The next measurements were taken again on axis. This time with the diffracting edge of the baffle and not the driver. The microphone is still perpendicular with the baffle. The geometry is different as will be seen in the plots. The timings however confirm the occurance of the diffraction and the fact that the diffracted energy folds over and diffracts at least one more time at the driver's side edge.
The red trace is measured at 6 feet, the white trace at 3 feet. Height of the microphone is 3 feet of the floor. (The microphone mount echo is again present int he signal ;-)
Because of the different geometry, the time alignment of all "firing" sources is different. The folded over diffraction needs to be triangulated with the microphone position, so the second pulse travels this time also. The first pulse travels because the driver needs to be triangulated with the microphone position. So first and second diffraction impulse are linked in this geometry.
The first diffraction signal is harder to catch in the red trace. It calculates at 1.6 milliseconds and it is there, the jump down followed by the high frequency baseline signal. This impulse shifts to 1.3 milliseconds (at a measuring distance from 3 feet) in the white trace. This impulse is merged with another stationary echo, but the time relation ship is still correct. Same for the second impulse that shifted also 0.3 milliseconds.
Thee is a slight time discrepancy for the second impulse in that it occurs later in all cases than calculated. Implies that the speed of the soundwaves slows down each time they are diffracted.
Copyright (c) 1997-98, by Rudi A. Blondia, ALL RIGHTS RESERVED. Last update: February 19, 1998