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Recommended distance of a boundary behind a PCC-160

I have heard that when using PCC mics (specifically the PCC 160), that in order to properly create the polar pattern you should leave about 8 inches of boundary space behind the microphone so that sound can cancel internally. Based upon my understanding of how the PCC works and the work I have done with them, there is no need for the boundary space behind the mic, and the supercardiod pattern will exist normally on its own.  Is it true that you need this rear boundary space, and if so can you explain how it works with the mic?
Tom Blanford

Reply:  By "boundary space," do you mean the distance from the mic to the nearest vertical boundary behind it? Or do you mean, the length of the horizontal boundary behind the mic? If you mean the latter, a PCC-160 at the edge of a stage floor would have no boundary space behind it. In that case, the mic capsule's supercardioid polar pattern should not be changed by the lack of a boundary behind it. I have not actually measured a PCC-160 in that situation, however.

If "boundary space" means the space between the PCC and the nearest vertical boundary behind it, the results are different. I'll explain this below.

In general, to avoid degradation of the PCC's polar pattern by a vertical boundary panel behind the PCC, the distance from the panel to the PCC should be at least 2 times its height. So a 4" high panel should be at least 8" behind the mic. That will prevent the panel from degrading the mic's polar pattern. You can have less spacing between the panel and the mic if you are willing to live with the altered performance.

Please see the figure below. I measured the anechoic rear rejection of a typical PCC-160 without a panel behind the mic, then measured it with a 4" high panel placed 8", 4" and 0" behind the mic.

Results: The closer the panel is to the PCC, the worse is the rear sound rejection. That's because the sound waves are forced to enter the mic capsule more from the side than from the rear. The side attenuation of the PCC's supercardioid capsule is less than its rear attenuation.

For example, if a 4" vertical panel is 8" behind the PCC-160, the rear rejection at 200 Hz is 14 dB. If the panel is touching the mic, the rear rejection at 200 Hz is 8 dB. That might be a difference you can live with.

On a stage floor, the PCC-160 should be placed as close to the actors as is practical to maximize gain before feedback. That requirement might result in the PCC being 8" or more from the stage edge. But the 8" horizontal boundary space is not required.

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