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RE: Two microphone stereo recordings

Posted by tomservo on May 31, 2012 at 06:28:27:
Hi Tony
This is a topic I love!
It is true, if you take two microphones there is no combination of spacing that permits the genuine item to be captured accurately.
The Blumein technique comes a lot closer, it is a special case of two vertically stacked elements (with open backs usually) and unlike a simple pressure mic have a figure 8 pattern.

This does two important things, one, it is “mono compatible”, a single sound source produces the same signal phase / time on both mics.
This means you can sum them and not have all kinds of comb filtering that is present when you sum two or more mic signals that are in separate locations. In the horizontal plane, they are in the same location in time.
Two, the figure 8 pattern provides amplitude shading according to horizontal angle and this is why a signal to the left is louder on the left channel. Two pressure microphones in very close proximity produce the same signal but have no directional discrimination.

The weakness in that design is that that each mic has a figure 8 pattern while what you would like to capture is only what is in front, not the rear. Also, the shape of the figure 8 only allows a fixed degree of amplitude shading vs angle and so is limited to a two channel system as is used currently. Cardioid mics can be satisfying but their patterns change a lot vs frequency.

The microphone array thing I am working on is along this line but done an entirely different way.
It allows the sound to be detected as if it were from one point in space but can be divided into a large number of channels if needed.
Also, where the Blumein is not coincident in the vertical plane, this can be made to capture a full hemisphere from one point in space.
As you enjoy “stereo” image, please pop on a set of headphones and try a couple of the recordings they put on the web site at work.
These have no compression so you may have to turn the volume up to get to a real sounding level. Try the Harley or Trains and keep in mind this is a work in progress and only the front image, more or less the width of your vision.
Let me know what you think.
Best,
Tom
Recordings at bottom of the page;
http://www.danleysoundlabs.com/technical%20downloads.html