BLH damping & bracing tips - diyAudio
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Old 19th July 2006, 08:18 AM   #1
Klimon is offline Klimon  Belgium
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Default BLH damping & bracing tips

Hello,

I just finished a good-sounding tube-amp so now I can't blame bad things on the amp anymore...

My speakers (Beyma 8ag/n based backloaded-horns:http://www.hifi-forum.de/viewthread-104-1913-1.html ) are suffering from a few annoying and quite audible distortions: the main cause being vibrating sidepanels of the hornmouth in the high bass / low midbass region. I've done a lot of reading the last couple of days on BLH damping and bracing and plan to use several steel rods to 'tighten' the hornmouth walls to eachother.

The other problem (I think it's unrelated to the distortion caused by the resonating hornmouth) is a low-midrange lift that gives an unnatural low-end veil to male vocals... One forummember wrote that he solved that problem (on his - different - BLH's) by stuffing the hornmouth with wool. Does anyone have a clue what causes this (rather broad) low-mid peak? Is it some kind of standing waves or rather the hornmouth that's too short so needs dampening (as a means of artificially increasing it's length by adding wool = slowing airspeed down)?

Did anyone experiment with lining one side-wall with damping foam or felt?

I've done a lot of succesfull tweaks already which have sorted out midrange and treble problems to a large extent so I'm very keen to solve the remaining low-mid and bass distortion.

So far I've damared the cones, removed the whizzer and coaxially installed a neodymium magnet tweeter (first order filter) using the Beyma's natural roll-off, added a zobel network, added a felt-lined compression chamber, dampened the driver basket... For now I only use a sheet of damping material behind the driver + a little wool in the hornthroath. Cabinet is 3/4" MDF, voids filled with sand.

If anyone feels tempted to share his knowledge and give me a hand I'ld higly appreciate it; good advice can considerably focus the experiment-proces so I've learned

Cheers,

Simon
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Old 19th July 2006, 01:08 PM   #2
Nortp is offline Nortp  United States
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if you are getting midrange out of the hornmouth it may be caused by a too small coupling chamber. If the chamber is not sized properly it won't provide the correct high frequency cutoff and allow unwanted higher frequencies to come through. Two things that you could try is to stuff the coupling chamber with polyfill, or equivalent, or try stuffing some of the upper reaches of the horn path. You'll need to experiment with the amount and density of the stuffing. Too high a stuffing density will negatively affect the sound.

PJN
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Old 19th July 2006, 01:42 PM   #3
Nortp is offline Nortp  United States
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I've also had good results lining the horn walls with carpeting, dampens vibs and provides absorbtion of HF.
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Old 19th July 2006, 08:13 PM   #4
Klimon is offline Klimon  Belgium
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Hi PJN,

You may be right. originally I didn't think it was a problem of midrange leaving the hornmouth, the horns were designed (not by me) using AJhorn and originally didn't have a compression chamber, I added one (few liters, mainly to increase the distance between the backside of the driver and the backpanel) so am 'ahead' of the original design. I use a small amount of wool in the hornthroath which gives much dryer bass + partly solves the low-mid bump. Stuffing the CC sucks too much life out of the music; will experiment some more with stuffing then.

Good to know also that lining the walls with carpet can help (have some felt carpet I've been saving for this purpose); another trick up the sleeve when all else fails

Cheers,

Simon
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Old 20th July 2006, 04:45 PM   #5
Nortp is offline Nortp  United States
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Here is how I lined and braced my horn
Attached Images
File Type: jpg jhorn(2)1.jpg (60.1 KB, 420 views)
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Old 20th July 2006, 06:33 PM   #6
Dumbass is offline Dumbass  British Antarctic Territory
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Quote:
Originally posted by Nortp
Here is how I lined and braced my horn
Show-off!


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Old 21st July 2006, 09:50 PM   #7
Klimon is offline Klimon  Belgium
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Quote:
Show-off!


Not very didactical with regards to lining and bracing but it does look impressive! What drivers are you using?

My lining /bracing problem has been solved largely by removing the zobel + a little lower crossover point for the tweeter = 3db more efficiency (less need to crank it up untill the point where the cabinet starts to resonate audibly + better balance); iron rods still to come.

Simon
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Old 22nd July 2006, 12:16 PM   #8
Nortp is offline Nortp  United States
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The drivers are a Jordan JX92s running full range. Later in order to add a little more "air" I added a dayton 3/4" neo tweeter as a supertweeter, with just a cap coming in around 16 KHz. I'm also using a BSC compensation network, L = 1.5 mH, R = 5 ohms on the Jordan. The Jordan is a great sounding driver.
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Old 26th July 2006, 08:35 PM   #9
no xo is offline no xo  United States
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I have had some success using adjustable shower curtain rods to brace cabinets. The kind of rod that has a threads on one end allowed me to adjust the tension and you can move one or more of them around to find where they are most effective. Cut the aluminum rod to size and expirement with placement and tension. I have not done this to a horns mouth, but in a sealed box with serious resonances it worked great and did not cost much. I`ve thought about filling the rods with sand to further damp things but have not got around to trying this yet.
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Old 27th July 2006, 04:15 PM   #10
Klimon is offline Klimon  Belgium
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Hi no xo,

Meanwhile I've installed four threaded rods in each hornmouth and the obvious resonances and coloration are gone... even overkill if that exists in speaker-dampening.

I've also filled the hornthroath with wool and this significantly improved the male vocal problem, the output of the horn can still be distinguished from that of the driver on low male vocals, more experimenting to come.

Simon
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