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Surround Sound Microphone Techniques



Surround Sound Microphone Techniques


The advent of surround sound, and more recently that of immersive sound has created the need for recording methods that go beyond those microphone techniques used for stereo recording . Many of the techniques used to capture a surround sound recording of a group, such as a choir, orchestra, or other ensemble, are adaptations or extensions of stereo microphone techniques. Surround recording techniques can be divided into four categories: (1) those that use a front array of microphones placed in close proximity, (2) those having separate front and rear arrays, (3) special arrays developed specifically for recording just the ambience of a space, which are used in conjunction with a front array, and (4) microphone arrays assembled as a unit.

      Front Arrays



The front arrays are basically stereo microphone techniques used to capture the main stereo image of the musical ensemble. The well-known Decca tree method is the same technique that has been used for years to make stereo recordings. The other front arrays are similar to the Decca tree using a variety of microphones, angles, and spacing. For surround sound, the front arrays normally are used in conjunction with an ambience array, which will be discussed later as will various combinations of front and rear arrays.

Decca Tree
Decca Tree

Decca Tree

The Decca tree is a stereo microphone technique developed by Roy Wallace and Arthur Haddy at Decca Studios in London in the 1950s, and has been used for years for stereo recording of orchestras. It uses three omnidirectional microphones in a triangle. Two microphones are placed left and right approximately 2 m (6 ft) apart with the third placed in the center about 1 m (3 ft) in front, and approximately 2.5 - 3 m (8 - 10 ft) up. The side mics are panned hard left and right while the third mic is mixed dead center. This technique produces a strong stereo image. It is sometimes called A-B-C stereo.1

INA-3 or Ideal Cardioid Arrangement (ICA)

INA-3
INA-3
Ideal Cardioid Arrangement (ICA)

This microphone technique was developed by Günther Theile. INA is an acronym for Ideale Nieren Anordnung, German for “ideal cardioid arrangement,” and it is sometimes called by that name. The INA-3 is a technique for the three-microphone front array, and most commonly is used with the rear array in the INA-5, which is discuused below. The INA-3 uses a cardioid microphone for the center channel placed 17.5 cm (7 in) in front of two cardioids used for left and right channels, pointing outwards. The spacing between the left and right microphones is 35 cm (14 in). Distances between the front three microphones can be changed to adjust the ambient response.8

Optimized Cardioid Triangle (OCT)

Optimized Cardioid Triangle
Optimized Cardioid Triangle (OCT)

The optimized cardioid triangle technique was developed by Günther Theile and Helmut Wittek for the front microphone array. It uses three microphones in a triangle with the center cardioid mic facing forward about 8 cm (3 inches) in front of the two higher side supercardioid mics that face outward about 40 to 100 cm (about 15 to 40 in) apart, with recording angles from 160° to 90°. It produces high separation between left-center and right-center and good localization.2

Omni+8

Omni+8
Omni+8

Omni+8 is a microphone technique for the front microphone array, typically used with the Omini+8 surround technique, discussed below. It consists of two omnidirectional microphones spaced 2 meters (6.5 ft) apart for left and right, and a figure-eight microphone for the center placed 40 cm (16 in) forward. The omnidirectional mics provide a rich and extended impression with ample bass response, while the figure-eight mic provides a strong, stabilized center channel.10

The Disadvantage of Three-Microphone Techniques
While three-microphone techniques work great for traditional two-channel stereo, there is a problem when mixing it down to the three channels (L-C-R) used in surround sound. When a three-microphone recording is played back through speakers, it can create a “triple phantom sound source.” Because of interchannel crosstalk, each two-channel pair (C-L, C-R, and L-R) produces a different phantom image, which seems to move as the listening position changes, leading to loss of focus and clarity.3 Despite this problem many people still claim excellent results with these techniques. However, the following two somewhat lesser known techniques were designed to overcome this shortcoming.

Double X-Y
Double X-Y

Double X-Y
The double X-Y method is an extension of the X-Y stereo tehnique that uses two coincident pairs. Each pair of cardioid microphone are angled at 90° to 120° from each other and are placed approximately 8 m (26 ft) apart. This technique works well for recording large orchestras, because the large distance results in minimal crosstalk between the left and right channels. However, it is not very effective for recording small groups or individual instruments such as a piano. The three channels are derived as follows: LL and RR are panned left and right. The center channel is the sum of LR plus RL, reduced by 3dB. This method is sometimes called a multiple X-Y technique.3

Double A-B
Double A-B

Double A-B
The double A-B method is an extension of the spaced cardioid stereo technique. (The origianl A-B stereo technique used omnidirectional microphones.) It uses five cardioid microphones spread in a line across the width of the stage, with the distance between each mic in the range of about 2.5 m (8 ft). This technique reduces the multiple phantom sound source problem. However, this technique is only useful for large ensembles. The three channels are derived as follows: Left channel = LL + L (reduced 3 dB), right channel = RR + R (reduced 3 dB), and center channel = C + L (reduced 3 dB) + R (reduced 3 dB). This method is sometimes called a multiple A-B technique.3

      Ambience Arrays



These are specialized arrays designed to capture the ambience in a space. They are normally used in conjunction with one of the front arrays described above.

Double ORTF
Double ORTF

Double ORTF

The double ORTF is an ambience technique based on the ORFT (Office de Radiodiffusion-Television Francaise) stereo technique. It consists of four cardioid microphones placed in a rectangular formation, with front and rear arrays angled at 110 degrees from one another and 17 cm (7 in) apart. The rear array is 25 cm (10 in) behind the front array. It is essentially two ORTF configurations placed back-to-back. Compare with the IRT cross, which some people have claimed as being a double ORTF.6

Hamasaki Square

Hamasaki square
Hamasaki square

The Hamasaki square is a ambience technique developed by Kimio Hamasaki, a research engineer with NHK Science & Technical Research Laboratories (STRL), which is a division of NHK (Nippon Hoso Kyokai), the Japan Broadcasting Corporation, Japan's national public broadcasting organization. It uses four bidirectional (figure-eight) microphones arranged in a square, spaced about 1.8 to 2 meters (6 to 6.5 ft) apart, facing outward with the microphone nulls pointing toward the main sound source so as to minimize the pickup of direct sound as well as the echoes from the back of the space. Ideally this array should be placed toward the back and high in the space. The back two microphones are mixed to the surround channels, while the front two channels are combined with the front left and front right signals. This technique is less sensitive to the distance between the main array and the ambience array than other ambience techniques. 9

IRT Cross

IRT cross
IRT Cross

The IRT cross is a ambience technique developed by the German-based IRT (Institut für Rundfunktechnik — Institute of Radio Technology). It consists of four cardioid microphones placed in a square formation, angled at 90 degrees from one another, 45 degrees off axis from the sound source, and 20 to 25 cm (8 to 10 in) apart. The outputs from these mics are routed to left, right, left surround, and right surround at an appropriate level compared to the front mic array. The main disadvantage of this approach is that it has a significant amount of direct sound. This technique resembles back-to-back near-coincident stereo pairs. Although some people have called it a double ORTF, it does not use the ORTF angle of 110 degrees and it is slightly larger. 9

      Combined Front and Rear Arrays



INA-5
INA-5
(Ideal Cardioid Arrangement

These techniques are complete surround sound techniques that consist of both the front array and the rear ambient array.

INA-5 or Ideal Cardioid Arrangement (ICA) Surround

INA-5 is a surround sound microphone technique that is an extension of the three-channel INA-3 system developed by Günther Theile. It uses five cardioid microphones with the center microphone placed 17.5 cm (7 in) in front of two cardioids for left and right channels, pointing outwards. The left and right microphones are spaced 35 cm (14 in) apart. Two additional rear cardioid mics are placed about 60 cm (24 inches) behind the front line, and 60 cm (24 in) apart, with an angle of 150° from the center mic. Distances between the front three microphones can be changed to adjust the ambient response.8

Fukada Tree

Fukada tree
Fukada Tree

The Fukada tree was developed by Akira Fukada, a recording engineer with NHK STRL. The front array consists of three cardioid microphones arranged similar to a Decca tree, with two cardioids spaced 1.8 meters (6 ft) apart, and the center microphone placed 1 meter (40 in) forward, producing a strong center image. Two additional omnidirectional microphones can be added on the left and right to enlarge the perceived image size or to better consolidate the front and surround channels. The rear array consists of two cardioids spaced about 1.8 meters (6 ft) apart. The left and right mics are angled at 45° from the center microphone and the rear mics at 135°. The surround microphones are positioned at the critical distance, the point at which the direct sound and reverberant field is equal. The front array should be placed several meters above and behind the conductor.2

Corey/Martin Tree

Corey/Martin tree
Corey/Martin Tree

The Corey/Martin tree is a surround sound microphone technique designed by Jason Corey and Geoff Martin consisting of three subcardioid microphones facing the front, and two cardioid microphones behind them facing the ceiling. The distance between the left and right front mics is 120 cm (48 in) with the center mic halfway between them. The rear surround cardioids are 30 cm (12 in) apart and about 60 cm (24 in) behind the front pair. The center microphone can be moved forward up to 15 cm (6 in), if needed. These distances can be adjusted depending on the size of the ensemble being recorded. A front spacing of 120 cm (48 in) is used for smaller ensembles, increasing to 180 cm (6 ft) for larger groups. However, with wider spacing the three front channels become more incoherent.8

A variation of this technique, developed by Mikkel Nymand, is called the Wide Cardioid Surround Array (WCSA). It uses matched cardioid microphones. This array is supposed to create an intense, dynamic, and enveloping sound character.4

Hamasaki System

Hamasaki surround
Hamasaki Surround

The Hamasaki surround system, which also called the NHK surround system, was developed by Kimio Hamasaki, a research engineer with NHK STRL. It uses 5 cardioid and two omnidirectional microphones. A baffle is used to separate the left front and right front channels, which are 30 cm (12 in) apart and angled at 45 degrees. Two omnidirectional microphones, which are low-pass filtered at 250 Hz, are spaced 3 meters (10 ft) apart in line with the left and right cardioids. The center mic is placed about 30 cm (12 in) forward. The rear pair of cardioids are placed 2 to 3 meters (6 to 10 ft) behind front array and 3 meters (10 ft) apart, at an angle of 135°.2

Double Mid-Side

Double M-S
Double Mid-Side

The double mid-side (M-S) technique is the mid-side stereo microphone technique expanded to surround sound. It uses two cardioid microphones, one facing front and one facing backward, each shared with one side-facing figure-eight microphone. The recorded tracks can be decoded by sums and differences of the microphone signals into the left front, right front, left side, and right side. This technique also allows for post recording changes to the pickup angle. Many of the other surround sound techniques occupy a considerable amount of space, making them inconvenient for field recording. However, the double mid-side technique is quite compact.2

Polyhymnia Pentagon

Polyhymnia pentagon
Polyhymnia Pentagon

The Polyhymnia pentagon was developed by Polyhymnia International (a recording company formerly known as Philips Classics). Five omnidirectional microphones are arranged according to the loudspeaker arrangement defined in ITU-R BS775, with the recorded signal from each microphone being panned into a corresponding speaker from this recommendation. The five microphones are placed on a circle with a radius of 2 meters (6.5 ft). The angle between the center mic and the left and right mics is 30°. The angle between the center and the surround mics is between 100 and 120°, with 110° being the most common setting. The length of the radius can be adjusted depending on the size of the ensemble, as well as the size of a room.8

Optimized Cardioid Triangle (OCT) Surround

OCT Surround
OCT Surround

The OCT surround technique was developed by Dr. Günther Theile. It uses the OCT technique for the front array plus additional surround channel microphones. The front array is designed to minimize crosstalk, by using supercardioids for the front left and right microphones spaced 40 to 100 cm (15 to 40 in) apart, angled at 90 degrees from the center cardioid microphone, which is placed about 8 cm (3 inches) forward. Two additional rear cardioid mics are placed about 40 cm (15 inches) behind the front line and an additional 20 cm (8 inches) farther out, pointing toward the rear. It produces good separation in the front, good localization, and a large listening area with a natural-sounding effect. 9

OCT+Hamasaki
OCT+Hamasaki Square



Optimized Cardioid Triangle and Hamasaki Square
(OCT+Hamasaki)


This is a surround sound microphone technique that combines the optimized cardioid triangle (OCT) technique front array with a Hamasaki square for the ambience channels. The OCT uses three microphones in a triangle with a center cardioid mic facing forward and two side supercardioid mics facing outward, similar to a modified Decca tree. The Hamasaki square uses four bidirectional (figure-eight) microphones arranged in a square, each facing outward with the microphone nulls pointing forward.



Optimized Cardioid Triangle and IRT Square (OCT+IRT)

OCT+IRT Cross
OCT+IRT Cross

This is a surround sound microphone technique that combines the optimized cardioid triangle (OCT) technique front array with an IRT cross for the ambience channels. The OCT uses three microphones in a triangle with a center cardioid mic facing forward and two side supercardioid mics facing outward. The IRT cross consists of four cardioid microphones placed in a square formation, angled at 90° from one another and 45° off axis from the sound source.



Omni+8 Surround

Omni+8 Surround
Omni+8 Surround

This technique is the surround version of the Omni+8 technique. It consists of two omnidirectional microphones spaced 2 meters (6.5 ft) apart for left and right, and a figure-eight microphone for the center placed 40 cm (16 in) forward. The left and right surround mics are omnidirectional microphones placed 4 to 6 meters (13 to 20 ft) behind the front array, and 2 meters (6.5 ft) apart. If there excessive room reflections, bidirectional (figure-eight) microphones may be subsituted for the surround channels.10





Cardioid Trapezoid

Cardioid Trapezoid
Cardioid Trapezoid

The cardioid trapezoid, known as the cardioid trapezium in the UK, is a surround technique especially designed for live pop music concerts, which do not require pickup of side reflections. It consists of four cardioid microphones facing backward, angled at 60° from one another, and 60 cm (2 ft) apart, in the shape of a trapezoid. It is effective for the pickup of both audience and ambience. Developed by Günther Theile, it is also known a Theile trapezoid.

      Surround Microphone Assemblies



This category of surround sound techniques consist of microphones with multiple capsules mounted at various angles.

Soundfield Microphone
Soundfield Microphone

Tetrahedral Microphone

The tetrahedral microphone a microphone consisting of four closely spaced cardioid microphone capsules arranged in a tetrahedron, invented by Michael Gerzon and Peter Craven. It can function as a mono microphone, a stereo microphone, or as a surround sound microphone. The microphone produces four signals, called the A-Format that must be processed via software or hardware into a second set of signals called the B-Format. It is part of the Ambisonics surround sound technology, but is not exclusive to it. It is sometimes called a SoundField microphone, which is a trademark currently owned by RØDE. Beside RØDE, Sennheiser also manufactures a tetrahedral microphone.10

Ball Boundary Microphone

ball boundary microphone
Ball Boundary Microphone

The ball boundary microphone consists of two bidrectional (figure-eight) microphones placed on each side of a sphere microphone facing the front. A sphere microphone has two condenser microphones flush-mounted in each side of a spehere to simulate a human head. The various channels are derived by combining the various signals as follows: Left = Ls + Lbi, Right = Rs + Rbi, Left Surround = Ls - Lbi, and Right Surround = Rs - Rbi, where Ls is the signal from the left sphere microphone, Rs is the signal from the right sphere microphone, Lbi is the signal from the left biderectional microphone, and Rbi is the signal from the right biderectional microphone.10


Spherical microphone array
Spherical
Microphone Array

Spherical Microphone Array

The spherical microphone array consists of a sphere into which are mounted a number of microphone capsules (as many as 24) pointed in a variety of directions, that is used to capture three-dimensional immersive sound of a live performance at a single point. The array includes processing to convert the multichannel sounds into some form of 3-D surround sound.

      Summary



When recording an orchestra or choir, it is fairly easy to try several stereo microphone techniques to see which is best for a given situation, simply by changing the spacing, the angle, or the type of microphone. However, the surround sound setups are much more complicated, using in most cases five to seven microphones that can take a considerable amount of space in a venue. The choice may be determined by the available space. However, most of the techniques to allow for some adjustment of spacing to obtain the best sound and to accommodate larger or smaller ensembles. Unlike the stereo techniques, there is no consideration of mono compatiblity.

If you are making surround sound recordings in the field, many of these techniques are too unwieldy. In that case, one should consider the double M-S or one of the microphone asemblies. While these techniques may not provide as spacious a sound, they are much easier to haul around and setup.

Last Updated: 09/01/2018


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