Scientific Techniques for Spacing and Angling of Stereo Microphones

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In order to get the perfect stereo effect from multiple microphones during recordings, placing of the devices is very important. Spacing and angling are two important concepts that can help you get the desired effect.

How do They Help?

The human ear perceives sound images from a combination of mics and the brain reconstructs the sound images to form an amalgamated sound. The time and sound level differences can be used in various ways to act together to create a lifelike realism.

Types of Techniques

There are many popular methods used with stereo miking. Each has its own acoustic benefits that you must understand to select the technique that suits your requirements the best. The following techniques are most common for two or more microphones:

1. Coincident pair

Here, the two directional mics are placed with their grilles touching, with an angle between the two, and their diaphragms one above the other. A wider angle symmetrical from the centre line means a wider uniform stereo effect. Popular variants include the Blumlein array that uses 90-degrees between the two mics, but other types use angles even up to 180-degrees. There are no phase cancellation problems in this case. This type provides great stereo localization and mono compatibility.

2. Near-coincident pair

The recording is done when the two mics are placed at an angle, but with some distance between them, in a bid to replicate the distance between the two human ears. Some phase differences creep in, and the mono compatibility is lesser than the coincident pair. Widely used in the ORTF method that places two cardiod mics at 17 cm apart and 120-degree angle.  The NOS changes the scape to 30 c, and the angle to 90-degrees, and the DIN it to 20 cm/ 90-degrees.

3. Spaced pair

This technique places the two mics a large distance (3 to 10 feet) apart, pointing straight ahead. Here spectral differences also come into play and the image produced overall is diffused. Any polar patterns can be used, but omni is most popular. This is good for a wide stereo spectrum, as in the case of an orchestra. Sometimes a third mic is also placed in between and mixed later.

4. Baffled pair

This technique inserts a hard and/or padded baffle between the two omni mics, usually spaced human head apart to add time differences. The baffle reduces the sound from the opposite direction and promotes channel separation. There are spectral differences as well. One variant uses a dummy head as a baffle with mics fitted on either side for recreating the performers location and the acoustic environment accurately. Others use a disk, sphere or an angled wedge along with a foam covering.

Usually the best results are produced when the two mics are matched for sharp imaging. The matched polar patterns, sound levels and frequency responses are widely used. The transducer types should also be taken into consideration for the chosen technique. The uniformity with frequency for the polar patterns and the off-axis phase shift is a must for the best results.