Why not HDF instead of MDF?

Discussion in 'Home Theater Projects' started by BrianAe, Nov 18, 2003.

  1. BrianAe

    BrianAe Second Unit

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    Just out of curiousity, why don't more people/companies use HDF to build their speaker/sub cabinents? I would imagine that the cost increase is minimal, that it would be easier to work with, and would give better results.
     
  2. Hank Frankenberg

    Hank Frankenberg Cinematographer

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    Brian, that's a question I've pondered and I hope someone answers with technical reasons. One problem with HDF is that it is difficult to find. I visited a company in San Antonio that builds precision router table fences and positioning machines as well as router table tops. They used HDF for the table tops and told me the only way they could get it is in truckload quantities. I asked a local high-end lumber dealer about it and he could special order it, but the cost was several times MDF.
     
  3. Dave Poehlman

    Dave Poehlman Producer

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    I think the main reasons are cost, availability, and ease of use.

    I thought I did remember someone here with a project that used HDF. Perhaps they can give a hands-on explanation.
     
  4. RichardHOS

    RichardHOS Second Unit

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    (1) It's expensive and more difficult to find
    (2) It's hell on tool bits compared to MDF
    (3) Cabinet response is all about stiffness.

    More about number 3: stiffness is governed by the material's modulus of elasticity and the geometry. For a panel, the bending stiffness goes up linearly as the modulus increases, but it increases with the cube of the thickness. Therefore, it is much cheaper and easier to simply use a thicker MDF rather than a thinner HDF panel. When in doubt, double up on the MDF... it would do more good than switching to HDF. The only exception is where space is at a premium and you need the stiffest material possible for a given thickness... but in that case you would be using plywood, not MDF. There are some additional factors, like mass and hysteresis damping, but those can be altered easily enough by adding anti-vibration adhesive mats for a specific application if required.
     
  5. BrianAe

    BrianAe Second Unit

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    Hmmm. My point is even at 3X the price it doesn't add that much cost to the overall price.

    Lack of availibility makes sense though.

    Richard, I don't know if I buy that last argument. I was thinking of equivalent thicknesses. I've seen good speakers made with 1" HDF. Don't know if someone would want to go 2" MDF.

    Anyone else care to chime in? Does anyone have experience building with HDF?
     
  6. Robert AG

    Robert AG Stunt Coordinator

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    >>...but those can be altered easily enough by adding anti-vibration adhesive mats for a specific application if required.
     
  7. RichardHOS

    RichardHOS Second Unit

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    In my limited experience, sufficiently stiff cabinet walls (i.e., sufficient thickness and bracing) do not need damping of the panel resonances. All you do by adding anti-vibration mats is to lower the resonant frequency, and also lower the amplitude of that resonance. For subs at least, the second effect is good but the first is not. For mid/high frequency enclosures things become more complex, and avoiding a resonance in the speakers operating band might not be possible or convinient. In that case, lowering the fundamental might be a good option, and lowering the peak certainly would be.

    For bass frequencies, internal damping by using a stuffing material is adequate.



    A bit more detail on how stiffness and mass interplay in resonant modes... There are many combinations (an infinite number, actually) of thickness, modulus, and mass for a given panel size that will produce the same resonant frequency. Increasing stiffness (combination of thickness and modulus) will increase the frequency; increasing mass will lower it.

    A stiff but heavy panel can have the same fundamental frequency as a lighter but more flexible panel. The difference is in the relative amplitude of the mode; the stiffer and more massive panel will have a smaller amplitude at resonance given the same input energy. Therefore, all other things being equal, for speaker cabinets it is more advantageous to increase modal frequencies by increasing stiffness rather than by dropping weight.

    Now, if you simply increase the modulus of a material the stiffness (for the same thickness panel) increases linearly. i.e., if you double the modulus of elasticity you double the stiffness, all other things being equal. If you increase the thickness, the stiffness increases roughly with the cube of thickness (dependant on exact panel size and shape). i.e., if you double the thickness the stiffness increase by roughly a factor of 8. Note that you also double mass, which drops the resonant frequency a bit, but the factor of 8 increase in stiffness dominates the response.

    That's why it's generally not a good idea to just add inert mass to a sub enclosure if the panel thickness is not increased to compensate. The goal is to drive the panel resonance out of the operating range of the sub. Better yet is to get the resonant frequency up there while keeping the panels massive, which means overly thick panels with inert mass added. Doing that will ensure minimal excitation of panel resonances from other sources of sound energy in the room (i.e., mid range drivers), as well as keeping panel flexing down under the pressurization of the sub (more of an issue in sealed designs).

    So is HDF good? Yeah, for subs at least its better than MDF (as far as cabinet reseponse is concerned). However, the same results can be achieved much cheaper using MDF by simply increasing the thickness of the panel.
     
  8. BrianAe

    BrianAe Second Unit

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    Thanks Richard. That's a great explanation.
     

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