OB noob question

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Pault

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First post here and it's a question(what else could it be?).It is my understanding that using a woofer in OB requires the driver to be of high Qts.Exactly how high is high enough?Am I missing something?Am I wrong?For example,an MTM OB configuration with 6,5" mids, what kind of drivers needs?What other specs of the driver are of big importance for OB designs?And what about 12" or 15" drivers?(Qts etc.).Thanks in advance!

Pavlos
 
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AndrewT

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big drivers with low to medium Xmax seem to perform well in OB.
sufficient small drivers giving adequate cone area should still be able to give good performance in OB.

Qts ~ 0.5 to 1 may work OK. But I've never built one for long term listening.

I have tried a 6inch coaxial designed for ceiling mounting in an 8foot by 4foot by 18mm board and was surprised at how well it played.
 
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tinitus

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ultrachrome said:
Your mid requirements are tied to the width of your baffle and your desired woofer crossover point.
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Expect a small driver to be almost "dead" or at least attenuated below 300hz, which is also the BSC area
Just small losses at these frequencies leads to thin and boring sound
I believe in the old days it was often wrongly called analytical sound, but probably still is

So you need some exstra SPL in the midbass
Either with EQ
Or multiple woofers
2.5way may work, or may not due to phase issues from mid rolling off
Or use a bigger woofer with higher sensitivity

Im not sure how I would do it
It could be a big true dipole
Or it could be a smaller hybrid with boxed woofer

But even skilled people who thought it would be easy, have realised that it can be quite tricky and take quite a lot of work
Others seem to have been more "lucky" without much problems
 
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tinitus

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Pault

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Joined 2008
First of all, thank you for your answers.
I asked specifically for the mids because the project I have in mind is a WMTMW speaker,where only MTM is OB.For the woofers I'm thinking of these http://tymphany.com/830667 in a 60 ltr box tuned to 33Hz with a 12" Dayton PR and crossed at about 250Hz.As for the tweeter I guess the Peerless HDS should be fine.The mids are still a mystery.What do you think of this project?Is it doable or I'll end up banging my head against the wall?
 
Rudolf

Rudolf

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Joined 2003
dipolplus.de
Pault said:
It is my understanding that using a woofer in OB requires the driver to be of high Qts.Exactly how high is high enough?
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You can build a dipole with any Qts you want. But: Practically you need to equalize any driver until its frequency response behaves like a driver with at least Qts=1. For Qts from 0.7 up the variable bass boost of many subwoofer modules will be sufficient. Drivers with Qts >0.9 probably will do without special EQ. When going above Qts=1.2 things will start to get worse again.

Am I missing something?
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The primary and limiting factor is not Qts, but the drivers ability to move air. The product of Sd and Xmax should be at least 4 times bigger than for the woofer in a CB you find sufficient for your room.

For example,an MTM OB configuration with 6,5" mids, what kind of drivers needs?
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In that configurations the mids are no problem. You will have a harder time to find an equivalent tweeter.

What other specs of the driver are of big importance for OB designs?
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You need to look for "silent" woofers. No ringing baskets and chuffing pole vents. There is quite some background noise that can be subdued by a box but might get annoying in an OB.

And what about 12" or 15" drivers?(Qts etc.).
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Serious OBs start with double 12" or single 15" woofers. If you want lots of headroom think about 21". 8" or 10" drivers may be an option for dipole line arrays.
 
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tinitus

Ex-Moderator R.I.P.
Joined 2005
Pault said:
BTW Tinitus these drivers seem quite interesting.As for EQing I'll probably use a Behringer DEQ.
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Whether those Omnes/TB drivers are adequate or not, they may tell you something about whats needed

Trick of it is as Rudolf touches...in a MTM its important to cross low, and the 2" TB may be used down to about 1khz, where impedance begins to rise, maybe best crossed just a little higher

A TB 4" ti ought to be good as tweeter too, and could b crossed even lower, but cost more
I have read that it sounds nice fullrange, but have to try it first...and I will
If unable to cross that low, I would consider a single mid

AND it would be true dipole, which a dome tweeter is not

I would maybe consider a cheap supertweeter like the small Fountec CD1.0MB(cost only 38EUR)


If you really like the HDS tweeter, it might work nicely with just a single Eminence Beta-8A...or maybe two in a 2.5way, and consider a small "waveguide" fore the HDS
But dont know really... :clown:
Would also work in a box, with actiive subs :D
 
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