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D75 chamber sizing

edited October 4 in DIY

I made a small 3" diameter x 1.75" long PVC chamber for my new (but slightly used) D75MX-21 midranges (thanks again, Dan!).   Below are a few construction pics with comments.  Qtc with this test chamber came out to 1.2 (see DATS V2 screenshot).  Is this an acceptable Qtc for a midrange of this size?  Should I make the chamber bigger or smaller?

When I removed the green felt along the perimeter to glue the PVC in place, I discovered a large number of vent holes. 

  The PVC tube blocked off these vent holes, so I ground clearance chamfers using my dremel tool.

I made a cap out of 1/4" masonite and then glued a white rubber bumper to the center of it to spread out internal resonances.

Finished chamber:

DATS V2 measured Qtc with test chamber sealed in place with caulking cord:

FR measurement at 31", midrange placed on floor flush with thick blanket:


hifisidejhollanderNicholas_23kennyk

Comments

  • For what it's worth, the Dyn D76AF chambers are about another half of that volume, and they are 3" domes, but I've had 2" domes also at about the volume you have there. Honestly, it depends on your wants/needs. 1.2 will provide an earlier rolloff, but bump the response a bit (not unlike a waveuide). This can be beneficial and allow smaller parts sizes for the same F3 of larger chambers.
  • That frequency response looks good.  Interested in what the HD measurements look like.
  • Thanks.  The DynAudio D76AF spec sheet shows a Qts of 0.9, so I think I need to go a bit larger.  I'll slice off a 3.5" length of PVC, re-measure, and report back.  I'll also run a few HD measurements.


  • What size pvc is that?  Instead of going to the trouble of cutting reliefs into the pvc, what about taking the mid to the local hardware store and going through their bin of pvc pipe and fittings.  Bet you'll find something that will work w/out all the fuss.  If you can't find it in the plumbing section, try the grey pvc in the electrical section.  
  • Kornbread said:
    What size pvc is that?  Instead of going to the trouble of cutting reliefs into the pvc, what about taking the mid to the local hardware store and going through their bin of pvc pipe and fittings.  Bet you'll find something that will work w/out all the fuss.  If you can't find it in the plumbing section, try the grey pvc in the electrical section.  

    You are correct.  This is  3" ID PVC (sch 40) and the 3" coupler used to join two 3" pipes together is almost the perfect fit to the magnet.  I bought one coupler, which, when cut in half, will work for 2 of the mids.  Thanks.
  • As another test, I doubled the enclosure size from 3" ID x 1.75"L to 3" ID x 3.5"L by stacking two enclosures end to end (have not used the coupler yet).  Qtc dropped from 1.204 to 1.029.  Fc dropped from 386.9 to 319Hz.  I ran an HD test with the 1kHz SPL level set at 95dB.  Microphone tip was 32 inches from the dome.  The attached HD graph is hard to interpret, so I pulled the following HD data using the mouse.  Graphs are also attached:

    Freq   Percent
    100Hz  10.75%
    200    5.52%
    300    2.19%
    400    2.43%
    500    2.29%
    600    0.79%
    700    0.52%
    800    0.64%
    900    0.52%
    1000   0.64%
    1500   0.62%
    2000   0.44%
    2500   0.35%
    3000   0.57%
    3500   0.43%
    4000   0.47%
    5000   0.28%
    6000   0.18%
    7000   0.21%
    8000   0.15%
    9000   0.13%
    10000  0.03%


  • I noticed that the response below 1kHz dropped off a little bit when I increased the enclosure size.  For my next test, I will cut the enclosure in half, raising to Qtc to 1.5 to see what happens.  Maybe this will give me the flattest measurement.   
    Nicholas_23
  • Wolf said:
    Fill it with Ultratouch,

    I have been using some light polyfill material so far.  Will test Ultratouch as well.
  • I would definitely recommend using some heavier damping material like the Ultratouch.  That or some fiberglass insulation.
    4thtry
  • The HD is sure telling you when these are not happy.
    4thtry
  • Yeah, 600-2.5k is the sweet spot for sure.
    4thtry
  • Lol I had these things for 5 years and never even took a frequency measurement.  Bill's had them for 2 weeks and already has them figured out!
    4thtry
  • Wolf said:
    Yeah, 600-2.5k is the sweet spot for sure.

    I agree.  I'm thinking a TMW or TMWW with a SB19 on top crossed with an initial target of 3k or so.  Then I will use one or two 5.25" woofers crossed with an initial target of 800Hz or so.   Maybe a single ND140 in a 0.75cu.ft cabinet, tuned to 40Hz or so.  Baffle would be about 17H x 6.5W.  Depth about 14".   Power handling would be limited, but I want to keep the baffle width to 6.5" and I have been unable to find two reasonably priced 5.25" 8 ohm woofers that would work well in a 0.75 cu ft box.   
  • PWRRYD said:
    I would definitely recommend using some heavier damping material like the Ultratouch.  That or some fiberglass insulation.
    I tested this using my 3x3.5" enclosure.   Qtc dropped from 1.2 to 1.1 when I inserted the Ultratouch.    So, Ultratouch must be a very reactive type material, similar to fiberglass.  I did not know this.  Thanks for the tip.  
  • I made a flange out of two layers of 1/8" thick tempered hardboard (HDF).  I used the type that is smooth on both sides.  Both layers are 5.25 x 5.25".  The top layer has a 3.625" hole cut to match the dome.  I partially rounded over the internal edge with a 1/4" round over bit.  The bottom layer has a 4.5" hole to clear the magnet and terminals.

    When glued together, this flange will be very strong.  I think I will drill 8 mounting holes instead of 4 to prevent flange distortion as I tighten the screws.  When finished in flat black with a satin top coat, it should look like a brand new driver!

     

    JasonPdcibelS7910kennyk
  • I glued the 1/8 + 1/8" flange sandwich together with Titebond II.  Then, to prevent warping, I inserted all the finished flanges into a makeshift "press" for drying.  Also, after several stages of glue "squeeze out" and clean-up, I placed a single layer of paper towel between each flange in the press to act as a moisture wick.  All flange surfaces were completely dry when I inserted these towels, so they should not stick and create a mess later on.   I'll probably leave the flanges in this "press" for a least a week before taking them out for painting.
     
    I made a total of 4 flanges.  This gives me one extra in case one gets messed up.


    PWRRYDJasonPkennyk
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