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It’s that time audio enthusiasts! Registration for the 2019 Speaker Design Competition is now open! Visit midwestaudiofest.com for details and to list your speaker project. We are excited to see all returning participants, and look forward to meeting some new designers this year, as well! Be sure your plans include a visit to the Parts Express Tent Sale for the lowest prices of the year, and the Audio Swap Meet where you can buy and trade with other audio fans. We hope to see you this summer! Vivian and Jill
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Rimne - Slot ported near-field monitors

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  • Rimne - Slot ported near-field monitors

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    This was a little weekend project I came up with to use some Dayton Audio ND65-4 I had laying around. I have previously used the ND65-8 in small portable speakers, and I have been impressed with their low frequency response. They measure relatively flat, but do suffer from some cone breakup around 7000 Hz.

    The idea was to use the driver in some small near-field monitors to put on your computer desk. I also wanted to try out a design with a slotted port since I’ve never done one before.

    A little information regarding the name of the speaker: Rimne is a jötun in Norse mythology. The e in Rimne is pronounced like the e in Ted. Other versions of the name are Hrímnir and Hrímne.

    I simulated several enclosures and ports combinations in VituixCAD. I decided on a 2 l (0.07 ft³) enclosure with a 0.8 x 8 x 18 cm (5/16 x 3 1/8 x 7 1/8 in) slotted port. This gave me an fb of 68 Hz and f3 of 63 Hz.

    I had to make the cabinet quite deep to accommodate the length of the port. However, the ported design of this speaker gives the low frequency response a boost. As such, I felt that this design didn’t need a baffle step compensation (BSC) circuit to measure flat.

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    The cabinets are constructed from 12 mm (0.5 in) Baltic birch plywood. All joints are simple butt joints. The baffle cut-out is a drilled hole, but I had to use a wood file to shape the cut-out so the speaker terminals would fit. I did use a router to camber the port and the back of the cut-out. I made the front of the speaker removable so that I could experiment with different amounts of fill. Two holes were drilled in the back plate for binding posts.

    The cabinet was lined with acoustic felt and stuffed with 40 g (1.4 oz) of fine sheep's wool. I had to use quite a bit of fill to tame the 1st and 2nd harmonic port resonances at 750 and 1500 Hz. The wool was pushed all the way to the top of the cabinet to not block the port.

    I decided to paint the front baffle white to contrast with the pin-cushion driver frame. The rest of the cabinet was given several coats of beeswax and then polished.

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    The speaker measures relatively flat. The frequency response is near-field below 350 Hz in the graph. There are dips and peaks in the response such as at the 1st harmonic port resonance and at the cone break-up. But I think the response is quite decent considering this is a speaker you can make in two evenings, it is without a crossover, and that it uses one inexpensive driver per speaker.

    My listening impression is that they sound full-bodied and clear; the bass, mids and high are easy to pick out in different kinds of music. I also hooked them up to the TV in the living room and they work all-right for movies, but they are a lacking in the low frequency rumble as expected. However, I think they are a solid upgrade over any built-in TV speaker. Their main limitation for living room use is that they will distort at high volumes as the driver reaches xmax.
    Last edited by solbero; 05-06-2019, 01:54 PM.

  • #2
    Nice. Are you using any lining or stuffing in the box?
    Brian Steele
    www.diysubwoofers.org

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    • #3
      Did you measure your own T/S parms for these, or is the design done off mfr. specs?

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      • #4
        Originally posted by Brian Steele View Post
        Nice. Are you using any lining or stuffing in the box?
        I have lined all the inside of the cabinet with 1 cm (1/2 in) acoustic felt and stuffed it with 40 g (1.4 oz) fine, sheep wool. Unstuffed the port produced a lot of resonance at 750 and 1500 Hz. I tried to not block the port by compressing the wool to the top of the cabinet. I tried a uniform stuffing first, but an impedance sweep showed that the impedance peak of the port was almost flattened.

        Originally posted by Chris Roemer View Post
        Did you measure your own T/S parms for these, or is the design done off mfr. specs?
        I relied on the published T/S parameters when I designed the enclosure. I am still quite new to speaker design. Would I have gained anything significant by measuring the speaker?

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        • #5
          Even though Dayton's specs are typically MUCH closer (to real measurements) than most, it seems like the ND series have shifted around aLOT over time.

          "spec" / my DATS
          Qes 0.68 / 1.02 (<-THIS shifts Qts from 0.61 /to/ 0.82)
          Qms 5.6 / 4.3
          Fs 89 / 138
          Vas .019cf / .008cf

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          • #6
            That is quite a difference between the stated and the measured specs. When I have the time I will pull out my drivers and measure them for comparison. I felt a tickle of curiosity seeing your numbers.

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            • #7
              Looks deceiving, I wouldn't have guessed it is 0.07cuft, looks like my 0.18cuft ones good job

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