Open backed boxes ? Anyone ? - Page 3

Scottmoose · 4th September 2008, 02:22 PM

If you like them, that's all that matters.

I've designed & built a handful of OBs, and several of my friends run different types: 4 - 7ft electrostatics in two cases, Lowther EX4 supported by twin vintage 12in Goodmans drivers in another; a Quasar style U frame in still another. I've heard plenty of others too. They all sound very good, but TBH, I just don't like dipoles / OBs very much (with the exception of the 4ft 'statics. Box speakers are what I like. To each their own poison.

That said, perversely, I was plotting an open-back OB focused array for a future build when this thread started, just for the sake of it really, but I'll stick a back on it when I'm done.

skorpion · 4th September 2008, 03:25 PM

Scottmoose,

You are more than welcome anyway ! Would be a pleasure to exchange views and experiences.

But of course you are right. In the case of the 'Volks-OB' Beranek's law rules.

/Erling

GM · 4th September 2008, 03:26 PM

Quote:

Originally posted by skorpion

The argument about standing waves, resonances etc in 'Topless-U's has been covered by MJK (with argument) and myself (with measurement) in some threads over at the Audio Circle.

Link?

TIA,

GM

MJK · 4th September 2008, 10:56 PM

Quote:

Originally posted by GM

Link?

TIA,

GM

GM,

I think this is the discussion Erling was referencing.

http://www.audiocircle.com/circles/i...5690#msg315690

skorpion · 5th September 2008, 12:37 PM

GM,

MJK is quite right in his link and there was also some earlier discussion.

However my measurements have occured here: http://www.audiocircle.com/circles/i...?topic=40827.0 . and here: http://www.audiocircle.com/circles/i...1154#msg441154 .

/Erling

Scottmoose · 5th September 2008, 02:05 PM

Ah! Now we're getting somewhere. Possibly. I think the telling phrase in those links is 'proper dimensions.' Surely proper dimensions are dependant upon the design goals? I can certainly buy & never doubted that the flat-front OB sheet can be used to model an open topped U-frame assuming you happen to be sizing it in such a way as to ~mimic the behaviour of a flat baffle. However, I'd be cautious about applying it as a universal rule, given that not every design actually has that particular object in mind. Martin, Greg? Any thoughts? Or am I just worrying about nothing (& hearing problems that aren't actually there)?

MJK · 5th September 2008, 02:12 PM

What I have done to model an OB with wings that extend back at some angle is to try and bound the results. I model the minimum baffle case where only the front flat baffle is included and then I model the maximum case where the wings are assume to be perfect flat extensions of the flat front baffle. My assumption is that reality is between these two cases.

Wings that are folded back will impact the sound of the OB system due to the geometry, however I do not believe they impact the sound due to resonances between the panels. As with everything else in audio, the wings may improve or degrade the performance. I would not draw a blanket conclusion about their impacts.

Scottmoose · 5th September 2008, 02:29 PM

Makes sense.

Cheers Martin.

REC1 · 5th September 2008, 04:13 PM

It just appears to me as the maximum resonance between two (2) panels is when both panels are 90 degrees to the main baffle and providing a total reflection surface..
As the panels are folded towards the main panel the resonance should decrease by 1-(sin angle) as a percentage of the zero ( perfect reflector) degrees. A Z shaped(two Ls) baffle might cure this.

Just a thought.

Been away for awile due to health problems.

One thought (while i was ill, so take it for whats its worth). Looking at Scotts excellent half Chang what if the unit was rotated so the mouth was @ the top then install the driver on the top horz surface with a 45 degree reflector. This would produce sound from a singular area throught the band. Added to another smaller reflector mounted in the mouth and mids/highs would be distributed by a two stage reflection action. This might cure the rising response with HE SD low Qts. Tuning would be by damping the side walls (HF) or the panel above the smaller deflector (mids).

Dave, i havent forgotten about you.

ron

Scottmoose · 5th September 2008, 05:19 PM

Heck yes! I like that. Oh, I like that a lot Ron, if we're thinking on the same lines (something like the attached?) We'd be talking a simpler baby version of the old Lowther-Voigt horn, or possibly the JBL Aquarius 3 minus the weird LF loading. And here's me with a pair of B20s to play with, plus tweeters. This I have to try.

Sorry to hear you haven't been well mate. Take care & get yourself right.

Scott

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4th September 2008, 02:22 PM	#21
Scottmoose diyAudio Member Join Date: Mar 2005 Location: UK	If you like them, that's all that matters. I've designed & built a handful of OBs, and several of my friends run different types: 4 - 7ft electrostatics in two cases, Lowther EX4 supported by twin vintage 12in Goodmans drivers in another; a Quasar style U frame in still another. I've heard plenty of others too. They all sound very good, but TBH, I just don't like dipoles / OBs very much (with the exception of the 4ft 'statics. Box speakers are what I like. To each their own poison. That said, perversely, I was plotting an open-back OB focused array for a future build when this thread started, just for the sake of it really, but I'll stick a back on it when I'm done.

4th September 2008, 03:25 PM	#22
skorpion diyAudio Member Join Date: Oct 2004	Scottmoose, You are more than welcome anyway ! Would be a pleasure to exchange views and experiences. But of course you are right. In the case of the 'Volks-OB' Beranek's law rules. /Erling

5th September 2008, 12:37 PM	#25
skorpion diyAudio Member Join Date: Oct 2004	GM, MJK is quite right in his link and there was also some earlier discussion. However my measurements have occured here: http://www.audiocircle.com/circles/i...?topic=40827.0 . and here: http://www.audiocircle.com/circles/i...1154#msg441154 . /Erling

5th September 2008, 02:05 PM	#26
Scottmoose diyAudio Member Join Date: Mar 2005 Location: UK	Ah! Now we're getting somewhere. Possibly. I think the telling phrase in those links is 'proper dimensions.' Surely proper dimensions are dependant upon the design goals? I can certainly buy & never doubted that the flat-front OB sheet can be used to model an open topped U-frame assuming you happen to be sizing it in such a way as to ~mimic the behaviour of a flat baffle. However, I'd be cautious about applying it as a universal rule, given that not every design actually has that particular object in mind. Martin, Greg? Any thoughts? Or am I just worrying about nothing (& hearing problems that aren't actually there)?

5th September 2008, 02:12 PM	#27
MJK Account disabled at member's request Join Date: Feb 2002 Location: Clifton Park, NY	What I have done to model an OB with wings that extend back at some angle is to try and bound the results. I model the minimum baffle case where only the front flat baffle is included and then I model the maximum case where the wings are assume to be perfect flat extensions of the flat front baffle. My assumption is that reality is between these two cases. Wings that are folded back will impact the sound of the OB system due to the geometry, however I do not believe they impact the sound due to resonances between the panels. As with everything else in audio, the wings may improve or degrade the performance. I would not draw a blanket conclusion about their impacts.

5th September 2008, 02:29 PM	#28
Scottmoose diyAudio Member Join Date: Mar 2005 Location: UK	Makes sense. Cheers Martin.

5th September 2008, 04:13 PM	#29
REC1 diyAudio Member Join Date: Jun 2008	It just appears to me as the maximum resonance between two (2) panels is when both panels are 90 degrees to the main baffle and providing a total reflection surface.. As the panels are folded towards the main panel the resonance should decrease by 1-(sin angle) as a percentage of the zero ( perfect reflector) degrees. A Z shaped(two Ls) baffle might cure this. Just a thought. Been away for awile due to health problems. One thought (while i was ill, so take it for whats its worth). Looking at Scotts excellent half Chang what if the unit was rotated so the mouth was @ the top then install the driver on the top horz surface with a 45 degree reflector. This would produce sound from a singular area throught the band. Added to another smaller reflector mounted in the mouth and mids/highs would be distributed by a two stage reflection action. This might cure the rising response with HE SD low Qts. Tuning would be by damping the side walls (HF) or the panel above the smaller deflector (mids). Dave, i havent forgotten about you. ron

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