Stereo Recording, XY coincident microphones

 

Stereo recording XY coincident

Whether you record live or in the studio, stereo recording can heighten the realism of your productions because simply put, we humans have two ears, so a stereo recording sounds more natural to us than mono. It's therefore pretty obvious that most stereo-miking setups grew out of attempts to approximate the positioning and functioning of our ears and over the years there have been a number of interesting stereo microphone configurations developed, all aiming to give the listener a realistic representation of the sound environment in front of them. As with everything some are more realistic than others, though of course sometimes its actually preferable to be less realistic and more flattering as nowhere is the room or location more important in shaping the sound. There are numerous forums where people will argue into oblivion about their preferred method and how every other set up is flawed but pragmatically which set up I favour on the day is down to the quality of the sound source and the room. I honestly feel that the forum experts should get out and do some more recording instead of talking about it. To start stereo recording you will need a pair of microphones to represent your left and right ears and the type of mics and their relative positions have led to 5 main stereo recording configurations, each with strengths and weaknesses.
vocal mic

The first and most obvious stereo recording set up is an XY configuration where a coincident pair of cardioid microphones are positioned so that their capsules are as close to each other as possible without touching. The angle between the capsules (called the mutual angle) is usually set between 90 and 135 degrees, with the greater angles creating a wider stereo picture. Directional, cardioid microphones are most sensitive to sound coming from directly in front of the capsules (on axis) and are less sensitive to off-axis sound. In other words, the microphone "hears" lots of sound coming from the front and much less from the sides. Therefore, sound sources positioned in the centre of the angle between the two mics will appear in the centre of the stereo field, whereas sources off to one side or the other will appear more on-axis to one microphone and thus will be reproduced louder on that side.
The XY setup generally gives you a workable but narrower, more focused stereo image than other stereo-miking techniques and there is often a 3 dB drop in the centre of the image as the centre sound information is hitting both mics off-axis. I have had some pretty successful results with a crossed pair of Rode NT5s recording small choirs and instrumental groups and you certainly can get a convincing stereo picture but it's only when you compare it to other techniques that you feel its perhaps a bit pinched or restricted. On the plus side any one can now afford to buy a pair of decent quality condenser mics and you can either use two mic stands or fit them easily onto a stereo bar on one stand. Rode have taken the idea to its logical conclusion and produced the NT4 mic which has the two capsules engineered at 90 degrees and mounted on a single mic body. It is really good value and very popular particularly as a stereo camera mic. When you're recording multiple instruments, you can widen the stereo spread by arranging the players in a semicircle around the XY pair and XY pairs also work well for close-miking solo instruments particularly the acoustic guitar and piano.
Developments on the coincident XY theme have produced a number of near coincident techniques where a pair of cardioid mics are slightly spaced apart to create a wider more engaging stereo picture and perhaps the most effective is the ORTF configuration developed by French radio (Office de Radiodiffusion-Télévision Française) in which a pair of cardioid mics are angled at 110 degrees apart with 6.69 inches between the capsules (the average distance between your ears) in an attempt to mimic human hearing. I always prefer the sound of ORTF over straight XY and have a stereo bar set up with clips at the correct angles so that I can just clip in a pair of NT5s or more recently Hebden Sound 3020s. The great advantage for me with this system is that if you have a room that isn't particularly nice sounding you don't want to capture a lot of it and I am always happier to get a tight stereo picture and add some reverb back in the studio. (That strange sound you might hear will be the gasps of the purist nerds)
Another near coincident set up is the NOS (Nederlandse Omroep Stichting=Holland Radio) Stereo Technique which uses two cardioid microphones spaced 30 cm apart and angled at 90° to create the stereo image. I have tried NOS a few times but one of the characteristics is if used at larger distances to the sound source it loses the low frequencies due to the nature of pressure gradient microphones and the influence of the proximity on these type of microphones. However close in on something like a piano the NOS technique can sound pretty good. Try it against ORTF and see which you prefer.

 

 

 


 
 
Microphones and recording 2010. XY Coincident Microphones, Stereo Recording