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Old 30th December 2003, 11:06 AM   #1
Misa A. is offline Misa A.  Russian Federation
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Join Date: Dec 2003
Location: Moscow
Default 2 more TL questions

Hello, friendly and knowledgeable people at diyAudio.com. Happy New Year to all of you.

I am building my first TL speaker, very similar to the D’Appolito’s Thor kit ( http://www.madisound.com/thor.html , http://www.madisound.com/audioXpress...r%20Review.pdf ), but using SS 8545 x2 and SS 9500 instead of Seas Excels.
The geometry of the line was slightly modified using Mr. MJK’s MathCAD worksheets.
Everything promises to work, though I am trying to build well sounding speakers with as low as possible bass extension and keep them as small as possible at the same time.

Can anybody please answer the following two questions:

1. I suppose that it is a good idea to put some sound absorbing material on the back and side walls of the first section of the line, along with staffing the TL, to reduce as much as possible reflections from the walls back to speakers and to suppress cross resonances. I am going to use cotton sheet wadding approx. 1 cm thick. My prototype Thor project does not say a word about this.
The question is:
How much will thickness of such coating affect the geometry of the TL which was optimized by wood-to-wood dimensions? And won’t it affect the basic performance of the transmission line?

2. Very much respectable Mr. D’Appolito in his article above says that he came to conclusion that the best solution is staffing the whole length of the line with the uniform density. Many other sources and my sense say that it’s better to staff more heavily the first 2/3 of the line to effectively suppress higher modes with minimal affecting the main 1st mode.
Which way should I follow? Unfortunately, I don’t have too much time for experiments.

Thank you in advance for answers

Misha
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Old 30th December 2003, 02:33 PM   #2
MJK is offline MJK  United States
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Join Date: Feb 2002
Location: Clifton Park, NY
Hi Misha,

"1. I suppose that it is a good idea to put some sound absorbing material on the back and side walls of the first section of the line, along with staffing the TL, to reduce as much as possible reflections from the walls back to speakers and to suppress cross resonances. I am going to use cotton sheet wadding approx. 1 cm thick. My prototype Thor project does not say a word about this.
The question is:
How much will thickness of such coating affect the geometry of the TL which was optimized by wood-to-wood dimensions? And won’t it affect the basic performance of the transmission line?"

(MJK) The most effective way to damp out reflections and standing waves between the sides and the front and back walls is with stuffing in the middle of the volume where the velocity will be highest. I think that putting fiber along the walls is an inefficient solution for this particular problem.

I doubt that it will change the required areas for the design and it will provide some amount of damping to the line's axial standing waves. The most efficient solution is to make sure the fibers are in the middle of the line cross-section.

"2. Very much respectable Mr. D’Appolito in his article above says that he came to conclusion that the best solution is staffing the whole length of the line with the uniform density. Many other sources and my sense say that it’s better to staff more heavily the first 2/3 of the line to effectively suppress higher modes with minimal affecting the main 1st mode.
Which way should I follow? Unfortunately, I don’t have too much time for experiments."

(MJK) I agree with your stuffing arrangement. Why not start with the first 2/3rds stuffed and only add stuffing to the last 1/3 if the line is underdamped? It should be easy to just reach in and add additional stuffing to the end of the line if reqired.

Hope that helps,
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Old 30th December 2003, 03:13 PM   #3
Misa A. is offline Misa A.  Russian Federation
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Join Date: Dec 2003
Location: Moscow
Thank you, Martin. Your opinion is of great value for me.

I am going to use Dacron II pillow stuffing with density of about 0.7 -0.8 lbs/cu.ft for beginning, and just was not sure it will provide enough damping without padding the walls. I will leave in place wadding which I already put on the walls and stop on that.
Can’t wait to finally finish cabinets and begin optimizing TL and crossovers.

Wish you a good and happy New Year !
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