How To Get The OB Sound From A TLine Or Sealed Box?? - Page 2 - diyAudio
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Multi-Way Conventional loudspeakers with crossovers

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Old 29th April 2008, 06:32 AM   #11
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hezz,

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I have tried damping near the cone but never found that approach sounds good. I think the problem is that the damping material can acoustically couple with the cone. I have always found damping sounds the best if it is at least 3-4 inches back from the driver. Somehow the driver needs a few inches of free space to breath or something.
Interesting. I shouted into the U-frame numerous times while changing damping methods until I got no or the least amount of echo. Perhaps it depends on the materials and the density of the materials. I damped all sides leaving only holes about the size of the drivers at the back of the magnets. We need that much damping at least for a U-frame to be used for midrange. Adding additional damping at the back of the magnet changed the sound. It became clearer, cleaner but lack of atmosphere due to reduced backwave bouncing off the front wall.

ttan98,

When I have time I will make separate posts.

Duke,

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In my opinion, the openness of a dipole system is largely due to the addition of a significant amount of reverberant energy whose onset is late enough that it doesn't fuse with the first-arrival sound. This benefit accrues if the backwave has to travel a good five or six feet to reach the wall behind the speaker before reflecting back towards the listener.
perhaps only at higher frequencies. At low frequencies the sound would wrap around the baffle and add to / cancel the front waves.

Regards,
Bill
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Old 29th April 2008, 07:01 AM   #12
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Bill/HiFiNutNut,

The wrap-around energy from a bipole's rear-facing woofer is a complication, and several different techniques for dealing with it have been used by various designers. Describing these techniques would take a while and be something of a thread hi-jack.

With appropriate design, the wrap-around can mitigate both the baffle step and the floor-bounce notch, at least in the frequency domain. And some approaches (such as Definitive Technology's patented side-firing woofer bipole) do not produce a wrap-around notch. The in-phase reinforcement in the bass region helps offset the relative lack of boundry reinforcement from placing the bipole out in the room a ways.

Duke
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Old 29th April 2008, 05:19 PM   #13
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Quote:
Originally posted by HiFiNutNut
hezz,



Interesting. I shouted into the U-frame numerous times while changing damping methods until I got no or the least amount of echo. Perhaps it depends on the materials and the density of the materials. I damped all sides leaving only holes about the size of the drivers at the back of the magnets. We need that much damping at least for a U-frame to be used for midrange. Adding additional damping at the back of the magnet changed the sound. It became clearer, cleaner but lack of atmosphere due to reduced backwave bouncing off the front wall.


Bill

Ya, I think this illustrates that there is a trade off between the two qualities. However, I think that one of the benefits of this midrange loading is that you can adjust it relatively easy to get the best balance that you like and that works well in the room you have the speakers in.

Ben,

I think some kind of a hybrid approach is going to be the only thing that gets you close to what you want. Because of this a three way is the best approach. The woofer will need a more conventional loading to keep the cabinet of modest size and I think an TL would blend the best but make the cabinet too large if you want deeper bass. Therefore I suggest an aperiodic woofer loading to complement the midrange OB sound.
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Old 4th May 2008, 10:38 AM   #14
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This is my recent attempt. While a full-OB sounds nice, the looks leave little to desire in a small room

Amazing after having OB for months now and without that "OB section" any listening gets really fatiqueing very fast. Perhaps the brain is looking for a "missing information" which the OB is...

Click the image to open in full size.

Admittedly the bass is not OB, but the plus is now I have SLAM
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Old 5th May 2008, 01:08 AM   #15
badman is offline badman  United States
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midwoofs that small do not get to have SLAM
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Old 5th May 2008, 02:33 PM   #16
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It does if equalised ...

Sorry I have to take that design back. It's crap sound compared to real OB. Back to the big one now:

Click the image to open in full size.



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Old 9th May 2008, 03:46 AM   #17
SamL is offline SamL  New Zealand
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HiFiNutNut,
Looks like you using 2 front chn vs 3 for HT. Or there more speaker to come? Any speaker at the rear?
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Old 10th May 2008, 01:29 AM   #18
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I have been thinking about what would make the ultimate open back loading and I have come up with some ideas.

Since I am not an expert on horn theory perhaps someone can correct any misstatements that I make.

First, is the lowest possible crossover frequency for the driver in the open back topology. And perhaps someone can pipe in here so we get a more accurate information. As I see it the crossover must be a bit higher than the wavelength of the frequency which cooresponds to the baffle width for a open baffle design. But for the open back we have some backwards baffle that wraps around the side. Can we add the two distances of the front baffle to the depth of the side baffle or do we need to make the calculation based on which of the two is the longer? Or is the path calculated as a straight line from the center of the driver cone to the first open edge at the back?

Second, It seems like from a resonance point of view in the open back cavity that some kind of an expanding horn shape would have fewer resonance problems and not emphasize one or two frequencies. However, it is my understanding that as the sound wave expands it creates a low pressure area near the cone which is what we don't really want since this will reduce the cones ability to damp it's own resonances when it needs to stop.

B&W have long claimed that gradually reducing the energy of the back wave is the best approach and this requires an inverse taper to the expanding horn loading. The problem is with the B&W approach is that they used a closed design which does not end up working as good as the theory predicts.

I therefore have this idea that we could have some kind of an expanding horn shape out the rear of the cabinet which at the terminus has an area at least two or three times the driver cone area. And then an inverse horn shape is formed inside this expanding horn with thick sound absorbing materials like rockwool which expands and gets thicker towards the terminus of the horn. As the standing waves get lower in frequency the damping material becomes thicker and works better lower in frequency. Then the size of the terminus should be adjusted so that it creates some small degree of spring effect for the driver. Perhaps an open area half the size of the cone area would be a place to start. It might be possible to critically damp a driver this way by optimizing the terminus area of the cavity.
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