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  • How to measure absorption and vibration?

    Two separate questions, but obviously related to cabinet construction. Thinking about SIMPLE analyses.

    1) How would you measure absorption? I'm thinking it could simply be placing a speaker at a certain distance and angle to a panel covered in absorbing material and placing the mic somewhere strategic. Then look at SPL and/or reflections? 1/4" felt, 1/2" felt, 1/4" foam, 1/2" foam, 1" foam, etc. Simply, (1) is there a measurable difference? (2) if so is it significant or insignificant?

    2) Same thing with vibrations. 1/4" ply, 1/2" ply, 3/4" ply, 1/4" mdf, ... bracing, etc. (2) is there a difference between materials? (3) How significant? 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).

  • #2
    I have wondered the same thing. I did run across this from Liberty Audio that recommends the ACH-01 accelerometer and even shows a power & buffer circuit - http://www.libinst.com/accel.htm
    Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.

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    • #3
      I use a DATS and tap on the cabinet at about the same time I begin an impedance sweep. The cabinet vibrations will show up in the impedance plot. Not perfect, it takes some practice but you can learn a lot this way.

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      • #4
        Originally posted by a4eaudio View Post

        1) How would you measure absorption? I'm thinking it could simply be placing a speaker at a certain distance and angle to a panel covered in absorbing material and placing the mic somewhere strategic. Then look at SPL and/or reflections? 1/4" felt, 1/2" felt, 1/4" foam, 1/2" foam, 1" foam, etc. Simply, (1) is there a measurable difference?
        It's been done, there are charts out there. The material with the highest coefficient of sound wave absorption is rigid fiberglass boards, like Corning Type 700.

        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          Originally posted by billfitzmaurice View Post
          It's been done, there are charts out there. The material with the highest coefficient of sound wave absorption is rigid fiberglass boards, like Corning Type 700.
          Yes, I know of those. I have something more specific I want to test.

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          • #6
            Originally posted by Billet View Post
            I use a DATS and tap on the cabinet at about the same time I begin an impedance sweep. The cabinet vibrations will show up in the impedance plot. Not perfect, it takes some practice but you can learn a lot this way.
            This is interesting. But I need something more "repeatable" with some precision. But it got me thinking... What if I got a Dayton Audio exciter, say one of the 10w ones for $10. Then I got the PTmini-8 planar tweeter for $16 that has a a flat impedance. Find some suitable test signal and predetermined output level to run through the exciter attached to the interior cabinet wall and attach the planar to the exterior wall. Run DATS for impedance. It wouldn't mean a lot in terms of what may be audible, but it would show a qualitative comparison, such as 1" well braced MDF does a lot compared to 1/4" ply.

            Another thought...the vibrations really only matter if they are creating audible noise outside of the cabinet. I could just run a loud sweep on one side of the test material with a mic near field on the other side. I wouldn't really care about the exact db difference, just whether certain materials do a lot better or not much difference.

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            • #7
              Looks like Meniscus sells what they call an Acceleration Sensor for $26.
              Co-conspirator in the development of the "CR Gnarly Fidelity Reduction Unit" - Registered Trademark, Patent Pending.

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              • a4eaudio
                a4eaudio commented
                Editing a comment
                Thanks, at that price I'll try it.

            • #8
              Originally posted by a4eaudio View Post
              Two separate questions, but obviously related to cabinet construction. Thinking about SIMPLE analyses.

              1) How would you measure absorption? I'm thinking it could simply be placing a speaker at a certain distance and angle to a panel covered in absorbing material and placing the mic somewhere strategic. Then look at SPL and/or reflections? 1/4" felt, 1/2" felt, 1/4" foam, 1/2" foam, 1" foam, etc. Simply, (1) is there a measurable difference? (2) if so is it significant or insignificant?

              2) Same thing with vibrations. 1/4" ply, 1/2" ply, 3/4" ply, 1/4" mdf, ... bracing, etc. (2) is there a difference between materials? (3) How significant? 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).
              Measuring the radiated sound from a speaker cabinet reasonably accurately requires either expensive hardware and computation (what industry does) or cheap hardware and an enormous amount of time and effort (some student projects). Neither is simple or particularly attractive to home DIYers.

              What a home DIYer can do is get incomplete partial measurements that provide some clues (e.g. knock tests, attaching strain gauge,...). A problem with this is that without the scientific/engineering understanding of what is going on the information is likely to be misinterpreted (e.g. bracing the hell out of everything makes a cabinet quieter being a common misconception among DIYers). If you opt for this route I would recommend trying to learn about the physics of how a speaker cabinet vibrates in order to better understand what your partial measurements are indicating about the sound radiated by the cabinet.

              Not sure I understand your questions or perhaps the motivation. Why not look up the material properties of plywood, MDF, foam or whatever rather than seeking to measure them?

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              • #9
                Originally posted by andy19191 View Post
                Not sure I understand your questions or perhaps the motivation. Why not look up the material properties of plywood, MDF, foam or whatever rather than seeking to measure them?
                I want to test somethings that I don't think have been used before. So I just want to be able to compare it to commonly used materials. For example, material X is as good as 3/4" ply, or a little better, or a lot worse. Just some not-very-rigorous relative comparisons.

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                • #10
                  I think a measurement microphone and a wooden mallet would be as good as anything at the hobbyist level. If you have a driver installed, a click test with a battery and a light switch might be better than the wooden mallet.

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                  • #11
                    Originally posted by a4eaudio View Post
                    I want to test somethings that I don't think have been used before. So I just want to be able to compare it to commonly used materials. For example, material X is as good as 3/4" ply, or a little better, or a lot worse. Just some not-very-rigorous relative comparisons.
                    An approach that is reasonably DIYer friendly might be to compare the vibration of a bending panel. Gives relative mass, stiffness and damping in bending. Some care needed when extrapolating to speaker cabinets because the shapes of the low frequency noisy modes often have little to do with the bending of individual panels. Nonetheless could be useful in comparing the effectiveness of various DIYer friendly composite panels.

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                    • #12
                      Originally posted by a4eaudio View Post
                      Another thought...the vibrations really only matter if they are creating audible noise outside of the cabinet.
                      This is the perspective I took when I started a project to investigate the same things. I think it better connects to the real world, and gives you a quantifiable SPL result. Whereas with an accelerometer, if you are damping impulses, how low is enough? You can see how I built my box and the results here https://www.somasonus.net/box-construction-methods
                      ~Brandon 8O
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                      • #13
                        I asked a similar question about measuring vibration in panels few years ago on this forum. The best advice was the "knuckle test". Just rap on a cabinet or a panel with your knuckles.

                        I'd add that making a prototype cabinet and rapping on it, then listening carefully to various pieces of music provides a way to compare the sound of the knuckle rap to the actual sound of music playing.

                        The results I like best are panels made of good 1/2" Baltic Birch combined with either 1/2" or 3/4" MDF, depending upon the volume of the cabinets to be made. The BB is exterior layer, usually veneered.

                        I've noticed that some cabinets have had an unexpected resonant sound when rapped, despite using panels that I expected to have a "dead" sound. I suspect most of the resonance, which sometimes seems more of a "ring", results from the volume of the cabinet rather than the panel construction.

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                        • #14
                          Also worth mentioning that panel vibrations and internal reflections are two different things. Which are you trying to measure?
                          Isn't it about time we started answering rhetorical questions?

                          Paul Carmody's DIY Audio Projects
                          Twitter: @undefinition1

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                          • #15
                            Regarding the knuckle rap test; a completely empty enclosure sounds quite different than one with the walls lined with a damping material such as recycled denim, foam, or fiberglass. Even enclosures with tons of bracing. It goes from a crack to a thud sound.
                            Craig

                            I drive way too fast to worry about cholesterol.

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