Bass Trap idea..

Discussion in 'Archived Threads 2001-2004' started by Travis Cain, Mar 18, 2003.

  1. Travis Cain

    Travis Cain Stunt Coordinator

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    16" or so sonotube with a million 4" holes cut with a hole saw.. Stuff well with fiberglass(roll it up semi tightly and stack the rolls inside the sonotube).. Then wrap the outside of the tube with poly batting, cover it with burlap or similar..

    How do the experts on bass traps feel about that idea?
     
  2. ThomasW

    ThomasW Cinematographer

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    It would work to some degree.

    I use 1/4 cylinders made from wire mesh with a radius of 24" filled with glass for my bass traps.

    The best bass traps use a flexible membrane, that is tuned to the target frequency. This graphic from realtraps.com is an example

    [​IMG]
     
  3. Chris Carswell

    Chris Carswell Supporting Actor

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    Dang Thomas, you always have a good link for us [​IMG] One of these days I'll find something before you do [​IMG]

    So if I had some scrap pieces of 24" sono tube (which I do) I could cut them into 1/4's & make something similar to what have? I could get off my but & go buy some wire mesh but it's fun being different. At least that what my "special" teachers always told me........... J/C, & no I didn't ride the short bus to school.

    Another question; What denotes the frequency at which they (traps, treatments, etc) become the most effective? I assume it would be something with the surface area-size & depth correct? Would it be somewhat similar as to how one would calculate basic rectangular room mode frequencies? (f=C/2L - I think that's the right formula)
     
  4. Brian Bunge

    Brian Bunge Producer

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    Chris,

    I wondered why I see all those short buses whenever I drive through Gainesville! Now I know! I always thought you Gators were a little slow. [​IMG]
     
  5. Troy R

    Troy R Stunt Coordinator

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  6. ThomasW

    ThomasW Cinematographer

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    Chris

    Using sonotube (even perforated) isn't the best idea for a bass absorber.

    My 1/4 cylinders are all metal mesh except for the OSB top/bottom plates separated by 2"X2"s

    With the fiberglass cylinder designs just make them as thick/deep/big as possible given the limitations of the particular install. Mine have a 24" radius, similar construction to the cylinders shown on the Risch site.

    I'm not sure how to calculate the Fs for the plate type trap shown in the graphic. I'd just go with a trial/error and see what happens. On the products & pricing page of the RealTrap web site they give the some of the dimensions and frequencies absorbed.
     
  7. RichardHOS

    RichardHOS Second Unit

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    Calculating Fs of a plate trap... rough approximation.

    Fs = (1/2pi)sqrt(k/m)

    k = (48EI)/L^3

    I = (1/12)WT^3

    m = dLWT

    Therefore, Fs = (1/2pi)sqrt(4ET^2/dL^4)

    where:

    E = elastic modulus of plate
    T = thickness of plate
    d = density of plate
    L = length of plate between spacers

    This assumes that the spacers are roughly pinned connections. I make no guarantee that I didn't screw up the algebra somewhere... I often do. [​IMG]
     
  8. RichardHOS

    RichardHOS Second Unit

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    I don't know why exactly, but it just occurred to me that I did make a mistake. My advanced vibrations professor would have been ashamed. [​IMG]

    For a vibrating beam, the "mass" in the sqrt(k/m) isn't the total mass of the beam, but rather the "effective" mass. The total mass is distributed along the beam, and as you go toward the boundaries the mass has a progressively smaller effect on the resonance characteristics. If you treat the beam as a massless spring and calculate its stiffness, then you want to know what part of the total mass is the "effective" mass that represents a lump mass directly in line with the spring.

    I think it works out to 1/3 the total mass, though I'm going from memory here.

    The corrected equation for Fs would be: Fs = (1/2pi)sqrt(12ET^2/dL^4)

    (I still haven't checked for Algebra mistakes.) [​IMG]
     
  9. Dennis XYZ

    Dennis XYZ Stunt Coordinator

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    The DIY version of the traps Thomas showed is at http://www.ethanwiner.com/basstrap.html . Pretty simple. That DIY page has been there since before he started his Realtraps company and he was nice enough to leave it up. Scroll to the bottom for the plans link.

    He says his commercial ones work better. Maybe he consulted somebody like Richard for his commercial ones. [​IMG] Check out Richard's equation Fs = (1/2pi)sqrt(12ET^2/dL^4) (I haven't checked the math either but it looks reasonable.) So, T is the plywood thickness and the resonant frequency goes up as you make it thicker. We all know that makes sense from building speaker boxes. But, with the DIY trap plans, Winer uses 1/8" ply for the midbass traps and 1/4" ply for the deep bass traps. Oooops, backwards.

    One of the coolest ideas I've heard lately was over on the Mad board. Somebody just screwed a 2x2 to the top of the wall behind the speakers. Then he fastened carpet to it and let it hang down the wall, spaced out 2", not quite touching the floor. He says he knocked down some peaks by 10dB which is great performance. I imagine it works kinda like a cross between a limp membrane and a resistive trap.
     
  10. RichardHOS

    RichardHOS Second Unit

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    Thickness isn't the only factor there... you also have the length of the plank. We all know longer strings have lower natural frequencies (with equal tension), and the same is true for wood or fiberglass or whatever. A 1/4" plank could have a lower resonant frequency than a 1/8" plank if the former was sufficiently longer.

    I still haven't checked my algebra though. [​IMG]

    BTW, if anyone were to use such an equation to calculate resonant frequency, especially if you planned on using it to actually build a trap, I'd suggest using a material that is homogeneous or at least is clearly orthotropic along a preferred axis. Examples of homogenous would be MDF or a plastic like UHMW or some such; examples of the latter would be a birch plank or similar.

    Something like plywood acts like a composite with quasi-isotropic properties, and establishing the modulus will be difficult if one is not provided for the material.
     

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