Following the first day of outdoor measurements with Rudi Blondia, I started creating different series of dipole baffles. Both classic two-sided and the new one-sided baffles were measured in my listening room. For all the Listening Room measurements a 2nd order high pass active filter crossing over at 150Hz (-3dB) was part of the measurement circuit. These measurements are semi-anechoic on a ground plane, but are short duration (7mS), and influenced by wall (sonic images) effects in the listening room.
Frequency response measurements were made at a 2 meter distance on 11 different variations of rectangular and trapezoid baffles. These baffles were all of the classic 'symmetrical' variation, in which the driver is mounted centered in the width of the dipole baffle.
A significant feature of the baffles is that most are 'thick', i.e. the same thickness as the RD75 driver, which is about 1.5" thick. This feature enhances anti-diffraction qualities of the baffles.
The result of this portion of the RD75 Dipole Baffle Study was the identification of "John's Bump", so labeled by Rudi Blondia. The 'bump' is a plateau in the frequency response of the RD75 driver below about 800Hz when it is mounted symmetrically in a 'two-sided' dipole baffle.
The origin of the 'bump' appears to result from the additive effects of virtual line source signals that emanate from the baffle's edges.
Rudi Blondia identified the significant effect upon the frequency response of the RD75 driver when it is mounted in a so-called 'one-sided' dipole baffle. This discovery came as an evolutionary response to the identification of "John's Bump" in the data on the 11 two-sided baffles above.
A one-sided dipole baffle mounts the RD75 driver at the extreme edge of one side of the baffle. The exposed mounting flange on the RD75 driver is finished as a 1/2 round section. Significant improvements in frequency response are seen - John's Bump disappears in one-sided dipole baffles.
A rethinking of the measurement possibilities in my listening room results in relatively clean 7mS duration measurements at 1m, 2m, 3m and 3.5m.
The Listening Room Measurements - The Active Filter Response.
The Gymnasium Measurements - Comparison to Listening Room Data.
The RD75 Dipole Baffle Study - Table of Contents
Acoustic Line Source Research - Table of Contents.