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nyt

2x 18" LMS Ultra 5400 in 27 cubic foot 15hz vented enclosure

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Hmmm, how far away is the MIC from the sub?

 

about a meter.  I just lengthened the port.  Will test more after I eat something.

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that's peculiar because the measured martysubs with slot ports came in at about a 20% adjustment factor.  maybe the turbulence around the corner is the difference.  the marty's were just straight slots.

 

the phase goes 180 at the tuning frequency, if i understand that right, so based on your phase response, it looks like you have about a 14hz tuning or so.  do you have a high pass filter engaged somewhere?  something seems a little off.

 

in any case, nice work, as usual.

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Ok, quickly eyeballing this and using rew, tuning looks to be around 14.7hz.  I'll nail it down exactly tomorrow.  It's late and I think I was making neighborhood dogs go crazy.  I found some rca to xlr and hooked up an FP14000 to it.  I was worried the garage was going to go to shit and my lights fall down.  There might be some port noise when pushing it near the limits, but I can't tell yet.  Too much other noise to discern anything.  I haven't routed any of the edges yet either.  I'll see about testing more in the day tomorrow, but it's supposed to rain, so this limits my measurement opportunities.

 

Looks like winisd was within a couple of inches.  The corner may have something to do with it as well.  As I said, I rounded nothing.

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Nyt,

Do you have any familiarity with doing impedance measurements? That's really the best way to determine tuning. A woofertester would work or you can make a simple voltage divider. REW can do it as can ARTA for next to nothing.

 

Option#2 would be to use the signal generator in REW and watch driver excursion at a moderate output level and look for the point that the cones move the least.

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Nyt,

Do you have any familiarity with doing impedance measurements? That's really the best way to determine tuning. A woofertester would work or you can make a simple voltage divider. REW can do it as can ARTA for next to nothing.

 

Option#2 would be to use the signal generator in REW and watch driver excursion at a moderate output level and look for the point that the cones move the least.

 

I've been using option #2, it's somewhere between 14.5 and 15hz.  Weather was shit today, so I didn't do anything.  I'm going to drag it outside tomorrow along with a sealed cabinet and take some measurements.

 

You have a schematic for a voltage divider setup?  What resistors do you use?  Not terribly important since with drivers with this much excursion and big amps, it's easy to eyeball.

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Here is a schematic from the (ARTA,(LIMP)) manual...REW also has information on this in their help manual.

 

All you really need is a good quality low tolerance resistor, a DMM, some speaker wire, an amp, a soundcard, a splitter (to split the output from one channel of the soundcard to both input channels for the channel calibration), and a couple of cheap cables that can be chopped up for the network and will plug back into the soundcard inputs. A handful of the stackable, screw down 5 way banana plugs work well to wire things up and make changes quickly. You likely have the majority of this stuff already.

 

All you really need to be careful about is the level going into your soundcard input as they will only handle a volt or 2 and make any changes to the wiring with the amplifier off.

 

 

 

Is the 14-15Hz tune before the extra port length or after? You know with the way you have the vents setup you could plug one or two of the vents and drop the tuning if you wanted to maintain a little higher tuning for more music output. You would lose some port area but unless you are realliy cranking on them I doubt you would ever notice any issues with it. Just a thought.

post-5-0-21097000-1388422762_thumb.jpg

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That was after the length was added.  When I get a few minutes today I'm going to take it outside with one of the sealed enclosures and take some sweeps.

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What was the tuning prior to extending the port length?

It was around 17hz.  I posted some graphs in here somewhere.

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I have an impedance how-to over at AVS. I'm working on an improved approach, as the Behringer UCA202 has a bad crosstalk issue. It works well enough for most folks, but the amplitude of the measurements below 20 Hz are inaccurate due to the crosstalk.

 

As ever - the issue is finding an affordable USB sound card with a real line in that isn't junk for less than $40. The Griffin iMic 2 I grabbed a while back seems to work OK, and it has a lot less crosstalk, but it does roll off a bit earlier than the UCA. The iMic lacks a powered headphone output though, so for power, I used an inexpensive headphone amp kit from Parts Express powered off a USB plug. Still sorting out the bugs with the REW approach, and not working on it all that fast to be honest. Too much other stuff going on, and I have plenty of other ways to measure impedance accurately.

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Here we go, sealed vs ported.  I unplugged one from the amp, plugged the other in.  Same wiring configuration, nothing changed in between.  Measurements at 3 feet.

 

NuIOAY9.png

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 ~10dB - 15dB centered around tune.

 

How is this relative to what you expected?    

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It's a bit better than expected.  Tuning came in a touch lower, all good though.

 

There's some slight port noise from inside the cab, going to round the edge with some PVC I think.

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Hmmm, you "might" notice the difference...

 

+15 dB at 15 Hz? From where you were already? That should be fun.

 

Nice work.

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This is how it modeled with same input signal.  A bit difference from reality, where spl increases more as frequency increases, otherwise very similar.  Less of a hump on the ported output also.

 

 

z9JAYKP.png

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There's something drastically amiss with your measurements, so I wouldn't be analizing them in such deatail. Pasting Josh's GP of the LMS sealed onto your graph... even at a glance the sealed trace is nearly a 4th order roll off.

 

Can you snap a pic of the 2, side x side?

post-28-0-19933300-1388441467_thumb.jpg

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Snap a pic of what?  The subs?  I already rolled the sealed back into my house and slid the monster ported one back into my garage.  It's not the easiest thing moving a giant 400lb sub around.  Taking these measurements sucked bad enough, was just above freezing :(

 

The gains over the sealed are as expected.  It might have been too close to my house or something *shrug*.  I don't think there was anything amiss in the measurement chain as it behaved as expected.  I used one of my linkwitz wm61a mics to measure.  I'll double check what calibration files are applied.  SPL was calibrated @ 100db at 50hz to a rat shack meter.

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Just remeasured, same results with a properly calibrated input and using my wm61a with calibration applied.  If the curve is not as expected, probably due to the outdoor conditions.

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