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Old 17th February 2017, 10:00 PM   #1
DonVK is offline DonVK  Canada
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Default OmniDirectional - work in progress

I've been working on a pair of Omni's. I'm looking for a different listening experience that is closer to the way real natural instruments project sound and use the room. I am willing to post pictures, and test results to share what works or doesn't, hoping for some insight and suggestions to improve it.

The starting point is this simple 2way setup. There were initial problems with them, but I'm enjoying them now, wanting more improvements. I'll post the those issues and my solutions up to where I am now.
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Old 18th February 2017, 01:10 AM   #2
DonVK is offline DonVK  Canada
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Default Drivers and CrossOvers

The XO is an 2 way LR4@1.8KHz + Zobel comp. connected to a Silverflute W20RC3808 (200mm woofer) and a Dayton DC28F-8 (28mm tweeter). These sound OK and are suitable for my sandbox.

First issue. The LR4 is constant amplitude but not constant power. So I have a -3db dip at 1.8Khz. Not a big deal but it will show up in all freq resp curves until I fix it.
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Old 18th February 2017, 01:58 AM   #3
DonVK is offline DonVK  Canada
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Default Bass reflex cabinet issues

I used HornResp to determine the cabinet volume and tuning port size. I tried to "stretch" things a bit and get fc=40Hz from a SilverFlute 8". It came up with 32L and 75mmx150mm pipe. The mfg parameters are off a bit, as I discovered later when searching for test results, but that is not the problem here.

I made the cabinet with internal ring bracing but mounted the woofer at one end so the back of the woofer saw the entire length of the cabinet. The bracing (I think) made it look like a back loaded horn and I got numerous resonances and possible standing waves that no amount of polyester fill could fix (see "before" images).

The cabinet was internally reduced to 16L via new partition and a new 25mmx140mm pipe installed. There are no "ring braces" in this new chamber and the response is pretty much what Horn Resp predicts and it extends out to 1.8KHz and down to 60Hz (when the port resp is included). The high freq tail is due to tweeter bleed through (forgot to disconnect it, like the before image).
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Old 18th February 2017, 03:59 AM   #4
DonVK is offline DonVK  Canada
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Default Is that lower woofer reflector cone necessary?

When I looked at various omni designs, some used a lower reflector cone (Deuval) and some didn't (Linkwitz). So I tested with and without cones to see for myself. In my case the woofer is always below the listening level. In my measurement graphs , the room acoustics are included so you'll see a bumpy curve from reflections.

Turns out that below 1Khz for a 200mm (8") it does not seem to make much difference. Regardless, I'm keeping mine as a "visual aid" and to support the upper cones. The woofer reflector is also not sensitive to surface shape or smoothness as I tried rough->refined->smoothed out and saw virtually no difference as I tried to make a better surface. Since my measurements are at 1m you can see the effect of the reflex port extending bass response.

Notice what happens to the high freq response when the upper reflector is in place. That's the next issue to deal with.
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Old 18th February 2017, 04:54 AM   #5
DonVK is offline DonVK  Canada
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Default The high freq section needs work.

I have 2 reflectors for the tweeter in an attempt to make a hybrid waveguide and reflector. In playing with a ripple tank simulator, it looked like the exit path needed to be controlled to prevent different path lengths from the tweeter and destructive interference. It seemed to work for an ideal point source, which sadly does not seem to apply to a 28mm dome tweeter.

The top "doughnut" was removed entirely as it caused interference between the upper doughnut and middle cone. The sound was distorted and no amount of frequency equalization could fix it. Just did not work.

With only a single middle reflector in place it seems to work OK but not great. It's usable with equalization and boosting. There are two issues 1) uneven response and 2) overall attenuation.

The overall attenuation (I think) is a result of increasing the projection cone angle from from 120deg (direct tweeter) to 360deg (cone) which lowers the pressure by approx 1/3 or 10*log(0.33)=~-5db. The tweeter is spec'd at -2db less efficient than the woofer. There is also absorption factor (0.15) from polystyrene or additional -1.0db. The total damage is -8db reduction in virtually everything from the tweeter w.r.t woofer levels and you can see it in the graphs and hear it. I need a higher efficiency tweeter (compression driver + waveguide) or active XO with seperate amps. I chose (placed the order) for the compression driver and conical waveguide which should arrive this week.
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Old 18th February 2017, 11:26 AM   #6
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Always interesting to see another omni build, considering a cheapy LX mini type build myself.

Any thoughts on the sound quality now?
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Old 18th February 2017, 01:19 PM   #7
DonVK is offline DonVK  Canada
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Quote:
Originally Posted by ewollowe View Post
Always interesting to see another omni build, considering a cheapy LX mini type build myself.

Any thoughts on the sound quality now?
At this time the dome tweeter is still installed. So I boost the HF using an equalizer as a temporary fix.

They are very enjoyable to listen to, I've spent the last week trying different program material. I think they are especially strong with acoustic instruments (guitar, piano, string quartets, orchestra) mostly because these instruments primary freq are <2KHz and the music pace and dynamic range allows my ears to pickup the room reflections so the speakers tend to disappear and the sound is spacious and enveloping like an instrument in the room.

They are not so good (at this time) for music like hard rock or high intensity low dynamic range (everything mastered at one high wall of music level). I haven't figured out if this problem is room reflections or a weak HF section or sensitivity or distortion. I'll post results when the waveguides are installed.
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Old 18th February 2017, 06:26 PM   #8
system7 is offline system7  United Kingdom
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Don, a fascinating project, and well documented.

I had a read around, and the prognosis for omnidirectional doesn't look good to me: Sonab OA5 mkII

I seriously wonder whether wall-mounting might not work better than floor mounting. It kind of ends up in the same place with adjacent boundaries. You possibly know about the Allison effect with the first reflection from the nearest boundary which creates a suckout.

I do appreciate the secondary sound of the room as an important part of the sound of a loudspeaker. Especially if you are not tied to a sweet spot for listening.

BTW, to correct the power hole in LR4, you just overlap the bass and tweeter more.
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Old 18th February 2017, 06:57 PM   #9
graaf is offline graaf  Poland
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try it without diffusors, just try, why not?
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Old 18th February 2017, 06:59 PM   #10
DonVK is offline DonVK  Canada
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Quote:
Originally Posted by graaf View Post
try it without diffusors, just try, why not?
I did. See earlier in the thread. Is a lower reflector needed?
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