In January 1998 the Study entered a new phase when the use of a very large high school gymnasium became available for testing. This venue provided the opportunity for 'semi-anechoic' measurements (on a ground plane) of great precision - and to the acquisition of distance measurements out to 8 meters. A much clearer understanding of dipole baffle operation is emerging, with the phenomena of 'moving dips' in the frequency response having been identified through measurement.
The "Introduction" below was written on March 25, 1998, and the situation is somewhat changed since that time. Significant elements (moving dips in frequency response ) of the RD-75 Dipole Baffle Study have not yet been posted to this site (but were released to BassList members in private communications). As time permits, I will attempt to recast the Introduction in light of the passage of time. [JRW 6/17/2001]
Prior to January 2, 1998 all measurements had been made in my backyard or in my listening room. Contained here is the record of those measurements made in the listening room as converted into test chamber. The first 'two-sided' baffle series only had measurements at 2 meters. Then I found a way to make measurements out to 3.5 meters distance for the 'one-sided' baffle series. The 2nd order high pass active filter was used in all of the listening room measurements - it's response should be considered as part of the frequency responses shown in this section only.
Sixteen baffles are detailed with frequency response plots and photos in this section.
The Gymnasium's great size allowed for very long period data acquisition times without sound reflections in the data. This gave very clean 'semi-anechoic with ground plane' measurements for the large dipole baffles. Distances out to 8 meters at different heights are measured.
Several important theoretical issues were either identified or confirmed in the course of the gym measurements. The -3dB fall in amplitude per doubling of distance for a line source was confirmed to an 8m distance. The phenomena of 'Moving Dips' in the frequency response, as a function of both height and distance from the baffles, was identified
The RD75 Dipole Baffle Database contains comprehensive distance and height measurements for the baffles which were measured in this very large venue.
The Auditorium measurements were the last large venue measurements made on the RD75 driver and associated dipole baffles. Access to both the Auditorium and Gymnasium was denied after that date, and it was decided to terminate the Dipole Baffle Study at that point.
Although three new 'Equal Area Baffles' were fabricated and ready for measurement at that time, including the new Compound Shape Baffle, it was decided that the measurement session would be devoted mostly to primary theoretical measurements in which free air measurements of the bare driver and several dipole baffles were compared with equivalent groundplane measurements. These measurements represent a stunning confirmation of the differences between the one-sided and two-sided dipole baffles; and, as such have a significant theoretical relevance for line source drivers in dipole baffles.
Limited measurements were taken on the new three-dimensional Compound Shape baffle. They indicated that it represents a breakthrough in frequency response linearity compared to the previous 'flat' baffles. Subsequent subjective testing showed that the Compound Shape baffle was relatively diffractionless and generated a superior soundfield. It is unfortunate that the denial of access to the large measuring venues ended the RD75 Dipole Baffle Study, but the depth and breadth of the information contained therein should be sufficent as a guide to those who would create their own RD75 dipole baffles.
The following pages are slight reworkings of project reports to the DIY Loudspeaker List.