Why is it important to use a matched mic pair for stereo recording?

Every manufacturer has production tolerances on finished products to allow for variances in purchased and manufactured parts, as well as in manufacturing processes. Microphone manufacturers are no different. The tighter the tolerance, the smaller the yield of acceptable parts, and so the higher the price of the finished product. It's a tradeoff between price and tolerance.

Crown's microphones are mid-priced (about $250 street price for a Crown CM-700 cardioid condenser mic). Our tolerance of +/- 2 dB for sensitivity and frequency response reflects what we are willing to release based on the selling price.
Matched pairs of CM-700 mics (CM-700MP) are matched within +/- 0.75 dB in sensitivity and frequency response. This permits sharper phantom imaging and less left-to-right image shift than if the mics were within +/- 2 dB. To create a matched pair, we measure several units and match them with software. This extra step increases the cost of the microphone pair.

Matched frequency response is important when you are using a main stereo mic pair and you want the sharpest possible imaging. The more closely the left and right mics are matched in frequency response, the better the image focus or localization. For example, suppose the left mic is +5 dB at 150 Hz relative to the right mic. For an instrument in the center of the musical ensemble, its reproduced low frequencies will shift partly toward the left, while mid frequencies will remain in the center -- causing some spread or blurring of that instrument's reproduced phantom image.

Matched sensitivity is important when you want center instruments to be heard from the center between your stereo speakers, rather than shifted slightly left or right of center. A level mismatch also can change the musical balance between violins on the left and basses on the right, for example. (However, you can compensate for mic sensitivity differences with your recorder's level controls or mixer's pan pots).  

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