Stereo Microphone Techniques
Guide to the Pros and Cons of Different Methods of Stereo Recording
Mar 24, 2009
No one solution to placing stereo microphones is right for all situations. Techniques fall into two classes, spaced and coincident.
As a generalisation, spaced microphone techniques offer a wide soundstage which can lack precise imaging and definition in the centre. They are also problematic if the signal is mixed to mono, where the intermicrophone delays can give a filtering effect akin to flanging.
Coincident Microphone Stereo TechniquesCoincident microphone techniques try to place the microphone capsules as closely together as possible, deriving from the approach adopting by Alan Blumlein in the early 1930s.
Since the microphones are close together, there is no time difference between sound sources getting to them, and stereo information is encoded as intensity difference between the channels only.
- Blumlein stereo. Two figure of eight microphones are mounted one above the other facing 90 degrees apart (one 45 left, the other 45 right). This can sound excellent, though it picks up a lot of room acoustic and has some issues with out-of phase signals from the back.
- XY stereo. Two cardioid microphones are placed one above the other angled 135 degrees apart. This picks up a wider stereo image from closer in than the Blumlein arrangement. Typically an orchestra would be recorded from where the conductor stands using XY, so less room ambience gets recorded.
- Mid-Side stereo. A cardioid microphone is mounted facing forwards, and the signal is combined in a matrix with a figure of eight microphone placed to pick up from either side. This is used a lot in broadcasting.
Coincident microphone placement tends to have sharper stereo imaging to the central positions, and there is no problem mixing to mono, hence they are often preferred by broadcasters.
Spaced Microphone Stereo Techniques
Spaced microphone techniques separate the microphones, leading to time differences between a sound impinging on the microphones, so there are phase differences as well as amplitude differences between the left and right channels.
- Spaced OmnisOmnidirectional microphones are rigged on a baseline between them of typically 50cm to 1 meter.
- Decca TreeAs spaced omnis above, with a typical 2m baseline. A third omni is placed 1m in front of the baseline in the central position, and mixed in equally to the left and right.
- ORTF stereo.A short baseline of 17cm is used, and cardiod microphones are placed with 110 degrees between their axes.
- NOS stereo.As ORTF but with a baseline of 30cm and an angle of 90 degrees between the microphones.
- Binaural Stereo.Small omnidirectional microphones are worn in the ears of the recordist, or placed in the ears of a dummy head. A variant of this is the Jecklin Disc where a circular baffle is placed between two omnidrectional microphones 36cm apart. Binaural stereo works very well listened on headphones but less well on loudspeakers. The head-worn microphones are unobtrusive. This technique has a lot to offer stealth field recordists, podcasters and concert tapers.
Some listeners prefer the sound of spaced techniques on orchestral music, and a lot of classical music recording was based on spaced omnis. Some of that preference may be because omnidirectional microphones tend to be the most neutral sounding microphones, but there is no coincident microphone technique available for omnidirectional microphones, since two coincident omnis have no way of discriminating direction.
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