Not Your Daddy's Dipoles

[matevana]

Just finished an OB design that incorpoartes some interesting concepts. I spent the weekend listening and I am favorably impressed.

Type: OB dipole up to approx 5.5k Hz. Biamped helper woofer, actively crossed and roll-off compensated. Coaxially mounted pro audio mid w/traditional 1" fabric dome tweeter. Acoustically dampened and isolated panels w/closed cell suspension. Visco-elastic feet.

Helper Woofer: Peerless 10" Nomex XXLS woofer. 250Hz lowpass, 24dB/octave.

Mid: Eminence Acoustinator 8" poly mid range, coax 1 1/8" 18TPI threaded, allowed to roll-off naturally.

Tweeter: Morel MDT-20 mounted coaxially, 1.8mf single cap, 6dB per octave.

Initial crossover points surprisingly good. Will spend some time tweaking. The drivers produce some respectable SPL's w/o strain. These were initially designed with many genres in mind. The acoustinator mid is a hair dark, but still makes for a realistic presentation. The voicing is very pleasing to me. More to come.

[gainphile]

Hello, I assume you use them for PC speakers? Great work !

How far do you place them from the rear wall?

[matevana]

Originally Posted by gainphile

Hello, I assume you use them for PC speakers? Great work !

How far do you place them from the rear wall?

They seem to be somewhat less sensative to rear wall interaction. I have tried them anywhere from 1.5 to 3.5 ft. w/o any smearing of the image, to my ears. Currently I have them at 2', which is more for aesthetic reasons than anything else. They are also at least a few feet away from the nearest corner boundary.

Interestingly I also tried them outdoors to listen w/o any boundary effect. They sound surprisingly full and warm, possibly due to the high excursion capabilities of the XXLS (13mm) and a higher than typical 250Hz lowpass filter. The XXLS remains clean much higher than that, but the 8" Emi is no slouch either, so a higher xo point is not desired. The low to upper mids are actually quite satisfying, which make them just as apt to reproduce electronic music... something I have found to be lacking in many dipole designs. This was a primary design goal for me... thus the title of the thread.

[mayhem13]

Some details of the dome mounting please? Didn't think that was at all possible.

[matevana]

Originally Posted by mayhem13

Some details of the dome mounting please? Didn't think that was at all possible.

I've been doing it for years and it's actually quite easy. Use a standard 18tpi to 2 bolt adapter. Selenium makes a metal one that is quite good and inexpensive.

Selenium ADM25-25 Horn Adapter | Parts-Express.com

It's easiest when the drivers flange diameter is 4" or greater. Align the tweeters center with the 1" throat and make a reference. I drill three holes through the horn adapter first, making sure they pass between the tweeter's outer flange diameter and body, away from the terminals. Then I clamp the tweeter back in position and use the adapter's pilot holes as a template, drilling straight therough the tweeter flange. Finish with flat head screws, locking washers and nuts, with the nuts facing outward. 10/32" screws work well.

Acoustically, the tweeter's dome should wind up in the same relative position as the element of a HF horn driver on a similar mid. If matched correctly, the single point source geometry is quite effective, using the mid driver's cone geometry as a wave guide. Driver selection is key, and with the proper x/o dispersion and off axis response aren't bad at all.

[djn]

I just finished my first pair of OBs and from all the reading and talking I did, a wide baffle and having the driver off center is needed to ensure the lower freqs are not cancled. How does that work with such a small baffle on yours?

[matevana]

Originally Posted by djn

I just finished my first pair of OBs and from all the reading and talking I did, a wide baffle and having the driver off center is needed to ensure the lower freqs are not cancled. How does that work with such a small baffle on yours?

On the contrary, many successful dipole designs using 10" woofers have a baffle width of only 12-13 inches (i.e., SL Orion). Remember this particular design uses some EQ to address dipole cancellation so a wider baffle is not necessary.

Offsetting the driver speaks more to retaining a proper stereo image than cancellation. In my test baffle I tested on center, 1.5" and 2.5" off center. With my single point source combination there was almost no distinguishable difference. I actually have 3 front panels cut with varying driver offsets. They are interchangable and can be swapped by removing 4 bolts.

Let me assure you that the quality and quantity of bass is not lacking. Unlike many dipoles I have heard, they excel in the 150Hz to 500Hz range; a solid presence w/o sounding boomy. In my opinion this is key to enjoying OB speakers with more modern music.

[djn]

That makes sense. I am new to OB and used the EDGE program to figure out my baffle size. I opted not to use the EQ the EDGE provides and went with big baffles. My wife and I are both artist and we are going to take one baffle board each and use them for canvases. Should be interesting. Here is a pic of my speakers. I'd never heard OBs before I built them and I am hooked.

[Godzilla]

djn, i am looking forward to seeing the art from you and your wife on those speakers.

matevana... i drink you every once in a while and my kids enjoy your hint of chocolaty flavor.

These two examples of open baffle speakers are a perfect example of how there are different ways to achieve similar goals.

Zilla

[matevana]

Originally Posted by djn

That makes sense. I am new to OB and used the EDGE program to figure out my baffle size. I opted not to use the EQ the EDGE provides and went with big baffles. My wife and I are both artist and we are going to take one baffle board each and use them for canvases. Should be interesting. Here is a pic of my speakers. I'd never heard OBs before I built them and I am hooked.

Great idea on the graphics! Please post again when they are finished. If it were me I would paint a picture of my living room so the speakers just disappear It is true that once you hear the 'correctness' of dipole bass, it's difficult to listen to monkey coffins.