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-   -   How To Get The OB Sound From A TLine Or Sealed Box?? (http://www.diyaudio.com/forums/multi-way/122152-how-get-ob-sound-tline-sealed-box.html)

Ben Reginato 28th April 2008 10:28 PM

How To Get The OB Sound From A TLine Or Sealed Box??
 
What tricks can I use to get a sealed box or TL to sound as close to an open baffle as possible.
The speaker is WWMTMWW with the MTM part in sub enclosure. The Mid to Woofer
Crossover is 200 Hz, its the mid section that I what to have the OB sound.
Is the elimination of the back wave the reason OBs have the open and airy sound?
Should I make the inner box sort of a rectangular pyramid shape?

Thanks Ben

Cal Weldon 28th April 2008 10:41 PM

2 Attachment(s)
Hi Ben,

I think you are asking a lot. The reason you go to an OB is 'cause you like the sound over a box unit so I can't see what you're asking. Can you leave the back off the mid enclosure or is it fully contained within the main enclosure? If so, it's back to the drawing board. I have had good luck with both OB and open back mids such as this. Maybe you could do something similar?

Hezz 29th April 2008 12:38 AM

Re: How To Get The OB Sound From A TLine Or Sealed Box??
 
Quote:

Originally posted by Ben Reginato
What tricks can I use to get a sealed box or TL to sound as close to an open baffle as possible.
The speaker is WWMTMWW with the MTM part in sub enclosure. The Mid to Woofer
Crossover is 200 Hz, its the mid section that I what to have the OB sound.
Is the elimination of the back wave the reason OBs have the open and airy sound?
Should I make the inner box sort of a rectangular pyramid shape?

Thanks Ben

There are some things you can do to get some of the open airiness but they will not be dipolar or as fast as OB speakers.

Some methods that help.

1. Make sure the mid bass in a three way is isolated in a large enough chamber so that it would sound good even if it where playing full range.

2. Do as much as you can to suppress standing waves and pressure inside the box. I like to use aperiodic alignments for this reason. They sound more natural to me. The resistive vent can help reduce pressure inside the box. This is the fundamental reason I do not like closed box systems unless they are critically damped.

3. Since cabinet pressures will be higher in a box speaker the cabinet needs to be significantly overbuilt to keep the cabinet from singing along.

4. Fill the entire box with fiberfill and experiment to get the amount just right.

One approach that might work well is to build a three way with the tweeter in an isolation cabinet and the midrange in it's own aperiodic or open back cabinet and the woofer in a good TL alignment. I think this combination would be killer if done right.

Ideally the midrange cabinet should have three qualities. Free from resonance, free from internal pressure waves and totally anechoic and devoid of echos. The only way that I can see to really approximate this is with an open back that is large enough for some serious deep acoustic foam on all the sides, lightly stuffed in the center with fiberfill and the back left open for pressure waves to vent. Or since the fiberfill may just slow down the rear wave it may create an out of phase rear wave that is to high in volume. It may be better to have only the sides deeply treated an a clear path out the rear so as not to cause any time delays for what part of the rear wave that does escape.

Ben Reginato 29th April 2008 01:17 AM

Hi Cal & Hess

Yes I know Im asking a lot but I dont want any of the negatives of the Open Baffle set up. Such as higher IM distortion, having to equalize the Mids not to metion the fact my speaker can only be out from the back wall 16-18 max. My better half is very visual and I have to fight for every inch.
What if I make the inner box sort of a rectangular pyramid shape feeding into a stuffed 4or5 Precision Port tube coupled to PVC pipe reducers i.e. 5to4,4to3 3 to2.5
Would that funnel the back wave away from the drivers, which is a 5.25 coated paper mid bass driver?

critofur 29th April 2008 04:09 AM

Cut a hole in the floor and attach about 4 large mufflers to the bottom of the speaker through pipes running through the hole in the floor. If each muffler is large enough for a good size V-8 engine then that setup should be able to damp the back wave without pressurizing the enclosure much.

If your wife won't allow that then tell her you're installing another heater duct, and make large fake heater with the whole thing filled with fiberglass, gradually denser and denser as you get to the far end (perhaps about 10 feet long) and attach the bottom of the speaker to your new fake duct. Ideally, the outside of the duct can be the regular galvanized steel (so it looks like a duct) then the inner walls could be something like 1 inch thick marble.

ScottG 29th April 2008 04:18 AM

See the solution employed here:

http://www.htguide.com/forum/showthread.php4?t=28728

HiFiNutNut 29th April 2008 04:44 AM

Hezz,

Thanks for your post. I agree completely.

I am undecided on whether to damp the centre (the area behind the driver magnets) or not on my OB U-frame MTM. I have listened to both methods of damping and they sound different rather than better or worse. I have not finalised the passive XO yet and a simulated XO is being used for test only.

Damping creates a low pass filter for the back waves. Can you describe more on the phase shift? Would the effect be the same as a longer D / depth in calculations? What harm would it cause?

Here are my new speaker photos:



http://home.iprimus.com.au/cbsoftware/HiFiRoom.jpg


http://home.iprimus.com.au/cbsoftwar...ofAthens_1.jpg


http://home.iprimus.com.au/cbsoftwar...ofAthens_2.jpg


Regards,
Bill

Hezz 29th April 2008 05:54 AM

2 Attachment(s)
Wow!!!,

Great work HIFINUTNUT. Beautiful setup. I don't have room for the larger cabinets that Open baffle or open back cabinets require but I have this idea that you could heavily damp the area around the back of the open back type of setup. The walls surrounding the driver but of course not the back. I think it would attenuate a lot of any near reflections. However, if too close it could acoustically couple.

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.

I use aperiodic alignments since I need to built smaller cabinets.

As far as I know when sound waves hit a filling material they slow down to some degree and are attenuated to some degree. My thinking is that for an openback you could vary some of the dipolar effect to some degree with the amount of stuffing. With a lot of stuffing you would be effectively creating an aperiodic tuning.

BEN,

One other thing that helps speakers to sound more open is to use the biggest cabinet that you can that still has good group delay properties.

For instance, say you have decided that your wife will only accept a 2 cubic foot cabinet. Then instead of stuffing a bunch of drivers into it try creating an ideal alignment for a 6 or 8 inch driver.

Here is an example of a modest sized three way floor standing cabinet that I worked up some time ago. About 34 inches tall it was designed for an 5 inch midrange and an 8 inch woofer in about 1.5 cubic feet. The idea was for bass reflex tuning of the woofer and aperiodic of the midrange but you could use aperiodic on the woofer also. I think you could build something like this and leave the upper cabinet open back and experiment with different damping materials on the walls and different filling until you found the optimum configuration. Also, for a closed box speaker I can't emphasis enough how important the cabinet construction quality is to get that more open sound.

ttan98 29th April 2008 06:05 AM

HiFiNutNut,

Can you give a brief description of your MTM open baffle?, eg

1. type of drivers used
2. x-overs frequencies.
3. depth of U-frame
4. low freq. response
5. any equalization

etc.

thanks.

audiokinesis 29th April 2008 06:19 AM

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.

A bipole interacts with the room similar to a dipole at midrange and treble frequencies, but doesn't require equalization in the bass region. My nomination for how to get a sealed or TL box to sound like an open-baffle speaker is to make it a bipole and then place it five or six feet in front of the wall.

Duke


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