The following plot shows the superposition of two virtual line source impulse responses. These were measured on axis on a RD50 with a one-sided baffle. The distance from the center of the ribbon to the edge of the baffle was 25.75" The total width of the assembly was 28.25". The total time interval in the graph is 6 milliseconds.
The top (red) response was measured with the microphone at 3 feet height at a distance of 6 feet. The bottom (white) response was measured with the microphone at 3 feet height at a distance of 3 feet.
The main pulse was positive going. Both measurements show a lot of diffraction impulses in the first millisecond. They are typically in on axis measurements for the slit diffraction and diffraction over the "horn" of the BG Radia series drivers. Additional diffraction can be seen from the boundary of the ribbon "horn" with the thin baffle itself. This particular baffle had the diffraction "maximized" for educational purposes.
Time analysis shows peaks for the 6 feet trace (red) at 2.29 milliseconds after the main impulse. A second "mystery" impulse can be seen at 5.6 milliseconds. The 3 feet trace (white) shows the first diffraction impulse at 2.52 milliseconds and the second "mystery" impulse stays at 5.6 milliseconds. Observe the polarity of the impulses.
Observe also the bumpy character of the first impulse instead of being a snappy impulse. Main reason is the fact that the diffraction impulse is measured off axis. Let's not forget that the energy content of this impulse has been measured to be mainly situated below 1kHz.
Geometry shows that in the 3 feet distance to the ribbon case one can expect the diffraction impulse 2.51 milliseconds after the main impulse. The measured impulse delay is 2.52 milliseconds.
The 6 feet measurement calculates for a diffraction impulse delay of 2.23 milliseconds. The measured delay after the main impulse is 2.26 milliseconds.
In both cases there is very good correlation between measured and calculated response taking into account measurement inaccuracies.
The sharp impulse seen in both plots at 1.41 milliseconds is the reflection of the microphone mount on the tripod, hence the stationary appearance.
What about the stationary "mystery" impulse occurring at 5.6 milliseconds after the main impulse. ? This impulse is also a diffraction impulse, be it diffracted twice. The first diffracted impulse at the edge of the baffle bends around the baffle , travels opposite direction and will diffract again at the edge of the ribbon driver. The distance traveled by the diffracted sound waves is pretty much constant for all measurement distances since the microphone is on axis with the ribbon, only 2.5 inches off-axis of the final diffraction edge. The calculated delay for this particular set-up was 5.4 milliseconds. Still close to the measured time interval of 5.6 milliseconds. It is my opinion that the soundwaves don't take the shortest possible path when bending around the baffle after the first diffraction.
The mystery impulse is measured on axis and shows more high frequency content as compared to the first diffracted response.
Copyright (c) 1997-98, by Rudi A. Blondia, ALL RIGHTS RESERVED. Last update: February 18, 1998