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Bass reflex bass bins, your advice re acoustic lining please

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  • Bass reflex bass bins, your advice re acoustic lining please

    I have a pair of original NHT Superzeros and I'm building a single bass bin to accompany them. I say bass bin and not subwoofer because the Superzeros have little to no real bass (see http://www.stereophile.com/content/n...r-measurements), so the bass bin will play up to just a little over 100 hz. For this reason I'm looking at using the Dayton Audio SPA250 plate amp because it's low pass filter is 24db/octave, thinking that the steeper slope will help with the increasing directionality (i.e., localization) of higher bass frequencies. Living in an apartment, economy of space is a high priority. It occurred to me to build a bass bin that would go under my old, large RCA console tv, exactly matiching the footprint of the oak swivel base. This necessitates a shielded woofer, which, as you may know, are now virtually unavailable. Thankfully Parts Express is again stocking the Dayton Audio RS180S-8, a 7" shielded woofer, which seems to meet my needs very well. I'll use two in parallel in a 65 liter box tuned to 29-32 hz with a 3" i.d. port (of appropriate length) for an extended bass shelf (EBS) alignment. With the two woofers sitting right next to the floor, I'm anticipating some room gain/boundary reinforcement will compliment the slight downward slope of the EBS alignment.

    Now to my question, or my request for your advice. Considering a range of approx 30 to 110 hz with a 24db/octave low pass filter and a bass reflex box with sufficient bracing and wall thickness to limit wall vibrations (also, the two largest panels will be "sandwiched" between a carpeted floor and the base of the tv), what, if any, acoustic lining do you recommend for the interior walls. Internet information regarding acoustic lining for bass reflex enclosures limited to bass frequencies (e.g. subwoofers) varies considerably and is contradictory. For example: don't use any acoustic lining that could interfere with the desired resonance(s) of a bass reflex alignment, line half of the interior walls including the back with a layer of fiberglass, line with open cell foam, line with closed cell foam, line with egg-crate foam, don't treat the walls at all but instead situate some fiberglass/polyfill in the interior of the enclosure directly behind the woofer(s), etc., etc. I don't need to make the enclosure "act" like a bigger box -- a reason for sometimes using stuffing -- as I'll be able to comfortably get the 65 liters I need. I simply want the box to work per the design and for the bass to be clean. So... what's your advice?

    Thanks in advance for any knowledgeable help offered.

    David

  • #2
    Re: Bass reflex bass bins, your advice re acoustic lining please

    All you're really trying to do by lining the walls is reduce the chances of internal noises escaping the enclosure. The better the material, the better the job. Mattress foam is cheap, easy, and should be effective enough for this application.
    http://jaysspeakerpage.weebly.com/

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    • #3
      Re: Bass reflex bass bins, your advice re acoustic lining please

      Originally posted by Jay1 View Post
      All you're really trying to do by lining the walls is reduce the chances of internal noises escaping the enclosure. The better the material, the better the job. Mattress foam is cheap, easy, and should be effective enough for this application.
      So you're suggesting mattress foam on all interior walls? Is there a particular kind -- open or closed cell, thickness, etc. Thanks for your response.

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      • #4
        Re: Bass reflex bass bins, your advice re acoustic lining please

        Only use open cell, close cell will simply reduce the box volume. The denser and thicker the foam the better, but you'll have to see what's available for what price locally. Lining all the walls is best, just dont block the port opening, or the rear of the driver.
        http://jaysspeakerpage.weebly.com/

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        • #5
          Re: Bass reflex bass bins, your advice re acoustic lining please

          Originally posted by dstmbgh View Post
          So you're suggesting mattress foam on all interior walls? Is there a particular kind -- open or closed cell, thickness, etc. Thanks for your response.
          Must be open cell (you can blow through it). Closed cell will reduce the volume of the box.
          2 inches all around, cheap Walmart foam should work fine

          I think I hear a difference - wow, it's amazing!" Ethan Winer: audio myths
          "As God is my witness I'll never be without a good pair of speakers!" Scarlett O'Hara

          High value, high quality RS150/TB28-537SH bookshelf - TARGAS NLA!
          SB13/Vifa BC25SC06 MTM DCR Galeons-SB13-MTM
          My Voxel min sub Yet-another-Voxel-build

          Tangband W6-sub

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          • #6
            Re: Bass reflex bass bins, your advice re acoustic lining please

            Originally posted by donradick View Post
            Must be open cell (you can blow through it). Closed cell will reduce the volume of the box.
            2 inches all around, cheap Walmart foam should work fine
            So let's say I do just that... 2" of Walmart open cell mattress foam (which I suppose is egg-crate) on all interior walls, do I build the box to the specified 65 liters or modify that volume to somehow adjust for the foam lining?

            Again, thanks for all replies.

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            • #7
              Re: Bass reflex bass bins, your advice re acoustic lining please

              Originally posted by dstmbgh View Post
              So let's say I do just that... 2" of Walmart open cell mattress foam (which I suppose is egg-crate) on all interior walls, do I build the box to the specified 65 liters or modify that volume to somehow adjust for the foam lining?

              Again, thanks for all replies.
              Build it to the max volume you can (65 liters), but you may be able to tune it a bit lower. The only way to be sure is to run an impedance
              check after you have stuffed the box. You can build an impedance jig for a couple of bucks and use the free LIMP software for the output.
              Or not worry about it. Tune it wth Unibox (vent length) and enjoy.

              I think I hear a difference - wow, it's amazing!" Ethan Winer: audio myths
              "As God is my witness I'll never be without a good pair of speakers!" Scarlett O'Hara

              High value, high quality RS150/TB28-537SH bookshelf - TARGAS NLA!
              SB13/Vifa BC25SC06 MTM DCR Galeons-SB13-MTM
              My Voxel min sub Yet-another-Voxel-build

              Tangband W6-sub

              Comment


              • #8
                Re: Bass reflex bass bins, your advice re acoustic lining please

                Originally posted by dstmbgh
                ...line all interior walls with carpet underlayment...[or]...line all walls with Walmart convoluted open-cell foam mattress topper...[then]...should I adjust the cabinet volume at all?
                I found some further information/testing relevant to my question. Few diy'ers have more experience building loudspeakers than Troels Gravesen. Here are his test results regarding adjusting cabinet volume and port length calculations based on real world outcomes. Very interesting and pertinent reading.

                http://www.troelsgravesen.dk/ports.htm

                http://www.troelsgravesen.dk/vent_tuning.htm

                His results indicate that it would be inaccurate to say that lining a bass reflex cab with air permeable damping will not change apparent box volume. In his tests merely lining (not filling) the interior walls of a bass reflex cab with either 30mm egg-crate foam or Monacor MDM3 (35mm thick blend of sheep's wool and polyester) increased virtual box volume 15%, thereby lowering the box tuning. In addition, he finds that box tuning port calculators are never in accordance with real world measurements, and thus port length always has to be shortened to actually hit targeted Fb.

                Interesting. Hard to argue with real world tests as opposed to theory, math that's supposed to be neat and tidy, or anecdotal experience.

                Further comment welcomed.

                David

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                • #9
                  Re: Bass reflex bass bins, your advice re acoustic lining please

                  Thanks for those links regarding ports & vent tuning. That finally sheds some light on why the specified vent length for my current build (Paul C Sunflowers) is about 30% shorter than calculated!

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