Port conversion, round to rectangular help

mapleglo

mapleglo

Warp Engineer
Sep 7, 2013
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phoenix, az
#1
I have these Peavey PA speakers, 112DL. They feature Scorpion cast frame woofers, and some sort of dual driver horns.

peavey112dl.jpg


I use them mostly for monitoring my Fender Rhodes and vocals in my little music room. They rattle pretty badly on the low notes from the Rhodes, and sound a little "boxy" overall. Pulling one of the drivers, I can see why. They're made from 5/16" plywood, with a few 3/4" braces, and no stuffing. Scorpions are pretty good speakers, and I was wondering why these cabs don't sound very good.

I thought to build a couple of replacement cabs out of 3/4" baltic birch plywood, but I don't want to use the multiple 2" ports. One in each cab has already fallen out, likely contributing to the poor sound quality.

Specs on the cabs are 1.7171 cubic feet (internally measured) with 4 ports, 2" in diameter, 1.5" long. Assuming these were designed correctly, if I wanted to make one shelf port, along the bottom of the cab, how would I convert the 4 to 1 port size/length? The interior width is 15 3/8", though the final interior width of a cab I would build out of 3/4" plywood (to maintain the exterior size which matches a few other cabs I have) would be 14.5" interior width.
 
#2
Just make the new port the same area as the total of the 4 ports. Depth of the would remain the same as the depth of the existing ports.

If the port is less than 1.25" high, you will want to block off somewhat of the width to make the port higher. Small dimensions deviate from the formulas somewhat.
 
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#4
And can't I increase the area of the port and increase its length to yield the same tuning frequency? Because 12.56" isn't as wide as the box, and a 1" high slot will like cause turbulence and noise?
 
#6
mapleglo said:
So do I calculate the area of each port (pi*R squared ~ 3.14*1=3.14) and multiply times 4 which equals 12.56?
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Correct

mapleglo said:
And can't I increase the area of the port and increase its length to yield the same tuning frequency? Because 12.56" isn't as wide as the box, and a 1" high slot will like cause turbulence and noise?
Click to expand...

Yes, you can recalculate as needed, but because of the factory tuning is why I mentioned about minimum port dimensions. It's not the turbulence and noise as much as it adds a non-linearity that causes (or can cause) the actual tuning to deviate from the predicted tuning as the power into the cabinet increases.

If it was me, I would duplicate the existing ports as is.
 
#7
The rattle may just be a warning, e.g. "Farting out"
Youmay have to turn down the bass, or add a subwoofer.

It may not have stuffing by design. Only lows go through the woofer, and one use of stuffing is to minimize the mids in the cab from bouncing out through the woofer. Or they may just be cheap
 
#8
salcott said:
Why not reglue the port tubes, make sure all the screws are tight, and put some stuffing in the existing cabinets first?
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^This. Also put glue on all the inside lines and support connecting points.

Little effort and if it works, you'll save some time and money.

Another thing to think about is that the Rhodes puts out some pretty good bass and possibly adding a sub-woofer or upgrading your cabs may be a better long term solution.
 
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#9
why? just repair the round ports. and line the boxes . you can also add dowels/brace from front to back panel, and side to side and would help with the buzzing.
1.5 inches is a very short high tuned cab, like disgusting high tuned. better performance for the roads if you made those things 5 or 6 inches in length.
a port in each corner is a better design than a single shelf, likewise easier to calculate.
 
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#10
Thanks for all the responses. These cabs are used at home - I doubt I'll ever gig with them, but, I want them to sound better than they do. Perhaps the best course is to re-glue the ports and add a bit more bracing to see if that solves the issue of rattling. It's funny, I picked up another set of Peavey cabs at a garage sale for $20.00 last week:

ampwall.jpg


These have the Sheffield woofers and piezo tweeters, but sound much better than the Scorpion loaded cabs, which doesn't make sense to me.

Perhaps I'll have to measure these Scorpions parameters and start from scratch with cab builds for the long run.
 
#11
the cabs tuned to 80hz so any bass signal will bring them to over excursion pretty quickly.
they are gonna be crossed over high at 3k or 5k for tweeter so a light lining with poly batting will improve midrange for the 12's
lower the tuning to 50/55hz and you wont fart out the woofers so easily and transients will improve.
1.5 inches to short
6 inches would be fine.

thats what she said ...dohhh
 
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#14
BogeyBass said:
1.5 inches is a very short high tuned cab, like disgusting high tuned. better performance for the roads if you made those things 5 or 6 inches in length.
Click to expand...

I don't think this is the case, within the limitations of the driver. The driver/box by design and be intent was never designed to handle and reproduce much below about 60Hz no matter what you do. The driver is pretty low compliance.
 
#15
yes understood, if high passed at 80 or 120 for use with sub. those frequencies are non concern.
and likewise using the crossover and reflex function to reduce cone excursion. yes design intent.

running fullrange and expecting to reproduce 80hz below its causing alot of excursion. so lowering the tuning will actually increase power handling.
for his application as far as running fullrange without sub and highpass on satellites.

sometimes i miss it too, remember cone excursion increases dramatically below FB, and yes high tuning can improve power handling. but if to high can cause over excursion if the speaker is expected to produce lower frequency.

once your 30 to 40 hz below tuning or Fb, cone excursion goes through the roof. so high tune at 80hz now your making 50hz go to high early in the game. lower tuning to 55hz now excursion is super low at those frequencies. cone movement is restricted at and around Fb.
using the reflex as a brake if you say.
 
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#18
mapleglo said:
FYI, the Fs of the Scorpion is 82.5 hz per the spec sheet I dug up on the web. Tuning the cab too far below that is not good, I think.
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ohhh #### really? good call
didnt know those things where that high. yes correct

well good vocal pa lol, good with a highpass as well at 80 or 120hz if combined with a sub, tuning way below fs starts getting into extended bass shelf territory. technically okish depends on design intent/application and driver. not so much here
 
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#19
mapleglo said:
FYI, the Fs of the Scorpion is 82.5 hz per the spec sheet I dug up on the web. Tuning the cab too far below that is not good, I think.
Click to expand...
Correct, which is why I was so clear with my suggestions. It's always good to understand the whole picture as the drivers and intent are important facts (as you posted). I would classify the Scorpion drivers as closer to mid-bass drivers, and they work very well for that application in the PA world.

I think much of the OP's challenges with these speakers is simply the wrong tool for the job.
 
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