In context of our mutual membership in the internet-based "Dipole Forum," I exchanged with Mr. Leslie Williams both the ex-Bohlender-Graebener employee's statement above and the plots on Page 3 of this report. On January 11, 2003 he kindly replied back to me with commentary and plots based upon measurements of the RD-75 variant driver by the Wisdom company in a custom transmission line baffle of his design. Below is a copy of the CSD plot and the relevant comments to the CSD plot he sent:

ETF derived CSD plot of TLD baffled 6-element Wisdom driver.

"So, hoping that it's not too foreign to your interest, I enclose some measurements (5) that I have from a take of the Wisdom in situ with TLD baffle via ETF software tool instead. Quite different from an RD75 in free air via IMP, but real. I cut the gate to what I thought you were using and tried to get similarity to your IMP plots."

"The driver is the 6 element Wisdom - without the passive crossover hooked up (hence the low pass). The mike is 2.5m away in a very well treated room. Baffle is transmission line dipole, whose front face is 5" wide."

[The]" high frequency decay (Decay) shows nothing of the 2khz aberration I see in your IMP plot. I see some evidence of the 5.5khz cavity, but to my eyes it seems low. Foster said that this had dropped dramatically vs the RD-75. I do note some problem ~15khz."

Leslie Williams, January 11, 2003.

The above ETF derived CSD plot of the Wisdom-based TDL system, showing the plethora of decay ridges starting at relatively low amplitude and duration after the impulse, really got to me. The fact that raising the screen gain in the IMP/MLS software to x5 from x1 showed so much more of the contaminating reflections made me want to go back and and raise the screen gain again to really get a handle upon the now evident 'reflection problem' in CSD FFT data processing.

RD-75 3m free air suspended impulse at a Screen Gain of x5, and 6.4mS duration.

On January 24, 2003 I sent Mr. Williams a long HTML-based e-mail message containing many plots including the impulse plot at left in response to his January 11 e-mail and the CSD plot above that he'd sent.

This impulse plot is important since the x5 screen gain windowed upper plot clearly shows the microphone stand reflection starting at approximately 2.59mS after the beginning of the main impulse. Furthermore, it can be seen that the microphone stand reflection is preceeded by about a millisecond's worth of serious ripple. The relatively undisturbed portion of the data after the impulse dies out only extends to about 1.66mS after the beginning of the impulse.

This is not good at all, horrible in fact; but, the best is yet to come!

Dipole Baffle Study Report#20.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 1.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 3.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 3.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 5 - NEXT PAGE.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 6.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 7.
Dipole Baffle Study Report#22.
The RD75 Dipole Baffle Study - Table of Contents
Acoustic Line Source Research - Table of Contents.