I thought with the upcoming release of the Diamond Jubilee 75th anniversary and of course the Fully Horn-Loaded, Vented Subwoofers KPT-1802-HLS and rare KPT-1502-HLS that some might be interested in reading the patent.   Note that it was filed on July 22, 1997 and the patent date of April 27, 1999.   Here is the PDF:  US5898138.pdf   miketn