Not Your Daddy's Dipoles

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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.
 

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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.
 
Coaxially mounting a hi-fi dome tweeter.

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.
 
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?
 
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.
 
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.
 

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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 :p It is true that once you hear the 'correctness' of dipole bass, it's difficult to listen to monkey coffins.
 
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

LOL, a little Teavana humor. Oh no Godzilla, there goes Tokyo. My uncle Buck (Dharma) would appreciate your name. :)
 
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.

Your experience is smiliar to mine. In fact I was so surprised that I wrote quite a bit on the experiece, compared to my usual one liner.

Perhaps the proportion between direct to reflected sound countributed more than what I understood. ie. More direct = better. Although rear wall distance is less optimal.

S11 OB - Bookshelf open baffle with Seas 8" and Tangband Full-range
 
Your experience is smiliar to mine. In fact I was so surprised that I wrote quite a bit on the experiece, compared to my usual one liner.

Perhaps the proportion between direct to reflected sound countributed more than what I understood. ie. More direct = better. Although rear wall distance is less optimal.

S11 OB - Bookshelf open baffle with Seas 8" and Tangband Full-range

Nice write-up on your desktop OB design! One thing I noticed with regard to reflected sound (and is often neglected in discussion) is the angle the sound reflects from the rear wall. While this may be secondary to distance, it seems to be an important factor as well. The reverberation seems to be more natural when the rear of the drivers are not parallel to the rear wall but at a slight angle. Of course this will usually toe-in (or out) the drivers themselves but in my case the room is not rectangular so this doesnt necesarily affect the driver position with regard to the sweet spot.

Speaking of the sweet spot, I also find that with most dipoles it helps to cross them a few feet ahead of the ideal listening position. Have you played with that in your desktop setup?
 
Just finished an OB design that incorpoartes some interesting concepts. I spent the weekend listening and I am favorably impressed.

Hi matevana,

Congratulations on such a nice build - and thanks for your listening impressions and little tip on using the tweeter mounted to flange.

From the photo's it looks as if you are using a main one piece baffle for mounting the speakers and separate baffles for the woofer and 8 inch on the front. I understand that this construction allows you to swap out different baffle sizes (that could be handy for room matching) but I also see a slight gap between the front and rear baffles. Is this gap perhaps some type of constrained layer design? If so what are you using for the constrained layer?

Some additional photo's showing the detail of your tweeter mounting and flange would be very nice and appreciated.
 
Hi matevana,

Congratulations on such a nice build - and thanks for your listening impressions and little tip on using the tweeter mounted to flange.

From the photo's it looks as if you are using a main one piece baffle for mounting the speakers and separate baffles for the woofer and 8 inch on the front. I understand that this construction allows you to swap out different baffle sizes (that could be handy for room matching) but I also see a slight gap between the front and rear baffles. Is this gap perhaps some type of constrained layer design? If so what are you using for the constrained layer?

Some additional photo's showing the detail of your tweeter mounting and flange would be very nice and appreciated.


Good observation. One of my design goals was to help isolate the mid/high frequency drivers from the woofer's moving mass. Notice that both drivers are mounted only to the front baffle. There is a seperate front baffle for each main driver and one common rear baffle. Each front baffle is isolated from the rear main panel by a sheet of 5/8" closed cell foam. Each front baffle is suspended ("floats") and is anchored using four 1/4" carriage bolts, rubber washer, fender washer, lock washer and compression nut, in that sequence, front to rear. There is a 1/4" gap between the upper front panel and lower front panel. Furthermore, while the bolt diameter is 1/4", the hole diameter is 1/2", so in theory the bolts themselves do not come in contact with the baffle board.

The concept is very similar to vibration isolation in home theatres using a "room within a room" theory. Vibration is lessend since no rigid parts interact directly. You can really feel the difference by touching the front lower panel (woofer baffle) and upper panel (mid/high baffle). A secondary benefit is the ability to change out drivers effeciently when experimenting. I have alternated between two completely different scenerios... the one in the photograph using a single point source coax setup and another housing a Jordan full range 4" driver. It takes about a minute to swap them out!
 
Thanks for the information about the CLD panels. :D Food for thought:xeye:


Well, maybe a poor man's version of CLD tiles. I use a 5/8" thick closed cell exercise mat made by Gold's Gym. It sells at Walmart for 15 bucks and has enough material to make one pair of my dipoles. I have also tried space foam and open cell materials. The best so far is a material called sorbothane but it is too expensive at the present. I use this material for the feet only, in small quantities. It is visco-elastic, having properties of both liquids and solids. Does wonders for vibration dampening in key spots.
 
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