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Old 26th January 2010, 12:00 AM   #1
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Default Dipole Sub in small room?

Hi All,

Amongs other things, I design & build turntable bearings. The significance of this, is that I have a particular requirement to monitor very low frequency information, very accurately.
I've got a fairly huge collection of interesting although not necessarily ideal drivers to draw on; 8 x B139 6171's, 4 x B139 1044's, a couple of Wharfedale Airedale 15" units, 4 x 18" pro-drivers, and loads of obscure stuff too. Amplification isn't a problem either.
Because I'll be needing to record results via computer much of the time I can employ a digital equaliser (got one of those behringer things).

My listening (living) room is tiny, and horribly laid oud, but I can't do much about that - it is ~5 metres square with a fireplace along one wall that juts out maybe 0.5M x 2.5M, and there's a rediculous internal porch in one corner that is 1M square. Tell it to my landlord, I know.

I've read some of the older posts with regards to small room dipole interaction, but they appear to be somewhat inconclusive or incomplete. My question is, in my tiny room; will I be able to cleanly reproduce frequencies down to maybe 15Hz using dipole subs?

Cheers,

Jeremy

Last edited by Jezz-the-Fezz; 26th January 2010 at 12:09 AM.
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Old 26th January 2010, 06:35 AM   #2
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For turntables, would it not be better to use something cheap, and simply watch the excursion? If you blow it, no matter because they're £5 to replace (well, maybe not, but you get the idea.

I don't think openbaffle/dipole will do what you're after, as those long wavelengths will cancel very quickly.
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Old 26th January 2010, 06:51 PM   #3
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Hi Chris, I really need to hear what's going on & I think your confirming my thoughts. I think the other option is to stuff all of the B139's in isobaric 200L enclosures. At -15dB@16Hz, room nodes will easily flatten the response at that frequency; not as pure as a dipole, but it will get there.
So far as watching the drivers excursions are concerned, that won't be necessary - it'l be going through a PC anyway. If I could do it with dipoles I would, but seems like I can't! Thanks for the input.
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Old 26th January 2010, 08:32 PM   #4
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Sounds like a pretty serious subwoofer. Being 15, I'm stuck with a pair of 8"s for a sub, then 2x5" woofers per side.

I'll keep an ear out - I'm not too far from Bolton.
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Old 27th January 2010, 06:30 AM   #5
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Yea, you can totally forget about getting any deep bass from OB w/o some serious fenangling like a seriously high Q, steep slopes, compensation, many woofer, lots of power, etc... Just not worth it.

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Old 27th January 2010, 05:33 PM   #6
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Dipoles are probably the most inefficient subwoofer design, which means that you need to throw alot of money at them to do what cheaper subs of a different design can do. Concerning the room interaction, as mr. geddes likes to say, the room dominates the sources, which means that statistically seen, the type of source doesnt matter. I would suggest a closed box or a bandpass sub, as you have lots of drivers around and dont need the more complex designs to get the most out of a small driver.
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Old 27th January 2010, 11:33 PM   #7
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Di-poles have a "semi-cardiod" dispersion pattern.

Sit in the wrong place or place them in a disadvantageous setting and they pretty much suck. Place them properly and they are quite forgiving, powerful and they don't "infect" the nearby areas with unwanted bass.

Check out Linkwitz site, if you don't quite get it, yet.
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Old 28th January 2010, 03:13 AM   #8
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Dipoles need a lot of displacement and path length to give useable output down to 15 Hz. This is especially true in a small room, where the distances to the room boundaries are probably less than 1/4 wavelength down at your target frequency so there is no possibilty of even beginning to pressurize the room down there.

If you are going to equalize and measure at a single microphone location, then a single large, low-tuned vented subwoofer in a corner is probably the most cost-effective approach. Use whichever model of the B139 has the longest excursion, and as many of them in the box as you can get away with while still achieving adequate extension.

If you want smooth in-room bass throughout the room rather than only in a single microphone location, then I suggest a distributed multisub system whether or not you equalize. I prefer asymmetrical distribution in all three planes.
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Old 28th January 2010, 03:46 AM   #9
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What about the effect of room gain? In a 5M x 5m room gain is likely to boost low
frequency responce. The old Absolute Sound did a review of an infinite baffel sub that had a fs of 30HZ with a QTS of .7 but because of room gain it was flat down to 20hz. Perhaps an infinit baffel with a fs of 25hz and a qts of .7 might be all you need?
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Old 28th January 2010, 11:42 AM   #10
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Default dipole subs

Quote:
Originally Posted by MaVo View Post

...which means that statistically seen, the type of source doesnt matter.
...
The type of source does matter. One interesting property of dipole radiators
is the ability to change the excitation of wanted/unwanted modes
very effectively by simply r o t a t i n g the sub(s) in the room.

That means more freedom in optimizing mode excitation, since in the bass
range the modal density of almost all home listening room is small and mode
excitation has to be optmized. Statistical room acoustics does not apply
to acoustically small rooms ("Schröder Frequency").

With monopolar subs the only thing you can do is move the sub to change
modal excitation. With dipoles you can move and rotate.

A different kind of fish are "very small rooms". If the lowest mode in the
room which can be excited is above say 50 Hz, the only technique
able to produce something like low bass reproduction is "room pressurization".

That is not possible with a dipole sub, and a (group of small) monopolar sub(s)
may in fact be superior. I personally do not like room pressurization and
would try to avoid it, since it is a kind of bass experience very very far
away from what you experience in a live concert (hall or open air).

By the way: That room pressurization will disturb your neighbours -
if any - to a much greater extent than modal excitation using
dipoles.

Kind Regards
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Last edited by LineArray; 28th January 2010 at 11:48 AM.
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