As I said, this is from a Tech paper from Speakerlab.

From Speakerlab's Technical Compendium TC-10 1979, by Pat Snyder

This paper, as well as about a half inch thickness of other technical papers, were what Speakerlab sent out as their "Speaker Design Book". Most were written by Pat Snyder, one of their engineers, and were papers presented at AES meetings.

If you wish, email me. I'll try to scan it, but it is printed on green paper. I'll see if I can lighten it up enough to read. I quoted Mr. Snyder accurately.