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  • Question regarding internal standing wave dampening

    Does putting some poly fill stuffing in the enclosure do the same thing as gluing dampening (carpet padding, open cell foam, fiber glass insulation, etc.) to the walls? Back in the day when we were building car audio enclosures I would spray adhesive that 1" thick yellow fiberglass acoustic insulation that you could buy at Radio Shack to the 5 walls (not the speaker baffle). If you stuck your hand down in the woofer hole and snapped your fingers, huge night and day difference with and without insulation on the walls. I understand the concept of stuffing the box to change the Q and effective volume (gain), but does the Accustuff material behind the midbass absorb the high frequency energy off the back of the cone?
    Craig

    I drive way too fast to worry about cholesterol.

  • #2
    Re: Question regarding internal standing wave dampening

    Originally posted by PWR RYD View Post
    Does putting some poly fill stuffing in the enclosure do the same thing as gluing dampening (carpet padding, open cell foam, fiber glass insulation, etc.) to the walls?
    Yes, it just does more of it because there's more of it.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      Re: Question regarding internal standing wave dampening

      Originally posted by PWR RYD View Post
      Does putting some poly fill stuffing in the enclosure do the same thing as gluing dampening (carpet padding, open cell foam, fiber glass insulation, etc.) to the walls? Back in the day when we were building car audio enclosures I would spray adhesive that 1" thick yellow fiberglass acoustic insulation that you could buy at Radio Shack to the 5 walls (not the speaker baffle). If you stuck your hand down in the woofer hole and snapped your fingers, huge night and day difference with and without insulation on the walls. I understand the concept of stuffing the box to change the Q and effective volume (gain), but does the Accustuff material behind the midbass absorb the high frequency energy off the back of the cone?
      I hope you decide on damping, not dampening your enclosure.
      If you want to stop the enclosure walls from resonating, you would attach damping materials to the walls. Something with mass and flexibility.

      If you want to damp a standing wave, the sound from the woofer bouncing around inside the enclosure, you would fill the enclosure with poly stuffing or fiberglas stuffing. If you have one very prominent standing wave then you only need a thick slug of stuffing right in the middle of the enclosure.

      Yes, high frequencies are attenuated when they pass through a fibrous tangle.

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      • #4
        Re: Question regarding internal standing wave dampening

        I hope you decide on damping, not dampening your enclosure.
        If you want to stop the enclosure walls from resonating, you would attach damping materials to the walls. Something with mass and flexibility.

        If you want to damp a standing wave, the sound from the woofer bouncing around inside the enclosure, you would fill the enclosure with poly stuffing or fiberglas stuffing. If you have one very prominent standing wave then you only need a thick slug of stuffing right in the middle of the enclosure.

        Yes, high frequencies are attenuated when they pass through a fibrous tangle.
        Fantastic! Yes, I am asking about how to "dampen" standing waves bouncing around inside the enclosure. Not how to "dampening" my enclosure from resonating. I certainly hope I have gone to great enough lengths to dampen my enclosure (window braces, shelf braces, 2 x 3/4" baffle, etc.).

        I am hoping that a little poly fill (maybe .25 lb/ft^3) will do the trick. I will make sure it does not impede air flow around the port's inside end and does not touch the backside of the midbass.

        Does this sound like a good plan?
        Craig

        I drive way too fast to worry about cholesterol.

        Comment


        • #5
          Re: Question regarding internal standing wave dampening

          ..If you have one very prominent standing wave then you only need a thick slug of stuffing right in the middle of the enclosure.
          Off center is better.
          "...this is not a subwoofer" - Jeff Bagby ;)

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          • #6
            Re: Question regarding internal standing wave dampening

            Originally posted by PWR RYD View Post
            Fantastic! Yes, I am asking about how to "dampen" standing waves bouncing around inside the enclosure. Not how to "dampening" my enclosure from resonating. I certainly hope I have gone to great enough lengths to dampen my enclosure (window braces, shelf braces, 2 x 3/4" baffle, etc.).

            I am hoping that a little poly fill (maybe .25 lb/ft^3) will do the trick. I will make sure it does not impede air flow around the port's inside end and does not touch the backside of the midbass.

            Does this sound like a good plan?
            It's OK if stuffing touches the backside of the midbass magnet, but not of course the cone. In fact that is probably the best place for stuffing, really close to the driver.
            A midbass driver is pushing midrange back into the enclosure as well as outside of the enclosure. The midrange frequencies will reflect off the inside enclosure walls and come back out through the midbass driver cone. So a wad of stuffing close to the midbass attenuates in both directions. The only thing you have to be careful with is the amount. Sometimes too much stuffing behind the midbass kills (damps) the midrange too much.

            You have to experiment with the amount of stuffing and hope that you don't kill the bass. Yes, always keep stuffing away from the port.
            Last edited by ; 08-14-2010, 02:05 PM.

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            • #7
              Re: Question regarding internal standing wave dampening

              Originally posted by critofur View Post
              Off center is better.
              The best place to attenuate a standing wave is at the node, wherever that may be. If you have a prominent standing wave that is directly related to the enclosure's longest dimension, then stuffing at the middle will have the greatest attenuation on that particular wave.

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              • #8
                Re: Question regarding internal standing wave dampening

                Originally posted by PWR RYD View Post

                I am hoping that a little poly fill (maybe .25 lb/ft^3) will do the trick. I will make sure it does not impede air flow around the port's inside end and does not touch the backside of the midbass.

                Does this sound like a good plan?
                Maybe, maybe not. Excess damping of a vented box can render it 'dead' sounding. As far as standing waves are concerned, that term is usually applied to longer wavelength reflections, at 300 Hz and lower, that aren't easily damped by an inch or so of damping material. They're best eliminated by proper cab design, which means keeping internal dimensions less than 1/4 wavelength within the passband.
                www.billfitzmaurice.com
                www.billfitzmaurice.info/forum

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                • #9
                  Re: Question regarding internal standing wave dampening

                  How does this stuff look for lining the wall(s) behind and next to the midbass?



                  It's $0.36/ft^2 carpet padding from the local box store. It's 3/8" thick, fairly dense, but open-cell in nature (I can blow air through it). If it's a good material for said purpose, than I should have enough here to do about 4 dozen pairs of speakers :eek:



                  Seriously though, I bought this stuff to put under an area rug in my little girls room since we have hardwood floors and she still has a little tip over incident. But since I have to trim it to fit the rug, I will have a few extra square feet. Should I 3M spray adhesive some of it to the wall behind the midbass?
                  Craig

                  I drive way too fast to worry about cholesterol.

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                  • #10
                    Re: Question regarding internal standing wave dampening

                    3/8" wont absorb very long wavelengths. Since this is the case it may not reach as low inf frequency as you will need to damp those standing waves.

                    I went that route and I can tell you that this material is far from ideal. I will also tell you that standard polyfill is also far from ideal and so are mattress pads. The problem lies in the density and thickness. For most people's buck, r19 insulation would be far more effective. Better yet, rigid fiberglass.

                    As a design philosophy, no cabinet is built without lining all of the walls with fiberglass. I usually use about 1"

                    Originally posted by PWR RYD View Post
                    How does this stuff look for lining the wall(s) behind and next to the midbass?



                    It's $0.36/ft^2 carpet padding from the local box store. It's 3/8" thick, fairly dense, but open-cell in nature (I can blow air through it). If it's a good material for said purpose, than I should have enough here to do about 4 dozen pairs of speakers :eek:



                    Seriously though, I bought this stuff to put under an area rug in my little girls room since we have hardwood floors and she still has a little tip over incident. But since I have to trim it to fit the rug, I will have a few extra square feet. Should I 3M spray adhesive some of it to the wall behind the midbass?

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                    • #11
                      Go

                      I usually use poly-fill but recently tried some fiberglass insulation. Fiberglass does a much better job.I haven't tried rigid fiberglass. Is it very expensive?
                      ________
                      Cheap Airsoft Holster
                      Last edited by dano; 04-28-2011, 06:52 PM.

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                      • #12
                        Re: Question regarding internal standing wave dampening

                        So lining 5 of the walls with that 1" thick yellow fiberglass insulation that Radio Shack used to sell wasn't such a bad idea after all.... I know at least a couple of the boxes I built like that for people won some trophies (probably more due to the other components in their systems) :p
                        Craig

                        I drive way too fast to worry about cholesterol.

                        Comment


                        • #13
                          Re: Question regarding internal standing wave dampening

                          Sorry for the noob question, but what is rigid fiberglass?
                          Craig

                          I drive way too fast to worry about cholesterol.

                          Comment


                          • #14
                            Re: Question regarding internal standing wave dampening

                            Originally posted by PWR RYD View Post
                            Sorry for the noob question, but what is rigid fiberglass?
                            Here is a link to Owens Corning panels.

                            Dave
                            http://www.pellegreneacoustics.com/

                            Trench Seam Method for MDF
                            https://picasaweb.google.com/101632266659473725850

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                            • #15
                              Re: Question regarding internal standing wave dampening

                              for pricing/a good place to purchase, i'd recommend ats acoustics. i've only bought rockwool from them, but they also sell the OC rigids, as well as some of their own stuff that costs less and supposedly performs as well or better.

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