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How to measure absorption and vibration?

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  • #16
    The knuckle test is not rigorous enough because I want to be able to create repeatable and comparable experiments.

    Originally posted by Paul Carmody View Post
    Also worth mentioning that panel vibrations and internal reflections are two different things. Which are you trying to measure?
    I want to test both, but I realize they are two separate characteristics and would need two separate measurement methods. augerpro's link is close in spirit to what I am interested in...he looks at both things (and more) in separate but standardized ways.

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    • #17
      From post #1:

      "I'm not interested on anything TOO scientific. I realize there are different things out there, accelerometer, vibration meter, etc., but not interested in anything too expensive (e.g., $100)."

      I suspect you'll need to develop a very scientific method and spend considerably more than $100. Perhaps there are studies at a university or materials testing facility that may address your interests.

      Much depends upon what you want to accomplish.

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      • #18
        Originally posted by TNA View Post
        I suspect you'll need to develop a very scientific method and spend considerably more than $100.
        Maybe, in which case I would just do something different. But Augerpro did some pretty detailed investigations on both vibrations and absorption materials in his link above which will guide me (all SPL based). I have a measurement mic and can build some sort of test cabinet. I'll buy the acceleration sensor for $26 that Meniscus has. I bought a $10 Dayton Audio exciter. Now I just have to get some time after InDIYana to play around.

        To be clear on how scientific I want to be...I just want to measure things relative to 3/4" MDF. So as long as I have SPL/volume, mic distance, etc. standardized then I can proceed to see if things are better, worse, or indifferent from 3/4" MDF. It's just that the knuckle test is too imprecise, because my results might be because I drank more coffee and knocked harder on 1/2" ply than the MDF.

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        • #19
          Originally posted by a4eaudio View Post
          To be clear on how scientific I want to be...I just want to measure things relative to 3/4" MDF. So as long as I have SPL/volume, mic distance, etc. standardized then I can proceed to see if things are better, worse, or indifferent from 3/4" MDF. It's just that the knuckle test is too imprecise, because my results might be because I drank more coffee and knocked harder on 1/2" ply than the MDF.
          This is an example of what tends to happen when people measure and the science/engineering (understanding what is going on) is considered relatively unimportant. The knuckle rap test (impulse test) usually doesn't contain useful information on resonance magnitudes between test cases. It provides useful information on the resonant frequencies and their damping but not the magnitudes. The useful magnitudes of the resonances require the structure to be correctly driven by the drivers and, if significant at low frequencies, the air within the cabinet. Rapping on the structure at different positions will excite the resonances differently. They will have the same frequency and Q but their magnitudes will change relative to each other. So how hard you rap doesn't really matter so long as it is not hard enough that the structure deforms in a nonlinear rather than a linear manner.

          Despite not being representative of the driven structure occasionally a calibrated impulse test is useful and in the case of speaker DIY this could be done by, for example, swinging a weight on a string. For a given position the kinetic energy on impact will be distributed among the resonances/modes in the same way so long as the deformation on impact remains linear. Don't rap too hard. This can be checked by testing when the distribution and magnitudes start deviating from being linear. The signal to noise tends to be fairly weak and so rapping as hard as possible while remaining linear is usually what is wanted. The sound made on impact and any bouncing back of the weight will take some of the energy and introduce errors. These are likely to change with both material and rap location lowering the accuracy of comparing magnitudes between tests.

          If you are not directly measuring what you want to know but instead some sort of rig intended to represent what you want to know then without knowing the science/engineering (i.e. what is actually going on) with respect to the cabinet vibration, the rig vibration and how the latter is related to the former then the chances of misunderstanding the information contained in the measurements grows considerably.

          I am not trying to put you off measurements, far from it, but rather to encourage useful measurements concerning practical materials that we DIYers could use in our garages. For example, how effective is the damping for various CLD panels that are practical for DIYers? I rather suspect that several of those used/proposed work little better than single ply or MDF but, as far as I am aware, reliable measurements demonstrating this are still absent. As are demonstrations based on the science/engineering but that is near the top of my todo list and being worked on in bursts.

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          • #20
            It will be interesting to see what you develop, and how your testing relates to your impressions of the effect different panel and cabinet construction produces on music played through the cabinets. Good luck with your project.

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            • #21
              From a purely anecdotal standpoint I have improved the results of the knuckle test by adding an additional brace to an existing side-to-side brace. The new brace goes from the wall at one end of the side-to-side brace to a box corner. I suppose the triangulation helps damping. No, I have not performed in-depth scientific measurements, nor has it been peer-reviewed lol. But it was easy, and cheap Perhaps the glob of PL Premium at each end helps damping.
              Francis

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