 7.6

Mic It! companion audio, Chapter 7

Spaced pair, AB

Usually two omnis. Spacing creates arrival-time differences.

Spaced microphone array and reproduced stereo image

Listen: Spaced pair

Does the center stay focused as the image widens?

Example 7.7

Example 7.8

Mic It! companion audio, Chapter 7

Mid-side, M/S

Mid faces forward. A figure-eight Side mic faces sideways.

Forward Mid and sideways Side microphone patterns

M/S capsule placement

M/S decoding

Left = Mid + Side
Right = Mid − Side

Send Mid equally to left and right.
Add Side on the left and polarity-inverted Side on the right.
Raise both Side feeds equally to increase width.

REAPER routing reference: M/S Mastery

Listen: M/S

How do source width and room sound differ?

Example 7.9

Example 7.10

Mic It! companion audio, Chapter 7

Decca Tree

Three spaced omnis feed left, center and right positions.

Decca Tree in practice

Angle, spacing and image width

Compare the arrays. How do angle and spacing change the stereo image?

Interactive stereo-array visualization

Pair task: choose an array

Record an acoustic duo in a small, reflective room. The mix also needs to work in mono.

Choose an array and sketch its placement.
Explain one advantage and one risk.
Name what you would listen for in a test take.

Exit check

Why can capsule spacing change the tone of a mono sum?
What remains when correctly decoded M/S is summed to mono?
Your recording has a weak center. What would you try first?