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Samps

Samps DS21 Build

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About half way done with my ported DS21.   Thought I'd share some pics.  

23.5 x 23.5 x 49.5. = good use of ply.  

13.4 internal

-1.0 for port

-.5 for driver

-.15 for bracing

= about 11.75 net cubic feet

port is 8 x 8.  Length is a guess at this point.  It's 22.75 in actual length, but the effective length I'm sure is going to be longer.   Hoping for 20-22 hz.  I built a jig to measure, but can't get a good reading yet.  

Going to be painted.  Bridged on a 3000dsp.  

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Looks good. Yes I would expect your port to act a bit longer than what you have calculated for the length plus 90deg turn since it is close to the back wall. I'd guess it acts a good 3" longer than expected. Impedance measurement should show where it lands.

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Make sure you put a finish on the cab before you install the driver.  I have had cabs that went without finish for 3 years, b/c after the driver goes in, it is so easy to just listen to it.

JSS

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I built and sold two THTs that were never finished.   I figure you have to test it before you spend all that time finishing, but then it never gets done.  

My impedance jig isn't working correctly.   It will make a sweep but the level is super low and the graph doesn't look anything like what I've seen before.   Pretty sure I built the jig correctly, but not sure I'm using the correct settings in REW and or computer.  Been testing a little bookshelf for practice.   

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6 hours ago, maxmercy said:

Make sure you put a finish on the cab before you install the driver.  I have had cabs that went without finish for 3 years, b/c after the driver goes in, it is so easy to just listen to it.

JSS

Three years.... Weak.... I am still rocking the OG MicroWrecker (since 2013)... No finish, not even sanded, still has big PL boogers on it. 

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5 hours ago, Samps said:

I built and sold two THTs that were never finished.   I figure you have to test it before you spend all that time finishing, but then it never gets done.  

My impedance jig isn't working correctly.   It will make a sweep but the level is super low and the graph doesn't look anything like what I've seen before.   Pretty sure I built the jig correctly, but not sure I'm using the correct settings in REW and or computer.  Been testing a little bookshelf for practice.   

PM me. I can help you work through it. 

They're pretty basic. Might be as simple as L and R channels swapped. 

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Would there be any benefit to adding a deflector to the back corner of my port?  

Currently there is a pretty big difference in volume of that corner vs the rest of the port.  

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3 hours ago, Samps said:

Would there be any benefit to adding a deflector to the back corner of my port?  

Currently there is a pretty big difference in volume of that corner vs the rest of the port.  

Possibly.

Looks like it is something where you could try it and see. Heck, just cut a piece and set it in place for testing. Won't be 100%, but will give you an idea.

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Yeah, that's what I was thinking.  Now that I can do the impedance test ( thanks Mike ) I feel like I can test things and make useful decisions.   I doubt it will make a big difference unless I'm pushing things pretty hard.   But it's simple enough to try.  

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Took a impedance sweep.  Looks like it's currently around 23hz.  Not sure if that might change once the driver gets broken in?   It may shift a little lower when the cab is standing upright.  It's currently laying on its back because I didn't want to screw in the driver.   Once it's standing up, the port may act a little longer since it's on the very bottom.  

I was hoping for a little lower but not sure if I want to mess with it.  22hz in the model works out pretty well with the inukes 20hz HPF. 

Adding length to the port isn't a problem, I left myself plenty of room to extend it.   I did add the corner deflector in the back corner, but didn't get the impedance test of the before and after.

ds21 impedance.jpg

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Driver break-in may shift the impedance peaks ever so slightly, but it won't affect the tune, which depends only on the cabinet volume and port characteristics.

Edit: I think you're closer to 21.5-22 Hz.  The tuning frequency is precisely where the phase crosses zero.

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Thanks, I'm going to put in a few screws tonight, and test it upright to see if that changes things.  

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Not having the driver bolted in will probably make the system a bit lossy too. I'd expect that standing the cab up and properly bolting in the driver and sealing it will probably sharpen the lower peak a little and perhaps lower it a bit more. 

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Yeah, that lower peak is a bit "soft", that generally means losses or leaks (or, as I learned, a really useless port). 

With a typical reflex box, we want the peaks to be about the same height. 

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I thought the peaks would only be the same height if the tune was at the drivers FS?   Winisd predicts similar lopsided peaks as I'm getting, but the lower peak is not quite as bad.   Hopefully it all works out once I add screws and stand it up.  

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The lower peak substantially depends on the suspension compliance, which is probably the least accurate T/S parameter given and the most likely to change with break-in as well as temperature and humidity.

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Looks better screwed in and standing up.   The trace has more wobble in it, but the impedance minimum came down to 21.27hz.   And the lower peak is a bit higher and sharper.   Also added a comparison between unscrewed laying down and the screwed standing up.  :)

 

ds21 impedance screwed.jpg

ds21 impedance compare.jpg

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Can you run a slower sweep?  That might help reduce the ripple.  Otherwise, looks good.  It looks like you hit the tune you wanted right on the mark.

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I ran the slowest sweep REW allows and the result was exactly the same.   Not sure where that waviness comes from.   It didn't show up in any of my tests of other speakers around my house.   Possibly still some air leaking out, I didn't use any gasket material under the driver since this is still under construction. 

I'm pretty confident this is working out as planned, so I'm going to move forward with sanding and paint.   It won't be a show piece, but decent anyways.  

Thanks for all the help!

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Wobbles could be reflections from surfaces and vibration/resonances. I put about 150lbs on my speaker stand to drop the vibration resonance of the top section down below 5Hz. Otherwise it shows up in the impedance measurements. You'd be surprised what adding some weight and turning the cab just 15 degrees might do sometimes. I bet that desk isn't as stable as you'd think. I usually put my cabs on a bar stool if measuring in my garage and angle them towards the far corner then add a couple 50lb dumb bells and some sandbags on top. Larger cabs like Skhorns, MAULS, etc. get done outdoors.  

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Lol, that desk is not stable at all.   Cheap IKEA type construction.   I added some braces to it just so I could make it useful, but I wouldn't call it stable in any way.   Maybe I'll try sitting it on the floor.  

I'm also planning on making my own grill for this, since nobody makes a damn 21" speaker grill.   I guess all the pro audio guys using these either have them in horns or make their own grills.  So, question is, how much clearance should I leave above the surround?  

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I'd do xmech rating +2mm or something. You'd never ever hit that but you don't want to take that chance either. 

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Admittedly, I oversimplified the statement regarding equal peak height above.

The relationship between the driver parameters and the box tune dictate the relative size of the two peaks. What matters most is that the peaks you're measuring are a decent match to what your simulation software is predicting. 

As far as the grille spacing? Yeah, make sure things won't hit. X-mech plus a few mm is a great start, I'd look at the thickness of nominal pieces of plywood or other scrap stock. Make sure the grille does not rattle. Foam weatherstrip and lots of screws.

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