Alvin Foster, Boston Audio Society, owns an IMP/MLS measurement system, from Liberty Instruments, in addition to more advanced measurement equipment. I thank him for making a significant effort to confirm my corrected results with his custom Wisdom-driver system. The Wisdom driver and the RD-75 driver are 'cousins' in that they both use the same forged steel clamshell driver enclosure. The Wisdom driver first came with a four-conductor planar membrane. Later a six-conductor model was released that used an external crossover to drive the conductor elements differentially. With the RD-75 all conductor elements are driven with the same signal.

Alvin has also provided two addition CSD comparison plots from other high quality loudspeakers. His comments are in Blue text below.

Below you will find the waterfall plot of three speakers. All three were measured with the microphone at 3' from the acoustic center of the driver and in a room. I have made an attempt to match John's waterfall specifications to almost the same anechoic conditions represented by his e-mail of 1/29/2003, anechoic measurements of the B-G RD-75 ribbon. No room boundaries were within three feet of the tested driver.

My diffraction measurements support John's graphs: the ribbon's diffraction results are not as bad as his previous IMP displays. My graphs also support the idea that the ribbon is very good but not the equal of the Paradigm 2-way dynamic speaker, a high quality speaker with an 8" woofer and 1" tweeter. The Wisdom ribbon, however, is almost the equal of the KOSS Electrostatic speaker. It is a 4' square design about 25 years old.

Alvin Foster, January 29, 2003.

Alvin Foster's Wisdom loudspeaker CSD measurement.

A most interesting comparison occurs with the presentation of this plot made by Mr. Foster as a result of my request for assistance. As I understand this CSD measurement comes from the same custom Wisdom driver loudspeaker as that of the plot displayed on page 4 made with ETF software. Alvin processed the impulse data in approximately the same duration (1.7mS) of my final free air RD-75 CSD plot on page 5 - which clearly elmininated the reflection ridges seen in the ETF plot.

Further grist-for-the-mill in regards the problems of reflections in the impulse data.

Alvin Foster's Koss loudspeaker CSD measurement.

Alvin Foster's Paradigm loudspeaker CSD measurement.

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 2.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 3.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 4.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 5.
Cumulative Spectral Decay Plots (waterfall) - Free Air, Bare Driver, Page 7 - NEXT PAGE.
Dipole Baffle Study Report#22.
The RD75 Dipole Baffle Study - Table of Contents
Acoustic Line Source Research - Table of Contents.