2 X 11 cu ft Q18 slot ported subs build - Page 7 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #61 of 237 Old 02-01-10, 02:02 PM
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Re: 2 X 11 cu ft Q18 slot ported subs build

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Moonfly wrote: View Post
I dont doubt you guys, I just prefer the sealed sound, and I get just the low end thump I desire an expect, and more so than I did with the ultra I had.

As long as we're all happy, thats the main thing
I'm in agreement. Sealed boxes may be less efficient but there is far better control of the driver due to the box being sealed. Instead of a 24Db octave (or greater) rolloff below the box tuning frequency that goes out the window, instead you get 12Db/octave & far less group delay
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gperkins_1973 wrote: View Post
Dan,

Agreed matey. Thats the important thing defo. The one thing I was going to say was even though my sealed sub had a high qtc, once EQd doesn't that pretty much go out of the window.
Not really no. To get a critically damped system Qtc needs to be around 0.5, anything above this will result in a slight "overhang" on any low bass note being produced. Just EQing won't get rid of that but you can get rid of it via some electronics. What you'd need is a Linkwitz Riley filter network, this can actually not only equalize the enclosure to a much lower frequency, but also get rid of that overhang in the driver response.

When i say overhang what i mean is the driver doesn't instantaneously respond to the signal applied & does stop as soon as the signal ceases. With some subtle electronics it can be made to do so

Qts of a driver is determined mostly by the strength of the magnetic field surrounding the voice coil. Low Qts drivers have a much better control over driver motion simply because the amplifier can act as an electronic brake or accelerator. With a high Qts or Qb the amp has less control & the driver can do what it likes in certain respects.

I always think about it in a way like cars. British cars tend to (sorry no offence meant at all) have good damping & do what they are told so react pretty well to changes (just like we'd like our speakers to do ). American cars on the other hand appear to wobble for a good 5 seconds when they stop, something like a waterbed & this is the same kind of effect that an underdamped (high Qts or Qb) driver will display (though not as badly - it's just to give you the idea). To some extent reflex enclosures whether they are low of high Qts drivers will exhibit the same effects. Better by far to use a driver in any reflex enclosure with a Qts of 0.38 or lower. Any higher & you'll get the waterbed effect, with it getting vastly worse the higher the driver Qts.

Lower Q will result in a faster more precise response & better musicality, lower phase shift & group delay will also promote the very same. You can go too far though in which case you end up with aneamic bass


Nice build by the way Hope it all goes to plan!
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post #62 of 237 Old 02-01-10, 02:09 PM
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Re: 2 X 11 cu ft Q18 slot ported subs build

I would just like to add that my drivers have a Qts of .36 , and my estimated QTC value is about .695, right where I want it . I did cost me more to use 2 drivers and go push/pull, but I really believe in the result. Its a personal choice thing though.

Graham, cant wait to get some feedback from 2 of those in your room, movies are certainly going to be powerful.
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post #63 of 237 Old 02-01-10, 02:27 PM
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Re: 2 X 11 cu ft Q18 slot ported subs build

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I would just like to add that my drivers have a Qts of .36 , and my estimated QTC value is about .695, right where I want it .

Graham, cant wait to get some feedback from 2 of those in your room, movies are certainly going to be powerful.
Powerful, no kidding I doubt you'll ever need to hit full volume with one sub, let alone a pair Remember we are talking UK front rooms here

Just had a quick peak at your sub build Moonfly, nice indeed. The drivers being driven in phase that way will result in lower distortion due to cancellation of certain harmonics. 0.707 Qb would be regarded as good with a sealed box, i'm sure it sounds great!
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post #64 of 237 Old 02-01-10, 02:42 PM
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Re: 2 X 11 cu ft Q18 slot ported subs build

Cheers for the comments.

Ive actually built a second one for a friend and it certainly is an animal, and runs clean at reference level. Like Graham though, he is bass nut (meant affectionately ), and wants 2 in is room.

FYI, The push-pull alignment was a very deliberate choice , but we digress . . . . . . .
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post #65 of 237 Old 02-01-10, 04:55 PM Thread Starter
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Re: 2 X 11 cu ft Q18 slot ported subs build

Guys,

I will have another play on Wednesday and EQ the one sub. I played with some filters and got it pretty flat. I am obviously limited on position but I wonder if the second sub may even out that dip at 55hz. I can reduce the peak easily enough. I am very impressed with the bottom end I now have which is great. My main problem with doing another sealed sub is that I would never have got down any where near a qtc of 0.7 unless I put the sealed sub in a 28 cu ft box but then you go over max excursion very easily so the ported route based on my drivers is probably the best way to go.

I will probably have some questions over the weekend when I EQ both the subs.

cheers

Graham
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post #66 of 237 Old 02-02-10, 03:06 AM
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Re: 2 X 11 cu ft Q18 slot ported subs build

I'm glad to see that going ported fixed your low bass wants.
Speaking of smoothing out graphs; hopefully I can order a AntiMode 8033 tomorrow or something for my room
I'm anxious to see if it will make a big difference or not.

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post #67 of 237 Old 02-02-10, 03:18 AM
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Re: 2 X 11 cu ft Q18 slot ported subs build

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sub_junkie wrote: View Post
I'm glad to see that going ported fixed your low bass wants.
Speaking of smoothing out graphs; hopefully I can order a AntiMode 8033 tomorrow or something for my room
I'm anxious to see if it will make a big difference or not.
Ive heard a lot of good thing about the antimode, but when I demo'd one I found it didnt make a whole heap of difference for me, either on the graph or audibly. I felt it made the timing of my then sub a touch better, but not enough to discount placebo.

I think the main reason for this was that I already had a good in room response, plus I was already using Audyssey. From that I decided that if you have a decent room, and already have eq in place resulting in a good response, its probably not really needed. If however non of that applies its probably a very good tool.

Just my ten pence.

P.S, nice build BTW.
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post #68 of 237 Old 02-02-10, 06:43 AM
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Re: 2 X 11 cu ft Q18 slot ported subs build

Graham, can you overlay the sealed graph with the latest so we can see the difference?




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post #69 of 237 Old 02-02-10, 07:16 AM Thread Starter
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Re: 2 X 11 cu ft Q18 slot ported subs build

Mike,

I will put up a pic when I get home. I did do it last night and there was alot more bottom end. I think it had a similar peak and null although my ported sub was slightly more I think. I haven't played around with positioning yet though. Probably best to wait until wednesday.

cheers


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post #70 of 237 Old 02-02-10, 01:58 PM Thread Starter
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Re: 2 X 11 cu ft Q18 slot ported subs build

Mike,

Here's the sealed and ported un EQd graphs. I still need to have a play with the sub facing into the wall or how it is now. I did try it facing into the wall quickly but the bass was over powering. Blue is ported and red is sealed.

cheers
graham
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