Stereo microphone techniques
I usually think that when a recording for an instrument, an ensemble or an orchestra is made, many of us are not very sure of which stereo mic technique to use or we just use the same one for everything. I would like to share with you the knowledge I have about this techniques for you to experience them in your recordings.
I have no intentions to convince you that any configuration is better or worse than another. I just think that we need to choose the one that works best according to the circumstances, considering all factors from budget or the available equipment to the music style, etc.
There are 4 basic elements to consider when choosing a technique:
1. Microphone polar pattern
2. Distance between the microphones (d)
3. Angle between the microphones
4. Distance between the source and the recording system. (D)
Some of the best-known stereo recording systems come from the combination of this 4 elements (it's possible that all techniques you know are not listed here)
1. XY (Blumein)
3. Stereo equivalent (ORTF, NOS, EBS)
There is a relationship between the position in which a virtual source appears between a pair of speakers and the difference in sound intensity (dB) for a stereo signal. This difference is achieved in stereo recording systems by the four elements previously discussed: polar pattern, distance and angle between the microphones and the source. (remember that for now we are always talking about stereo recording techniques)
For example, we know that to get a virtual source to be 100% towards one of the speakers the difference must be 18dB (or 1.5 ms), 75% must be 11dB, 50% must be 6.5dB, for 25 % must be 3dB and 0dB to be completely at the center.
These differences in level (dB) or in time (ms) can be achieved by manipulating either the distance between the microphones, the angle between the microphones or both. So the sound that reaches each of the microphones' capsules of the system, generates differences in level or time that are translated into different images in the speakers with different positions of the virtual sources and different image widths.
For example when the microphones are brought closer to the source, the image becomes wider in the speakers. Or if I decrease the angle between the axes of the microphones in a XY system, the image becomes smaller because the recording area is wider. In the same way, we can see image differences between each of the systems AB vs XY vs equivalent.
Specifically if we compare an XY system with a cardiode polar pattern to an AB one we could listen: