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Old 16th October 2010, 06:09 PM   #1941
pjpoes is offline pjpoes  United States
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Sealed subs distortion is much lower than ported below the port tuning. When subs are ported in things like an extended bass shelf, you tend to get higher distortion throughout that range, which means in many cases, really low tuning doesn't solve that issue. When used within their effective range, a sealed sub can have lower distortion than ported throughout their range (they will be the same above the tuning frequency). In many cases, the subs with really low distortion numbers on HT Shack are about the drivers rather than the enclosure. Certainly a port can give greater output, with lower distortion, at the given tuning frequency, but much below that the distortion will rise far above that of the sealed, and response drops like a stone. Often the difference is so small, and our hearing acuity for distortion at those frequencies so poor, I'm not sure it makes a difference.

I have two of Dr. Geddes bandpass subs for my main subs, I don't know their exact range, but they are the broadband type, and relatively small, about 2 cubic feet. I'm fairly sure they extend into the 100+hz range with no low pass on them. Their distortion measures very low between around 60hz and 100hz or so. I didn't measure much above that, so I can't say, and below that it started to rise, but I'm sure not in any audible way.

My ULF, so to speak, subs are a ported design in a 5 cubic foot box with a 12" TC Sounds TC3000 with a one-off motor design they were testing right before being bought out. Lower inductance, lower distortion, and optimization in larger boxes. The other uses dual dayton RSSH15HF subs in a push-pull sealed 2.5 cbft box (Previously I had these each in their own 2 cubic foot sealed boxes), and that sub has lower distortion below 25hz than the TC Sounds design. The difference is small, and the TC has much greater LF output abilities than the Dayton, but its all in the driver, not the enclosure. The TC has far more than 30mm of one way linear excursion and more than 3000 watts of power handling.
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Old 16th October 2010, 06:29 PM   #1942
pjpoes is offline pjpoes  United States
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On a completely separate note, Dr. Geddes I have a question? Do you think you would ever consider making a dedicated center channel which used two 12" drivers and a single 15" waveguide above the drivers. Using the setup to allow an enclosure that isn't as tall as the current setup, and also offering better on-axis response (not the driver setup, just crossover optimized for this and larger waveguide). It's probably wishful thinking, but I would love the ability to have a center that is about 4-6" shorter so I can mount my screen lower, and not use an acoustically transparent screen. I know you prefer them, from a sound standpoint, I do too, but I couldn't give up on the image quality difference in my non-dedicated room.
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Old 16th October 2010, 07:40 PM   #1943
noah katz is offline noah katz  United States
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Originally Posted by pjpoes View Post
Sealed subs distortion is much lower than ported below the port tuning. When subs are ported in things like an extended bass shelf, you tend to get higher distortion throughout that range, which means in many cases, really low tuning doesn't solve that issue.
As you said, above freq around tuning, sealed and vented behave the same.

It's only below Fb that vented excursion increases over sealed, and this is true for EBS.

So I disagree that distortion is higher in EBS shelf freq range.
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Old 16th October 2010, 07:54 PM   #1944
gedlee is offline gedlee  United States
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Why all the worry about distortion in a sub? It really isn't a factor. Flow noise in the ports is, because it's uncorrelated and goes higher in frequency, but THD type distortion in a sub is certainly not audible. At least this is true for a well designed sub. Some of the plate amp subs that I tested had incredibly high distortion due to the amp clipping at even reasonable levels. This is an issue. From my studies it was all about not overloading the design and having enough port area that didn't cause flow noise. Plate amps just do not seem to have the headroom that is claimed - they all seem to clip very early. I suspect its the amp, although with a plate amp installed in the system its hard to tell, but the distortion rises very quickly at some point, which is indicative of clipping, not a driver issue.

Matt - I'm not sure that I see the two 12's as being shorter. I do make a crossover modification for center channel use, write me if you want and I'll tell you what to do. I don't think that I was doing this when you bought yours. I would just turn the speakers on their side, which isn't ideal, but its not going to be a big issue either.
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Old 17th October 2010, 12:25 AM   #1945
paulspencer is offline paulspencer  Australia
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With a lot of comparative measurements of subs out there now, even the same driver sealed and vented, you can easily talk about distortion and also ringing and compression. The big question is how do you interpret the results in a meaningful way.
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Old 17th October 2010, 01:24 AM   #1946
gedlee is offline gedlee  United States
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The big question is how do you interpret the results in a meaningful way.
I think that is what I was getting at. Looking at THD at levels below clipping is meaningless and if the amp is clipping then thats simply a "use" issue - you are using it past its design intent (well not according to the marketing brochure!) At levels below the amp clipping, I have found that only port noise is an issue. Nothing else matters.

But nobody (except me, I guess) measures port noise, so that leaves you without much data.
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Old 17th October 2010, 02:31 AM   #1947
paulspencer is offline paulspencer  Australia
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Nothing else matters.
Not even compression?
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Old 17th October 2010, 05:05 PM   #1948
gedlee is offline gedlee  United States
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I have not found compression to be a problem at LFs. It appears to be a problem only in the higher frequencies where the signal transients occur and by necessity voice coils are smaller with less thermal mass. In a sub I don't see compression as a factor at all.
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Old 17th October 2010, 06:12 PM   #1949
pjpoes is offline pjpoes  United States
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Dr. Geddes, you previously sent me some sort of mod for using it as a center. It was a change in the trap values I believe, and a change in the cap value on the parallel wired cap and resistor in series with the tweeter.

I tried it on it's side, sounded terrible. Added a lot of harshness. I tried a lot of things to improve things, nothing worked as well as simply putting it back up right. On it's side everything sounded somewhat harsh and nasally. In my opinion, not a small difference either. I thought it might be reflections off the floor, so I placed 2" of rockwell on the floor all around it. It didn't solve the issue, so either it wasn't enough, or it wasn't the problem.

My thought was that two 12's would allow a shorter speaker than the Summa, but could still use the 15" waveguide that has less on-axis issues. The idea would be to place the 15" waveguide into the space or crevice made between the two 12's when next to each other. The two 12's would be together like a figure 8 on it's side, and then that v crevice would be where the waveguide is nested. The 12" waveguide would fit best, but then its just shorter, no better on-axis I would assume.
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Old 22nd October 2010, 07:46 PM   #1950
davebtw is offline davebtw  United States
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I would love to hear some Abbey's. However, I live here in the sticks, in the Salt Lake City area. Almost all of the DIY, and other speakers, for that matter are very hard to audition here.

Anyone with some Abbeys in Utah that would like to share an evening of listening?
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