Tweakers' Asylum

• Some Suggestions - Jagdeep 16:16:40 07/03/07 (1)
  In Reply to: RE: Is MDF airtight? (Long-ish) posted by Dyak on June 25, 2007 at 10:05:39

  It is gas permeable but.... very slightly.
  If you arew making a ported speaker ... don't even bother about it.
  For a sealed enclosure ... the way to go.... the speed of the pressure pulse changes inside a box are too fast with respect to the permeability of the MDF. A really simple solution would be to apply a layer of varnish/laquer to the inside of the box. That'll take care of any concerns you have.
  
  Don't forket to treat the inside corners with the soft squishy material (something like sorbothane but made in the UK)It is very effective.
  
  Make the back panel of the speakers a screwed on panel with a seal. This will allow you to tweak the speaker easily.
  
  
  Cheers
  Jag
  
  

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  • RE: Some Suggestions - Dyak 16:39:30 07/05/07 (0)
    In Reply to: RE: Some Suggestions posted by Jagdeep on July 03, 2007 at 16:16:40

    Hi Jag,
    
    OK, that makes sense...'slightly' gas permeable.
    
    They're sealed enclosures so I'll use the laquer or I already have an unopened / old can of fiberglass resin and hardener just sittin' about that I may still use...not sure yet...still have a few days to make up my mind.
    
    Interesting...the inside corners...the 3-way corners (ie top, side, back)or the 2 way corners (top, back), or all of the above?
    
    I have already have Parts Express "Sonic Barrier 3/4" 3-layer composite damping material" (linked below) that may (?) fit the bill. If I used this stuff, should it be 1) triangular in shape and pushed tight into the 3-way corners, or 2) two inch strips following the 2-way corners? I have not read of this technique before, do you happen to have a link that may provide me with some more info? Or maybe a link to the material you're referring to would be great too!
    
    I'll definitely consider the removable back, but am leaning towards gluein' & screwin' it as well. My hope is to have the cabinet super-solid...I've found that different portions of the cabinet sound different (to arap of the knuckles) when screwed-on only vs. being glued (or glued and screwed)...or is this some sort of psyco-somatic/biased perception of mine? Regardless, I am building the crossovers in an external box, so my thought at this point is that if I really build the cabinet "right" (or as right as reasonably possible) I should not have to worry terribly about getting back inside (sounds like some famous last words, eh!?!?)
    
    Granted this is my first time constructing a pair of speaks, so I'd love to hear any thoughts or rebuttals to my thought process.
    
    Thanks for passing on your knowledge!
    
    Regards,
    
    Doug
    

    • http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=260-530   (Open in New Window)

    
    

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• RE: Is MDF airtight? (Long-ish) - Dyak 13:49:39 07/02/07 (0)
  In Reply to: RE: Is MDF airtight? (Long-ish) posted by Dyak on June 25, 2007 at 10:05:39

  Hi Everyone,
  
  Thank you for your input!
  
  Kenster,
  
  Gotta love them parlor tricks....they look neat, but some how one still feels like a sucker afterwards! Thanks for calling a spade a spade (and then for helping me to understand why its a spade!)
  
  
  LKDog,
  
  Thanks, I took your plight to heart and glued a 1/2" piece of plywood to the very bottom of my cabinet which I'll veneer over....this should limit 1) the cracking/chipping of the MDF as I move it around tilt/slide/etc... and 2) if I ever end up w/a slightly damp floor it may discourage or hopefully slow down the potential (if not inevitable) swelling.
  
  
  Stuck,
  
  Thanks, interesting analogy. What you say makes sense....seems sound (pun intended) and I won't sweat it. Thanks for the input.
  
  
  Regards to all,
  
  Doug
  
  

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• RE: Is MDF airtight? (Long-ish) - kenster 20:18:12 06/30/07 (0)
  In Reply to: RE: Is MDF airtight? (Long-ish) posted by Dyak on June 25, 2007 at 10:05:39

  "Most people choose MDF for their enclosures. Believe it or not, MDF is not airtight. All the hours you have spent sealing your seams with Silicone, Latex Caulking, and the like, while important, are only half the problem. During a recent visit by Dave Gumienny of Kicker Car Audio, he reminded me about the breathability of MDF. He told a story of one of the training courses they did years ago when he worked for PPI, where 5 small pieces of MDF, 3/8 thick, were stacked, and all where lifted by a vacuum into the air."
  
  I gonna have to call "bullshit" on this. It is the vacuum that is created by the lack of air molecules between the layers, or sheets of MDF, that is causing the pieces of MDF to stick together when lifted.
  
  This same "experiment" can be demonstrated with plates of glass. It is the smooth surface of the pieces that allow the air to be displaced when the two pieces are placed on top of, or in very close proximity to each other, and most of the air molecules will be displaced from between the planes of glass/MDF, thus a vacuum is formed which will adhere the pieces together when lifted.
  
  The term "Vacuum" merely means the absence of positive pressure and can be confused with the "sucking" that is normally assigned to a vacuum cleaner so to speak. In other words, what U witnessed was a nice, but yet effective, parlor trick :-)
  
  Cheers
  
  
  
  
  
  
  

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• RE: Is MDF airtight? (Long-ish) - LKDog 12:35:44 06/25/07 (1)
  In Reply to: RE: Is MDF airtight? (Long-ish) posted by Dyak on June 25, 2007 at 10:05:39

  Well, it definitely is absorbent despite seeming so rigid.
  
  I have a small set of speaker stands I made for my rear 5.1 HT monitors.
  I had not yet put spikes on them to raise them off the floor slightly.
  I got a little water in the basement from a heavy Spring rain.
  One of the stand bottoms swelled up like a baby's diaper.
  
  Now the question is do speaker manufacturers wish to leave the surface absorbent, or do they wish to seal it so it is more reflective and airtight?
  Maybe post a question on the Speaker forum here might yield some input, too.
  
  
  
  
  
  

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  • RE: Is MDF airtight? (Long-ish) - stuck.wilson@gmail.com 16:22:38 06/25/07 (0)
    In Reply to: RE: Is MDF airtight? (Long-ish) posted by LKDog on June 25, 2007 at 12:35:44

    
    wouldn't it be more important that it retain ENERGY-- e.g. the backwave of a a speaker- in the SHORT TERM, rather than being airtight in the long term? really-- you're dealing with transient events-- not necessarily keeping 'air', per se, in.
    
    glass, in technical terms, is a suspension, and thusly not a solid, which is why old glass panes are thicker at the bottom than at the top. but it'll hold rain out of your house for 200 years before it becomes impractical to use it.
    
    i wouldn't sweat the theoretical lack of 'airtight'.. because you really don't have to worry about it for the length of time necessary to produce tones.. unless you want to!
    
    yrs
    
    d.
    
    

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