Tom Perazella's "Angel's Wings" dipole RD-75 loudspeaker and dipole mid-bass.

Tom Perazella authored a cover-photo article of the January 2001 "premier issue" of "AudioXPress" magazine (Vol. 32 No.1). Titled "On Angel's Wings, Pt.1", its second part appeared in the following issue. The articles dealt with his design and fabrication of a pair of RD-75 airfoil-style dipole baffles. Rudi Blondia and I were cited in the article for our prior work on RD-75 dipole baffles. Thanks, Tom.

Shown at left is a recent photo of one of his RD-75 dipole baffles, along with one of his new dipole mid-bass baffles. Tom is known for having a build-in infinite baffle subwoofer system that makes other systems seem bass deficient.

Tom writes (his text in blue):

... [below is] the waterfall plot of my RD 75s. The measurement time is 3.27ms. It is somewhere in the middle of the results [John] got. This was taken in my loft at a distance of 1M. The driver was in my baffle and was about 4' from the back wall. The ceiling is sloped and the floor is carpeted.

BTW, the baffle seems to be doing a good job as some guy named John in California gave me some good advice.

Tom Perazella, January 29 & 30, 2003.

Tom Perazella's "Angel's Wings" RD-75 loudspeaker CSD measurement.

Alvin Foster responded to Tom's message containing the CSD plot at left:

Tom,

Your results are great. So much so that I suspect that the IMP John and I are using may be a part of the problem. In my near anechoic waterfall plots, I compared my Wisdom ribbon with a high quality dynamic driver. The measuring conditions were about the same. By comparing the Wisdom to a dynamic driver, I figured the errors in my set up would be mutual. My results demonstrate that the dynamic driver was better on diffraction. Have you completed a similar test with a dynamic driver?

I believe, as you stated some years ago, the primary difference between our ribbons and high quality dynamic drivers are dispersion and lower distortion at high frequencies.

Alvin Foster, January 30, 2003.

In summary, the RD-75 driver appears to have a quite reasonable cumulative spectral decay response. Measurement issues with loudspeakers in general, and large loudspeakers in particular are often vexing. Despite the aggravation of the ex-Bohlender-Graebener employee's negative statements about the RD-75, overall the process leading to this correction report has been a postive experience.

For further perspective on the issue of measurement of the CSD of large loudspeakers, let me refer you to page 2 the "Stereophile" magazine archives for the July 2001 issue's review and profile of the Martin Logan "Prodigy" electrostatic loudspeaker. The link is immediately below:

Stereophile's archives, July 2001, MartinLogan "Prodigy" Measurements, page 2.

On that page John Atkinson states: " (The presence of reflections in this kind of waterfall plot is indistinguishable from delayed energy due to resonances.) " Note that the impulse duration for the Prodigy loudspeaker had to be severely truncated, as in this correction for my free air RD-75 measurements, due to ground reflections in the data. Mr. Atkinson further cautions, "As I have mentioned before, measuring large panel speakers is an exercise fraught with practical difficulties and, sometimes, frustration."

This correction report then is best seen as a cautionary call for skepticism when you see CSD plots with large numbers of extended 'ridges' or other 'hash' therein. Ask to see the impulse response from which the CSD plot was calculated - at a magnification sufficient to determine the presence of post-impulse reflections in the data.

John R.Whittaker

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 6.
Dipole Baffle Study Report#22.
The RD75 Dipole Baffle Study - Table of Contents
Acoustic Line Source Research - Table of Contents.