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Old 27th March 2010, 11:21 PM   #1
bwaslo is offline bwaslo  United States
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Default A 2-way Omni experiment

After reading Floyd Toole's book, I thought I'd make some back surround speakers for my setup and, for this, to try an omnidirectional design. Toole described some considerable research about audibility of resonances, specifically that high-Q resonances are relatively inaudible at least compared to low-Q resonances. Similar research pointed to general preference by most listeners to speakers that had a generally flat response with deep bass, but particularly those that had nearly the same response off-axis as on-axis. Since I also wanted the speakers to create rear channel ambience, it seemed like omnis would be the way to go.

The obvious choice for an easy to construct omni is Linkwitz' "Pluto", but what's the fun in EXACTLY duplicating someone else's design? I decided to go with something similar, but to try to have it omnidirectional over the whole spectrum (Pluto has a forward-only facing tweeter).

So this is another 'sewer pipe speaker'. I used a 7" Peerless driver mounted facing upward in a 6" diameter pipe, topped by a Peerles 1" 'full range' driver (in a short 2" diameter pipe) also facing upward but into a conic reflector. See attached photos. The idea was that, since the speaker would have internal amplifiers and an electronic crossover, I'd first find the arrangement that gave the most consistent spectral shape with mic variation in vertical orientation around the speaker. Flatness of response was not a criteria at this stage, as long as it gave enough energy for the crossover EQ to work with. And narrow response ripples or comb notches would be tolerated, if they couldn't be avoided.

I tried a lot of different reflector shapes and styles, curves, cones, balls, partially absorptive, small, large. I don't have the means (or patience) to try something like the Duevel, though that would probably be the best way to do it. The arrangement that worked best of the ones I tried was a small 3" diameter conic shape, with it's point cut off so that it could be positioned close to the tweeter's inverted radiator. That didn't give the flattest response (pretty terrible, actually) but the response it gave seemed the most consistent over vertical orientations. This was made using a sheet of black plastic (the cover of a spiral-bound notebook) cut out, wrapped, and heat-welded at its seam with a soldering iron tip. I mounted the cone on screws and standoffs that also mounted the tweeter to its pipe/enclosure. Aluminum rods were used to suspend this over the woofer. No name for these yet. Maybe "Saturns", because the reflector looks a little like a ring. Or maybe just "anonymous".

Like Linkwitz' Pluto, each speaker has a two-channel power amplifier inside with an op-amp based crossover/equalizer board. I ended up using a stereo TAS5630 based class-D amplifier with on-board SMPS power supply, made by Authlxl (who frequents the class D power amp forum here). This amp can produce 140W per channel into 4 ohm loads (both drivers are 4 ohm). 140W is probably seriously insane for a tweeter amplifier, but I wanted that kind of power for the woofer, and they came as a set, so that's the way it went.

The geometry of the arrangement makes omnidirectional characteristics horizontally around the speaker a given. Some plots are attached of the final responses at various vertical angles relative to the tweeter. The first shows raw curves, made quasi-anechoically using a 3msec window (and with non-anechoic response blended in at lower frequencies). The second plot has the same curves but with 1/3rd octave smoothing applied to show the general spectral balance through all the comb ripples. The yellow curve in each is the power average of all the curves, which should correspond to the response of room reflections from the speaker, assuming reflected response is the same as the one that radiates outward. That came out respectably flat.

I haven't had much of a chance to listen to these yet. I just got the EQ straightened out today (it looks like it still could use some tweaking around 2kHz), but can't be making much noise now because people here are napping. So I posted about it instead for now. I'll see what the pipes sound like as stereo main speakers, first, before moving them to surround duty. Assuming I don't blow the tweeters out before they get that far.

Cheers!
Attached Images
File Type: jpg Closeup 3.JPG (25.5 KB, 1942 views)
File Type: png Vertical responses.PNG (41.8 KB, 866 views)
File Type: png Vertical responses (O3 Smoothed).PNG (23.7 KB, 1857 views)
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Old 28th March 2010, 03:59 AM   #2
authlxl is offline authlxl  China
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ok!excellent works,we have to learn more from bill.it's so hard for me to know technology of speaker.
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Old 28th March 2010, 09:53 AM   #3
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Looks pretty cool, similar to something Visaton sell as a kit, I believe...
Visaton - Lautsprecher und Zubehör, Loudspeakers and Accessories

I hope you enjoy these speakers - have you tried them as main speakers in stereo? I've heard this can give interesting effects.
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Old 28th March 2010, 03:12 PM   #4
bwaslo is offline bwaslo  United States
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Thanks.

I'd never seen that Visaton kit before, yes it is very similar. I'm impressed that they managed to do it in a passive speaker. Wish they gave some response plots and vertical pattern information.

Today I should be able to do some listening.
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Old 28th March 2010, 06:24 PM   #5
bwaslo is offline bwaslo  United States
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Well, after several hours of listening to these as mains, I'm impressed. I could certainly live with these instead of what I've been using (line source dipole planar magnetics). I'm even considering making another pair for the fronts now, though that would be difficult now as the tweeters aren't available any more (sold out from a Parts Express closeout). These things sound really good with everything I've tried so far. I'm set up with subs, though, so I can't say what their low bass would be like on their own. Pipe enclosures really do a good job of not sounding 'boxy', they seem as open as the dipoles I think. And the treble seems really excellent, which is surprising considering the raggedness of the direct response. And I don't think I ever heard a speaker that vanished so completely. It may be a while before I move these into back channel service.

Has anyone here heard Duevals or that Visaton 'Fontana' omni? What do they sound like? Is this an effect that people get tired of?

I've always been of the opinion that highly directional speakers are best, or else dipoles which at least suppress energy going out at the sides, maybe I was wrong. My mind is changing maybe - high directivity, unless so strong it can swamp out all reflections, maybe isn't the way to go after all.

Or maybe it's just because they're new and different and the 'builder's preference' effect is in play?
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Old 28th March 2010, 07:39 PM   #6
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I think the sound would be more natural because it behaves more like a musical instrument.
Violins, guitars, saxophones, etc etc are all fairly omnidirectional. A speaker that is highly directional will never achieve the same sound.

I have considered building a budget version of the Visaton kit (one woofer being £70 means it's not a light decision - I have 4 £20 woofers that will do). The problem for me is that I'm in a bedroom, where things move around often. This means the acoustics are constantly changing, but altogether too congested to be able to fully utilise the idea.
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Old 28th March 2010, 08:43 PM   #7
bwaslo is offline bwaslo  United States
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correction -- that's a 2" (not 1" like I said) Peerless fullrange. There is a black coned version (830983) still available at Madisound and PartsExpress, so maybe I'll do another pair. I'm crossing it WAY below the freq range it was designed for, so overpowering it might not be an issue. Authlxl, I may be writing you for another pair of amp boards (maybe the Tripath version this time).

The woofer (835004 SDS) is the one that Parts Express is blowing out for $10.

Last edited by bwaslo; 28th March 2010 at 08:45 PM.
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Old 28th March 2010, 09:09 PM   #8
ScottG is offline ScottG  United States
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Quote:
Originally Posted by bwaslo View Post
I've always been of the opinion that highly directional speakers are best, or else dipoles which at least suppress energy going out at the sides, maybe I was wrong. My mind is changing maybe - high directivity, unless so strong it can swamp out all reflections, maybe isn't the way to go after all.

Or maybe it's just because they're new and different and the 'builder's preference' effect is in play?


If you have ever done any testing with real sources contemporaneously with a *radial* like you have here, then you would know that they are *more* correct in basic operation. (..they simply sound more life-like with respect to imaging and sound-staging.. provided you take some precautions with use.)

You can "get around" most of the worry of overly reflected sound by sitting closer to the speakers than they are to your listening-room walls. (..in other words move them closer to you.) On the other hand this aspect isn't a significant detriment when used as a side or rear channel.

You may even find that there is a particular distance between speakers that is more "correct" than others.

The problem with the design will be perceived dynamics and output (..there is only so much the drivers can do - even with some modest gain from the waveguide).
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Old 28th March 2010, 10:15 PM   #9
bwaslo is offline bwaslo  United States
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I doubt there is any gain from these waveguides since they splay energy over a wider range than it would cover otherwise. Not just the horizontal/radial pattern is uniform (not plotted), but the vertical pattern is substantially uniform, too, see the plots. So I think 'omni' is a better term in this case than 'radial', though that is correct also.
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Old 29th March 2010, 08:06 PM   #10
ScottG is offline ScottG  United States
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Originally Posted by bwaslo View Post
I doubt there is any gain from these waveguides since they splay energy over a wider range than it would cover otherwise. Not just the horizontal/radial pattern is uniform (not plotted), but the vertical pattern is substantially uniform, too, see the plots. So I think 'omni' is a better term in this case than 'radial', though that is correct also.
I think you might be surprised just how much gain the waveguide is providing in the top 2 octaves (..5 kHz up - with the higher the freq. the greater the gain). If you are really interested you can always measure it without the waveguide.
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