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First speaker project...help with low frequency fall off

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  • First speaker project...help with low frequency fall off

    Hey guys,

    I'm in the middle of my first speaker project. I'm using some Dayton Audio ND65-8's:

    http://www.parts-express.com/pe/show...number=290-206

    And a DTA-2 amplifier:

    http://www.parts-express.com/pe/show...number=300-385

    According to the specs on the speaker, and inputting the parameters into BassBox 6, I came up with a test enclosure made from 1/2" MDF. All joints are glued and pin-nailed except for the front so I can detach and make modifications inside:

    Click image for larger version

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    I started everything up and immediately noticed that it didn't have nearly the amount of bass I was expecting. I pulled my Radio Shack SPL meter (C weighting, fast response) and tested the frequency response between 40Hz - 300Hz:

    Click image for larger version

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    One problem is the noticeable dip between 150-190Hz...any ideas on this one?

    The rolloff below 120Hz pretty much mirrors the official spec sheet from Dayton. But according to Bass Box the F3 for this enclosure is right around 59Hz so I expected this thing to hold steady down to that frequency...maybe my expectations were wrong? Could my port be too close to the back wall (it's about 2.5 inches away) and I should be adding a 90 degree elbow in there? Could I have just enough tiny air gaps that the port can't perform it's duty correctly? Am I just expecting way too much from a 2.5" speaker?

    Here's what BassBox came up with for the box and port configuration:

    Click image for larger version

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    Another weird phenomenon was that stuffing the box with Acousta-stuf actually made the bass response much worse...it was noticeable by ear right away and the SPL meter confirmed: 50-60Hz dropped by 4dB. I had to just add a minimal amount at the very back of the box, and that was almost the identical response as having no padding whatsoever. Again, my expectations about the magic properties of Acousta-stuf were incorrect to begin with.

    On the bright side, I listed to "No Surprises" by Radiohead and heard details I've never heard before. I hooked up a couple other pairs of speakers to make sure I wasn't crazy and sure enough, they didn't reproduce that detail (you could hear the guitarist's fingers on the strings on the repeating four note intro). I just wish I could get the bass out of this thing that I was (perhaps over-enthusiastically) expecting!

    Any help would be greatly appreciated!

    Thanks,
    Rob
    Last edited by Cannikin; 04-27-2013, 03:34 PM. Reason: Added SPL meter details

  • #2
    Re: First speaker project...help with low frequency fall off

    Baffle step, leaks, suffing, expecting too much, etc, etc. Try running your sim through WBCD or Winisd and see what you get. Is the port blocked by the stuffing? The port has plenty of room to the wall, no issue there.
    https://www.facebook.com/Mosaic-Audi...7373763888294/

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    • #3
      Re: First speaker project...help with low frequency fall off

      One problem is the noticeable dip between 150-190Hz...any ideas on this one?
      Educated guess based on experience: Nearby boundary effect, causing a droop in response.

      Another weird phenomenon was that stuffing the box with Acousta-stuf actually made the bass response much worse
      I think that is a hint that the T/S params of your driver do not match spec, and the box you are using is likely too large for its actual spec.
      Audio: Media PC -> Sabre ESS 9023 DAC -> Behringer EP2500 -> (insert speakers of the moment)
      Sites: Jupiter Audioworks - Flicker Stream - Proud Member of Midwest Audio Club

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      • #4
        Re: First speaker project...help with low frequency fall off

        Originally posted by Cannikin View Post

        One problem is the noticeable dip between 150-190Hz...any ideas on this one?
        The first thing I'd consider is the measuring protocol. Chances are that's room related.
        according to Bass Box the F3 for this enclosure is right around 59Hz so I expected this thing to hold steady down to that frequency..
        I get an f3 of 45hz with 52Hz tuning in 2.4L, so either something's wrong with the build or that's a measurement protocol issue too. But don't expect a lot of low end out of it in any event, displacement limits it to 90dB maximum output.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          Re: First speaker project...help with low frequency fall off

          Originally posted by billfitzmaurice View Post
          I get an f3 of 45hz with 52Hz tuning in 2.4L, so either something's wrong with the build or that's a measurement protocol issue too.
          Oh measurement! Could it be affected by the weighting on the meter? He said he is using an RS SPL meter.
          Audio: Media PC -> Sabre ESS 9023 DAC -> Behringer EP2500 -> (insert speakers of the moment)
          Sites: Jupiter Audioworks - Flicker Stream - Proud Member of Midwest Audio Club

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          • #6
            Re: First speaker project...help with low frequency fall off

            Originally posted by JasonP View Post
            Oh measurement! Could it be affected by the weighting on the meter?
            Not above 50Hz.
            www.billfitzmaurice.com
            www.billfitzmaurice.info/forum

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            • #7
              Re: First speaker project...help with low frequency fall off

              The RS SPL meters only allow the use of A or C weighting. The A-weighting curve looks like this:


              The C-weighting curve looks like this:


              So, if the RS meter is set to A-weighting, then the bass response curve will be severely affected.

              Personally, I think the issue is likely related to A-weighting setting along with room reflections affecting the 150Hz region.
              DARPA Jr - 2015 InDIYana Winner - RS180-8 + RS100P-8 + ND25FA
              The Aria's - RS150-4 (or RS150-8) + XT25SC90
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              The Canzonetta's - RS100P-8 and ND16FA
              AudioSource AMP-100 Mods OR Pyle PAMP1000 Mods

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              • #8
                Re: First speaker project...help with low frequency fall off

                Anything under 300Hz in a room of reasonable size is absolutely moot, unless it's from a nearfield measurement.

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                • #9
                  Re: First speaker project...help with low frequency fall off

                  Originally posted by tyger23 View Post
                  The RS SPL meters only allow the use of A or C weighting. The A-weighting curve looks like this:


                  The C-weighting curve looks like this:


                  So, if the RS meter is set to A-weighting, then the bass response curve will be severely affected.

                  Personally, I think the issue is likely related to A-weighting setting along with room reflections affecting the 150Hz region.
                  I'm using C weighting, fast response.

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                  • #10
                    Re: First speaker project...help with low frequency fall off

                    Thanks for the suggestions guys! I'm going to do a little more testing in a better environment (everything was simply resting on my desk for this test) and I'll update this post with some more details. I'll also model the speaker in WinISD and see how that compares to BassBox and maybe make some adjustments.

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                    • #11
                      Re: First speaker project...help with low frequency fall off

                      The "rolloff" below about 300Hz should follow your box sim output, NOT Dayton's measured response (unless they specifically tell you test conditions, and that those were in a box pretty much like yours).

                      WinISD ( www.linearteam.org ) likes a box about double the size of yours, with about the same tuning. It yields about the same output down to 100Hz or so, and at 40 and below, but there's a slight lift at 80Hz, over +2dB at 60, and +3 at 50. As has also been mentioned, your SPL measurements might be suspect. Do you know what "baffle step" is? Also, MOST folks stuff closed boxes, but line vented boxes with convoluted (egg carton) foam. You can stuff a vented box to death.

                      That little guy is probably Xmax liimited with only 2 or 3 watts by the time you get down to 80Hz.

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