The Dipole Baffle Study has it's anniversary on May 17th. That was the day Rudi Blondia and I met for the first time, and we measured 4-BG RD75s and 2-Carver 60" planar-magnetics in my backyard. It's been a busy year, just ask Rudi or me. There have been moments of intense frustration followed by great insights into dipole operation.
My initial study of different 11 baffles last June/July led to the identification of what Rudi has generously labeled "John's Bump". Determining the origin of the bump led to Rudi's insightful analysis and identification of the Virtual Diffraction Line Sources (VDLS). VDLSs profoundly affect the operation and performance of large line- source dipoles. He provided direction for exploration of a new type of baffle (for me) - the 'One-Sided' baffle. The One-Sided baffle places the RD75 at the extreme edge of a dipole baffle, with the speaker side not attached to the baffle usually having a minimal diffraction treatment.
Instrumentation and measurement issues continue to vex and plague. The rude realization that livingroom measurements have their limitations resulted in the seeking of larger venues in which to measure. The idea being, that it's necessary to know the (semi-) anechoic response of a system in order to fully evaluate it's performance in a reflective environment such as a normal sized listening room. That is, how does one separate room response from speaker response.
Starting on January 2nd, access to several large-scale measuring venues led to efforts on my part to create a database of different dipole baffle measurements using a RD75 driver, for future study and modeling use. The search for the 'optimum', most linear dipole baffle shape, has continued. Insights into what 'optimum' means in actual usage have been gained.
On May 3rd, Rudi accompanied me to a new venue, the Auditorium. This facility provided means for acquisition of clean tutorial data on the phenomena that led to the indentification of "John' Bump". Free air measurements were made on a bare RD75 and examples of a Two-Sided and One-Sided baffle. Those measurements were repeated with the driver and baffles on a ground plane. They are directly comparable, and reveal many important and interesting facts about dipole operation. Presumably these results would also apply to any other large floor-standing line-source driver used in such a manner (are there any others available to DIYers)?
A year ago I promised to the Bass List a year's worth of research in this regard. That year has ended. Starting next week I will present the new information in a series of Dipole Baffle Study posts, with references to data and photos at my website, as a year-end report of this research project. The culmination will be the presentation of a new compound-shape baffle that appears to have the greatest linearity of any baffles tested so far.
Other projects demand attention, and there is the siren call of my 10 year old 72" true ribbon project - it's past time to finish it. Whether this is an end-game or merely a hiatus I do not know at this time.
Regards,
John Whittaker
The Gymnasium Measurements - The RD75 Dipole Baffle Database
Dipole Baffle Study Report#17
The RD75 Dipole Baffle Study - Table
of Contents
Acoustic Line Source Research -
Table of Contents.