Date: Tue, 30 Jun 1998 16:17:03 -0700
To: bass@mcfeeley.cc.utexas.edu
Does it seem possible? The last of the Dipole Baffle Study reports is at hand. It's been more than a year now since the start of the Study in May 1997. Access to the large gymnasium for measurement has been restricted. A pithy point indeed, what with three new baffles in the garage remaining to be measured. As Rudi Blondia and I lamented to each other last week, once you've had a chance to measure in such a large space returning to the livingroom for measurement is unsatisfying.
The Compound Shape baffle (1SCS), which has been mentioned previously, is the latest in my RD75 dipole baffle series and by all evidence the best. It was measured semi-anechoically (groundplane) in another large venue, the Auditorium, on May 3rd this year. As such, it's response should be directly comparable to the earlier gym measurements of other baffles.
What does it look like? Here is a series of four unsatisfying photos taken at the Auditorium. None of them particularly show the airfoil-like shape of the rear side of the baffle.
The large front view photo shows foam strips at the top and bottom of the RD75 driver. They were there for further testing of 'tapering' and 'shadowing' effects of the line-source. Please note that the adjustable tilt base and rear support brace are NOT part of the design. The baffle was built upon my new 'test baffle jig'. Conceptually, a production version of the baffle would not have those features.
Recently I was able to make a tracing/drawing of the top and bottom of the Compound Shape baffle and overlay them. The GIF file below contains that overlay. The complete baffle and driver assembly is ~17.75" wide and 5 5/16" deep at the bottom. One should imagine/extrapolate a smooth, tapered connection of the top to the bottom. Keep in mind that the baffle was based upon a found object, a 4.25" diameter cardboard carpet tube. Had I a 'Santa Claus Machine' the baffle would have been different. The 'bare edge' of the RD75 driver normally has a 1/2 round treatment applied over the mounting flange.
Schematic cross-section of Compound Shape baffle. ..(8K-GIF)
The Compound Shape baffle is part of the 'Equal Area Series' that I'd been measuring at the gym to document the effects of baffle shape while keeping the surface area of the baffle equal. In the comparisons below keep in mind that the baffles compared are all from the 'Equal Area Series'. The nominal surface area, including driver and edge treatements, is approximately 2500 square inches This the area of a 15" wide two-sided rectangular baffle (1.5" thick, and including driver width). It is the minimum baffle width recommended by Mr. David Graebener for dipole use of the RD75 down to it's 125Hz lower frequency limit.
Reading Bass List posts from the past 'dark ages' one must pity our Carver planar brethern in that they seemed to have paid little attention to the effects of dipole baffle shape (as least in the posts I've read from that period). To the question then, given equal surface area of a baffle, does it's shape effect frequency response?
Equal Area baffles: 3 meter comparison, 1/12 octave smoothed.
Equal Area baffles: 4 meter comparison, 1/12 octave smoothed.
The two other 'Equal Area' baffles are the 1S6T and the 1STR. Both are one-sided baffles, the 1STR is as perfect a right triangle as the RD75 driver allows; the 1S6T is trapezoid 12" wide at the top and 18" wide at the bottom. Both baffles are 1.5" thick. The comparison plots are 1/12 octave smoothed.
Is there any doubt that the new baffle produces superior linearity in the frequency response at 3 and 4 meters on-axis? The 'Moving Dips' in the frequency response of the dipole baffles can be seen with a vengance in these two plots. There are at least three 'dips' that can be seen moving through the frequency responses when the two distance comparsion sets are contrasted. Note what happens to the 6KHz cavity resonance peak at 4m!
For those who require their frequency response plots to be substantially flatter I supply this 3 meter 1/3 octave smoothed response with phase response. Do you know what you are seeing in the phase response? That is called 'minimum phase' behavior, and is a very good thing indeed. That is part of the reason why the RD75 sounds so good.
1SCS baffle: 3m/1m response, 1/3 octave smoothed.
There is much potential for further work in regards dipole development. It seems clear that there is a 'problem' in the 3-4KHz area that is associated with the dipole path around the 'bare edge' of the one-sided driver. The very good reasons to use a one-sided dipole baffle have been fully detailed earlier in this series.
I believe that the Compound Shape baffle demonstrates that relatively large scale anti-diffraction measures are required to maximize the linearity potential of the RD75. It appears, to date, that no special dipole implementation will eliminate all aspects of the 'Moving Dip' phenomenon. I know that Rudi Blondia has been working on this issue with some success in the lower frequencies. I am of the opinion, based upon the totality of the gymnasium measurements, that RD75 dipole baffle implementations should be selected/created for a selected listening distance and height. A discerning appraisal of the 'Database'(Gym Measurements) section demonstrates that the frequency response of the RD75 varies by baffle, distance, and height - sometime dramatically so! There will be no 'universal' dipole baffle for the RD75.
Subjectively, in direct comparsion to the 1S6T baffle, the Compound Shape baffle 'isn't there'. This effect has been heard and commented upon by completely naive listeners. The best analogy I've been able to come up with is in the visual realm, and is that of specular highlights from a shiny object, i.e. glare spots. The Compound Shape baffle does not sonically 'glare' as does the previous best baffle. This is surely due to it's enhanced anti-diffraction attributes. The soundfield it generates is both wider and deeper than from the 1S6T baffle; and, the sound is smoother as directly implied by it's frequency response. The Compound Shape baffle is no doubt as close to an optimum standalone baffle as I will develop.
Both Rudi Blondia and I have demonstrated that the RD75 can be successfully used in a two-way system incorporating a subwoofer with a crossover point in the 125-140Hz range. For best use the RD75 should be electronically equalized to eliminate the deterimental effects of it's required passive notch filter. Further equalization to linearize it's frequency response can also beneficial to it's phase response (as a minimum phase driver).
The RD75 is a heroically-sized driver that can create a heroically-size soundfield. It's not for everyone. As far as I know a pair of the drivers is still available for about $800. This is a bargain of large proportion. An inspired DIYer should be able to assemble a two-way RD75 system, bi-amped, for $2000 or much less depending upon existing resources (amps, subwoofers, etc.).
The Dipole Baffle Study has been an intense learning experience and a lot of damn work - but most of all it's been a 'rush' (the old hippy speaking). Hope you've enjoyed the ride? Any who remember my statement of purpose a year ago will know that I never intended to utilize permanently any of the baffles developed - at the start there was just the intent to explore an extreme concept, i.e. a 'really' full-range two-way system with an optimized standalone dipole baffle. The dearth of information about dipole baffle usage with line-source speakers irritated me then, as it still does.
Now I'm off to work on my 72" true ribbon project - we'll give the RD75 a tweeter maybe? Next week I'll bolt up the RD75s to the venerable DynaPleat Line Arrays for those last few dipole Hz in the low end that they might provide. I really want to explore the concept of 'Loudspeaker as Large Visual Offense'.
John Whittaker (Blond God of the BGs) Dipole Baffle Study Report#22.
Ad Astra, Per Ardum
The RD75 Dipole Baffle Study - Table
of Contents
Acoustic Line Source Research -
Table of Contents.
