Following paragraphs contain a history of ribbon testing under multiple situations on a set of Carver 60" and B&G RD75 "ribbons" This account is more or less on a day by day basis, some days however took a full week ;-) As time passes by the measurements got more accurate and meaningful, a matter of learning curve. The errors, wrong assumptions, all part of the learning curve, are not back-annotated. Just plain history.
A click on the links below will bring you to the right spot on the page.
The above measurements, with exception of the outdoor measurements of Day 1, are mostly "inroom" measurements that include some room interaction after eliminating and diffusing the obvious "hard" echoes. Following measurements are performed in pseudo-anechoic conditions in an empty space, larger than my living room, during Labor Day weekend.
Anyone familiar with the history of this pageorwho visited John Whittaker's page with the RD 75 Dipole Baffle Study is aware that there is a 6-8 dB bump below 1 kHz. ( Take a peek at "Day 20" if you forgot ;-) So far a lot of speculation has been going on,such as dipole reinforcement and floor-bounce. After discounting floor-bounce as the source for "John's bump" as I like to call it, I continued playing around with models and finally got my finger on the problem and an attractive, inexpensive, way to avoid the bump.
Day 1. John Whittaker and I got together on Saturday May 17, 1997 to run our collection of Carver and B&G "ribbons" through the IMP/MLS speaker test system. You'll find on the following pages some history with results. All tests were performed freefield, outdoors. Not directly under anechoic conditions but nevertheless with repeatable results. All by all a fun learning curve and I'm ready to expand on it in a future session.
Day 6. Thursday, June 12, 1997
Day 7. Saturday, June 14, 1997.
Day 8.. Sunday, June 15, 1997.
Day 9 and 10.. Weekend of June 28-29, 1997. I built a box for monopole ribbon testing and listening. Used MDF, rabbit joined, glued and screwed, braced. Stuffed it with hollow fiber (Dacron) and attached a BG RD75 driver as first test victim. (Carver to follow next week) All measurements are done in room with the microphone pretty much in a listening position height.
Although I'm a dipole fan because of the unequaled open sound, I'm pretty happy I've done the test and listened extensively to music on a boxed ribbon. I have to admit that "boxing" them in makes placement fairly straightforward. Make me wonder about getting my hands on a Carver 48" or a RD50 for my rears in my surround sound system.
Day 11-12. Independence Day weekend, 1997.
A weekend with fireworks and a lot of music listening. Had a monopole Carver built during the week + a quarter round baffle for a RD75 + a couple of trapezium baffles for a Carver ribbon. Some experimentation with different listening (measuring) distances + heights as I found out that my couches were definitely making a "difference". The delights of in room response testing ;-) My "living" room is by now converted to a semi-permanent testing field with echo-features sono-tec'd out. Need a good excuse to throw the couches out ;-)
Day 13. More work on asymmetric rectangular and trapezium shaped baffles. Higher resolution plots.
These and previous measurements (Day 11-12) make it clear to me that for a "flat" dipole baffle, an asymmetric trapezoidal shape, with different angles for both sides, is the preferable way to go. Some of the changes are subtle and may get lost in other listening room "interactions" but for anybody with a "perfect" listening room out there, asymmetric trapezoidal dipole seems a way to go. See below for the not-so-ideal-listening room.
Previous shows that it pays to follow "theory" even when the differences seem minimal with respect what room interaction can "ruin". Following the rules gets the more desirable curve.
Interlude: looking for the floor "bounce" effects.
I was puzzled by the peaks and dips in some of the responses because one day they are there, the other day they are kind of gone under different circumstances. Time to reconstruct and dig up all the old notes and go through some of the files I didn't publish. (I've accumulated so far over 8 megabytes of *.FR2 and *.IM2 data) Following data was accumulated a couple of days ago after John told me about his "bump". Measurements taken in a different spot in my listening space than usual, nevertheless some nice correlations with previous data. Please remember that this data is taken indoors, not in free field nor anechoic conditions.
The above tests show that floor bounce is definitely visible in the measurement. Secondly, the effect is different in a dipole system versus a monopolar system. (Backwave+backwave bounce versus no backwave+backwave bounce) The dipole system tends to keep the dips in the same location, at least for the low frequencies. The ultimate dip/peak magnitude variations seem to be function of the distance between direct and bounced sound reaching the measuring device, your ears. It is at least nice to see that John's bounce could be reconstructed at the same distance, other driver, other baffle, other location without the crossover.
Bonus, since some of you asked, a couple of plots showing the difference between 4k MLS and 16 k MLS, with echo's gated out (1024 point FFT, 21 milliseconds) or my typical in room response (4096 point FFT, 85 milliseconds.)
While we are on the subject, a plot showing floor covering effects and a close-up in the mirror-zone. It should be clear now that listening to those tall dipole drivers is best performed at a minimal distance of 1.5 to 2 times the height of the dipole. The monopole version doesn't show the same boost below 1 kHz but shows a more pronounced dipping in that area
Take also a look at what happens when you move the backwall that your dipole uses to bounce back.
Day 14 Just a couple of Carver 60" plots with small "horns" attached to the ribbon driver as an experiment to flatten response. More details on this technique will follow in a baffle construction article for the Carver 60" planar.
Day 15 August 7, 1997. Baffle with RD75, 10.5 feet up in the air, trying to out-smart the "bump".
Day 16 August 24-25, 1997. Comparison between impedance plot Carver 60" and B&G RD-75. Sure it is almost resistive but it shows us more.
Day 17. I actually performed these measurements a couple of weeks ago. Since the question was asked by a BASS-list member if slanting made any difference, I decided to post these two "slant" examples.
Day 18 A look at a RD75 baffle with a monopole section at the bottom to accommodate a sealed woofer. Nothing to worry about !
Day 19 Receive some e-mails regarding dipole placement in small rooms. Interesting discussions on the BASS-list. Prompted me to document this for fun. Worthwhile exercise !
Day 20 The large space allowed be to go for a set of distance measurements.
Day 21 The issue of floor bounce came up many times with respect to planar magnetic ribbons. Following test shows what is really happening. All explained using MLS impulse plots.
Day 22 After discounting the floor-bounce as the source for the huge and wide bump in the frequency/magnitude response of planar magnetic ribbons on a dipole baffle, more work needed to be done in order to come up with an answer. Following page documents the results which leads to an elegant solution, opening room for more study with a better knowledge on planar magnetic ribbon and dipole baffle behavior.
Following measurements were taken using a prototype CXR-22 active crossover with a crossover point of 150 Hz for the B&G's and 200 Hz for the Carver ribbons.
Copyright (c) 1997-2005, by Rudi A. Blondia, ALL RIGHTS RESERVED. Last update: March 17, 1998.