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Old 13th January 2010, 08:37 AM   #601
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Quote:
Originally Posted by stephenmarklay View Post
I am sure that you build them as well as anyone does. When you say "i can build boxes with no audiable energy exiting the wall panels" is that for BB only? If it were for others than you could not hear a difference no matter what the material was.
For the technique, light, stiff, and well damped are properties that make Baltic Birch (or similar many ply plywood) one of the very best practical materials.

Steel, aluminum can be made to work. Never been able to get anywhere near the same results with MDF. HDF laminated both sides with plastic laminate is OK, but sooo much work.

dave
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Old 13th January 2010, 11:02 AM   #602
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BB does appear to be the best all around choice I agree. I am doing a sandwich of 3/8 BB with Green Glue in the middle in my current project. I will brace is as much as possible but it is a mini monitor so won't require as much for stiffness. I am going to try to isolate the driver with a Sorbothane gasket but that is easy to undue if needed. I will also add some rigid fiberglass in two densities (not to damp the walls).
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Old 13th January 2010, 12:53 PM   #603
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First of all avoid any paralell walls. Use at least two or mor different materials that won`t sing together and last check ressonances up against drivers working range.
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Old 13th January 2010, 02:33 PM   #604
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Originally Posted by planet10 View Post
From these last 1/2 dozen posts it seems you guys need to go back & read the thread again.

dave
I actually read every post in this thread, of course I may have forgtten some of the info since it is rather long.

Now I'm a "prove it to myself" kind of person and believe in using a bit of science and math as a starting point to a design. What I found in this thread was lots of opinions with only a bit of proof. So I searched for as much info as I could find on the governing equations for deflection (the equation posted early in this thread was incorrect), composite plate design, resonant frequencies and critical frequencies and for material properties for the range of materials discussed so far.

I've proven (on paper) for myself that BB plywood is a better choice than MDF in that the properties are more consistent, the space between bracing can be larger and I know how much sound energy will be absorbed at a frequency range from 100 Hz to 4000 Hz. I do not like the rather large range of MDF properties I have found so far and while people have stated that MDF disspates more sound energy to heat than BB plywood I can find no data to support that. I suppose one could design the box using the worst properties but then the space beteen bracing gets a bit small.
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Old 13th January 2010, 02:43 PM   #605
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Originally Posted by planet10 View Post
but using the rationale that is embedded in this thread i can build boxes with no audiable energy exiting the wall panels,

dave
Dave,

Can you remind me what frequency range and dB levels your method applies to?

I cannot fathom how a speaker achieving 106.5 dB (my design upper limit) at 25 Hz (my proposed woofers, 18 Hz for the sub) will have no audible sound exiting through the enclosure walls no matter how they are braced and no matter what realistic material is used. My data shows that the transmission loss for 6 inches of concrete is 39 dB at 100 Hz. If the speaker can generate 106.5 dB into the air wouldn't it also generate 106.5 dB into the box? If so wouldn't the 6" concrete walls would be radiating sound at 77.5 dB at 100 Hz? That's still a lot of sound, causes hearing damage if you are exposed to it for a long time according to :
Sound pressure - Wikipedia, the free encyclopedia
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Old 13th January 2010, 02:44 PM   #606
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Originally Posted by PaleRider View Post
First of all avoid any paralell walls. Use at least two or mor different materials that won`t sing together and last check ressonances up against drivers working range.
I think that is is great idea under 2 circumstances :

1) you're not sure what the properties of the material are
2) you need to deal with a large range of frequencies such as in a full range speaker
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Old 13th January 2010, 03:00 PM   #607
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Quote:
Originally Posted by stephenmarklay View Post
BB does appear to be the best all around choice I agree. I am doing a sandwich of 3/8 BB with Green Glue in the middle in my current project. I will brace is as much as possible but it is a mini monitor so won't require as much for stiffness. I am going to try to isolate the driver with a Sorbothane gasket but that is easy to undue if needed. I will also add some rigid fiberglass in two densities (not to damp the walls).
Sorbothane may be a good choice. Had a look at their web site and it can damp out low frequency vibration better than anything else I've run across so far. I downloaded the design software and the guides and will use that to determine how thick the material has to be to be effective. Thanks for the pointer.
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Old 13th January 2010, 05:18 PM   #608
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Originally Posted by pietschu View Post
I think that is is great idea under 2 circumstances :

1) you're not sure what the properties of the material are
2) you need to deal with a large range of frequencies such as in a full range speaker


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Old 13th January 2010, 06:48 PM   #609
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It is great for us new ones to be able to learn from more experienced people.

So far I learned that I will using BB plywood for at least 2 of the layers of a composite panel and bracing about every 4 inches for the subwoofers and woofers. The other layers, ...... will keep pondering and calculating.
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Old 13th January 2010, 08:48 PM   #610
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Default lol mixed my numbrs

lol i have to admit i mixed up my numbers...MDF IS weaker in tension and compressive breaking stress, by a margin of 10-20% compared with super quality ply...although i still think MDF is stiffer, just ultimately weaker...after all birch ply really is very springy. I think this springiness is a major factor in its high tensile stress specs, although i wouldnt have though a springy elastic material wouldve been good at NOT oscillating otherwisde i mightve taken the glass filled nylon 66 welded together route....

whether springyness is an advantage is another debate altogether. i personally think it is the ONLY place where ply fails, other than the relative hardship of routing cleanly in the material. In these 2 ways only then i think its safe to say MDF is better...maybe a MDF-ply-MDF sandwhich would be a good comprimise, reducing the amount of routed cuts id need to use.
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