Blumlein pair

Stereo realism does not necessarily emerge from making an image as wide as possible. It emerges when the directional, tonal and reverberant cues in a recording support one another strongly enough to create a stable and believable acoustic space.

In the first article of this series, I explored how human hearing reconstructs a three-dimensional auditory world from differences in arrival time, level and spectral filtering at the two ears.

This second article moves from perception to production: how do different stereo microphone techniques encode spatial information, and why can the choice of array become especially important when a recording is reproduced through loudspeakers?

As a recording engineer and the founder of Direct Sound Records, my central objective is not simply to create an impressive stereo effect. It is to preserve the relationship between the musicians, the acoustic environment and the listener in a way that remains convincing during reproduction.

Among the many available stereo techniques, the Blumlein pair remains one of the most revealing—and one of the most demanding.

There Is No Universal “Best” Stereo Technique

Before considering Blumlein, an important distinction must be made: no stereo microphone technique wins in every situation.

The appropriate array depends on several factors:

• the size and arrangement of the ensemble;
• the acoustic quality of the room;
• the desired relationship between direct and reverberant sound;
• the required stereo width and localisation precision;
• mono compatibility;
• the intended playback system;
• and the artistic purpose of the recording.

A spaced pair may be ideal when a broad sense of scale and low-frequency spaciousness is required. ORTF can provide an effective balance of width, localisation and ambience. Mid-Side offers control over stereo width after recording. XY can provide a stable image with strong mono compatibility.

Blumlein has its own strengths and limitations. Its value lies not in being universally superior, but in the particular way it connects direct sound, room ambience and coincident stereo geometry.

How Stereo Microphone Arrays Encode Space

Stereo microphone techniques can be broadly understood according to the cues they create between the left and right channels.

Spaced Pairs

In an AB arrangement, two microphones are separated physically. A sound arriving from one side will generally reach one microphone before the other, producing an interchannel time difference. Depending on the microphones and source position, there may also be a difference in level.

Spaced arrays can produce a broad and enveloping presentation. They can be particularly effective for large ensembles, organs, orchestras and situations in which the acoustic environment is an important part of the experience.

However, the time differences between channels can influence mono compatibility and may produce frequency-dependent reinforcement or cancellation when the channels are combined. Increasing microphone spacing can also weaken centre localisation if the geometry is not appropriate for the source and listening conditions.

These are design trade-offs, not proof that spaced recording is inherently defective.

Near-Coincident Arrays

Near-coincident techniques such as ORTF deliberately combine microphone spacing with directional microphone patterns.

ORTF uses two cardioid microphones separated by approximately 17 centimetres and angled 110 degrees apart. The resulting stereo image contains both interchannel timing and level differences.

This combination often produces greater spaciousness than a fully coincident cardioid pair while retaining more definite localisation than a widely spaced AB array. It is one reason ORTF has remained a widely used technique for classical music, ensembles and location recording.

Describing ORTF simply as “phasey” overlooks the fact that its timing differences are intentional components of its spatial design.

Coincident Arrays

In a coincident array, the microphone capsules are positioned as close as physically possible to the same acoustic point.

Because direct sound reaches the two capsules at almost the same time, stereo direction is created primarily through differences in level and polarity rather than substantial arrival-time differences.

XY, Mid-Side and Blumlein are all coincident techniques, although they use different polar patterns and encode the surrounding sound field differently.

The coincident geometry generally provides predictable mono compatibility and reduces the possibility of time-delay-related cancellations when the channels are summed. It can also create a clearly defined centre image and stable localisation within the normal listening area.

Real microphones are not mathematically perfect points, however. Capsule dimensions, vertical displacement, polar-pattern differences and off-axis response mean that no practical array is perfectly coincident or perfectly phase coherent at every frequency.

What Loudspeaker Playback Changes

Headphone and loudspeaker reproduction deliver stereo signals to the listener in fundamentally different ways.

With conventional headphones, the left channel is delivered predominantly to the left ear and the right channel to the right ear. With two loudspeakers, each loudspeaker reaches both ears.

The left ear therefore hears:

• the left loudspeaker directly;
• the right loudspeaker through an additional acoustic path;
• and reflections from the listening room.

The right ear receives the corresponding combination from the opposite side.

This acoustic crosstalk is not an accidental failure of stereo. It is part of conventional two-channel loudspeaker reproduction. The brain uses the resulting combination of timing, level and spectral cues to perceive phantom images between and sometimes beyond the loudspeakers.

However, the reconstruction is sensitive to geometry. Moving away from the central listening position changes the relative distances from the two loudspeakers and therefore changes the arrival-time and level relationships at the ears. The phantom image tends to shift towards the nearer speaker.

The loudspeakers, room and listener must therefore be considered as one reproduction system. A recording does not carry an independent three-dimensional space that remains unchanged under every playback condition.

Enter the Blumlein Pair

The Blumlein pair uses two figure-of-eight microphones mounted coincidently and angled 90 degrees apart.

The technique is associated with Alan Dower Blumlein, whose pioneering 1931 patent described fundamental principles of stereophonic recording and reproduction.

In a correctly arranged Blumlein pair, the microphone diaphragms occupy almost the same acoustic point. Directional information is encoded primarily through the different levels and polarities produced by the two figure-of-eight patterns.

A figure-of-eight microphone is equally sensitive to sound arriving from the front and rear, while strongly rejecting sound arriving from its sides. Consequently, the array captures both the performance in front of the microphones and a substantial amount of acoustic information from behind them.

This is a defining characteristic of Blumlein—not a minor detail.

Why Blumlein Can Sound So Convincing

Coincident Timing for Direct Sound

Because the two capsules are positioned at approximately the same point, direct sound from an instrument reaches both microphones almost simultaneously.

This minimises interchannel arrival-time differences introduced by the microphone spacing itself. The stereo image is produced predominantly through level and polarity relationships.

For loudspeaker reproduction, this can create a precise centre image and clearly organised lateral positions, particularly when the ensemble and array are positioned carefully.

Strong Mono Compatibility

When the two channels of a coincident recording are combined, corresponding direct sounds normally align more predictably than they do in a widely spaced array.

This does not mean that every part of a Blumlein recording will combine perfectly. Reflections arrive from many directions and at many times, while real microphones have tolerances and frequency-dependent polar behaviour.

Nevertheless, the coincident geometry generally gives Blumlein excellent mono compatibility compared with arrays that rely heavily on microphone spacing.

Natural Integration of the Room

The rear lobes of the figure-of-eight microphones capture reverberant energy and sound arriving from behind the array.

In a good acoustic environment, this can create a remarkably integrated sense of depth. The room does not feel like a synthetic effect added behind the musicians. It becomes part of the same spatial event.

The direct sound establishes the performers, while the reflected energy communicates the dimensions, character and decay of the venue.

When those relationships are balanced correctly, the listener may perceive not merely a wide line between two loudspeakers, but a coherent acoustic scene extending behind and around the performers.

Spatial Information Without Microphone Spacing

Blumlein can generate a substantial stereo image without separating the microphones horizontally.

This is particularly attractive when the engineer wants clear directional information while minimising time-of-arrival differences between channels.

The result can feel cohesive because the direct sound and room information are captured from a single acoustic viewpoint.

What Phase Coherence Really Means Here

The expression phase coherence is often used loosely in audio. In the context of a coincident stereo array, it is more useful to speak about the consistency of interchannel timing relationships.

Blumlein does not remove phase from a recording. Every acoustic event contains complex phase relationships, and every room creates reflections with different delays, levels and spectra.

What the array minimises is the additional time difference that would otherwise be introduced by placing the two microphones at separate locations.

This distinction matters.

A Blumlein recording can still contain:

• phase differences created by room reflections;
• microphone-response differences;
• polarity differences inherent in the figure-of-eight geometry;
• and complex interference between direct and reverberant sound.

Its strength is not “perfect phase purity.” Its strength is that both channels observe the direct acoustic event from approximately the same point in space.

Why Blumlein Is Also Demanding

The characteristics that make Blumlein revealing also make it unforgiving.

The Room Must Deserve to Be Recorded

Because figure-of-eight microphones capture strongly from both front and rear, an unattractive room will not politely disappear.

Flutter echoes, mechanical noise, audience movement, heating systems and poorly controlled reflections can become prominent parts of the recording.

Blumlein works best when the acoustic environment contributes positively to the performance.

Placement Is Critical

The balance between ensemble width, direct sound and reverberation depends strongly on the distance and orientation of the array.

Positioning the microphones too close may produce an image that is excessively wide or exclude important sources from the useful recording angle. Placing them too far away may allow reverberation to dominate and reduce clarity.

Small movements can significantly change the result. This is why Blumlein rewards careful listening and deliberate placement rather than formula alone.

Rear Sound Is Part of the Recording

The rear lobes do not distinguish between beautiful reverberation and unwanted noise.

Musicians, audience members, equipment and reflective surfaces behind the microphones all become part of the captured field. The engineer must therefore consider the entire environment around the array, not only what lies in front of it.

The Listening Position Still Matters

Blumlein does not eliminate the limitations of two-loudspeaker stereo.

A listener moving significantly away from the central position will still experience changes in timing and level from the loudspeakers, and the stereo image will shift accordingly.

Coincident recording can provide a coherent source signal, but it cannot make conventional stereo reproduction independent of loudspeaker and listener geometry.

When I Choose Blumlein

In my work, Blumlein becomes especially compelling when:

• the musicians are acoustically balanced in the room;
• the venue has a distinctive and musically valuable acoustic;
• the ensemble fits naturally within the array’s useful recording angle;
• the intention is to preserve a complete performance rather than construct one later;
• and loudspeaker playback is an important reference.

I would not choose it automatically when the room is problematic, when strong isolation is required, when sources must be balanced independently, or when the ensemble geometry demands a wider or more flexible array.

In those situations, ORTF, AB, Mid-Side, XY, supplementary microphones or a hybrid approach may be more appropriate.

The technique should serve the acoustic event—not the engineer’s ideology.

Spatial Width Is Not the Same as Realism

A recording can create an enormous stereo image and still feel artificial.

Width may be produced by long interchannel delays, decorrelation, processing or exaggerated ambience. These effects can be exciting, but they do not necessarily communicate a believable relationship between performers and space.

Blumlein offers a different proposition. Its most successful recordings do not merely place sounds from left to right. They establish a unified perspective from which the listener can infer:

• where the musicians are positioned;
• how far away they appear;
• how the room surrounds them;
• and how direct and reflected sound belong to the same event.

This is why the technique can feel less like an audio effect and more like a view into an acoustic performance.

A Reference, Not a Religion

The Blumlein pair deserves its status as one of the foundational stereo microphone techniques. Its coincident geometry, figure-of-eight patterns and integration of direct and reverberant sound can produce extraordinary depth, localisation and spatial coherence.

But the strongest case for Blumlein does not require dismissing other approaches.

AB can communicate scale and spaciousness that a coincident pair may not reproduce in the same way. ORTF can offer a persuasive compromise between width and localisation. Mid-Side provides valuable control after recording. XY can be practical, focused and robust.

The real achievement lies in understanding how each technique encodes space—and choosing the one whose compromises best serve the music, venue and intended reproduction system.

For Direct Sound Records, the objective is not to manufacture an impressive stereo image. It is to preserve the acoustic relationships that make a performance feel present, intelligible and emotionally credible.

When the room, musicians and microphone position align, Blumlein can be one of the most direct ways of achieving that objective.

References and Further Reading

1. Blumlein, A. D. Improvements in and Relating to Sound-Transmission, Sound-Recording and Sound-Reproducing Systems. British Patent GB394325A, filed 1931 and published 1933.
   View patent
2. Eargle, J. M. “An Overview of Stereo Recording Techniques for Popular Music.”
   Journal of the Audio Engineering Society, 1985.
   View AES record
3. Ceoen, C. “Basic Stereo Microphone Perspectives—A Review.”
   Journal of the Audio Engineering Society, 1985.
   View AES record
4. Toole, F. E. “Loudspeakers and Rooms for Stereophonic Sound Reproduction.”
   Audio Engineering Society 8th International Conference, 1990.
   View AES record
5. Kendall, G. S. “The Effects of Interaural Crosstalk on Stereo Reproduction and Minimizing Interaural Crosstalk in Nearfield Monitoring by the Use of a Physical Barrier: Part 1.”
   Audio Engineering Society 81st Convention, 1986.
   View AES record
6. Lee, H. and Gribben, C. “On the Optimum Listening Position and Listening Angle in a Two-Channel Stereophonic Reproduction System.”
   Audio Engineering Society.
   View AES record

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An earlier version of this article was published on LinkedIn. This Direct Sound Records Journal edition has been revised, expanded and technically updated, with additional context and references.