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Question about Re-Tuning the 4430 Cabinets

Zonker92

Zonker92

All shiny and chrome
I understand that the 4430 cabinet, with two ports that are about 4" in diameter and about 8" deep, is tuned to about 34 Hertz. I have also read that it is is desirable to re-tune the cabinet to about 29-30 Hz, by closing off one port and shortening the second port to about 4" in length.

I don't want to cut up my port tubes, and it looks like removing one of them (to replace it with a shorter one) might be a little destructive.

What if I just blocked one of the ports and left the other alone? I did see Zilch suggested that as a possible approach. What would that do to the tuning?

I may just try it and see.

Any other ideas or thoughts?

(Disclaimer: OK, I admit it: I think they sound great as they are. I just like tweaking, but I don't want to damage the speakers in the process.)

(Confession: I posted this same question over on LH.)
 
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Here's a port length calculator:

http://www.psp-inc.com/tools.html

If your present ports are 8" long, and you really need a single 4" port, it will tune way low with one port. I'd also think about a flared port if you go to a single, since it will be moving the air that two ports used to. If you're worried a flared port is not JBL worthy, the D66000 Everest uses twin flared ports :yes:
 
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Interesting; thanks. I think the net volume of the cabinet is about 5 cubic feet.

I wonder if I blocked one port and stuck a flare onto the back of the other, which effectively shortens it. Playing with the numbers, that almost seems like it would work to tune it to around 28-29 Hz. I'll try to nail down the numbers.
 
Zonker92 said:
Interesting; thanks. I think the net volume of the cabinet is about 5 cubic feet.

I wonder if I blocked one port and stuck a flare onto the back of the other, which effectively shortens it. Playing with the numbers, that almost seems like it would work to tune it to around 28-29 Hz. I'll try to nail down the numbers.
Click to expand...

A flare on one end may be better than no flare, although it will generate some even harmonics because of the flow asymmetry. There's a JBL white paper floating around on the subject of optimum port flaring, I'll see if I can find it.
 
Found it. If you search Google on "Maximizing Performance From Loudspeaker Ports" you'll find the white paper.
 
Thanks. I just checked and per JBL, the net volume is 5 cf. I just measured the ports at 4.1" ID and 8.25" long. The result is that if I simply blocked one port the box would be tuned to around 25 Hertz. With the stock two ports, that's tuned to about 35 Hertz. So simply blocking one port would drop the tuning about 10 Hertz. Blocking one port and replacing the second one with a shorter port (4-5") does seem to drop it to around 29-30 Hertz.

Seems like the simplest thing is to buy some Zilch plugs (or just use an old rag) and block one port and see if I like it.
 
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Will be surprised if that doesn't introduce audible port noise... the single 4" port is going to have difficulty handling the increased air flow. You're getting the lower resonance by increasing the velocity of air through the port, that plus the halved surface area usually = wind noise.

You might try inserting cylinders to reduce the diameter of the existing ports, though at some point that has the same risk - but it gives you the chance to cut the cross-section by some factor less than 50% vs. losing one port completely.

John
 
Interesting; thanks. I ran the calculator and it looks like reducing each port to 3.2" might put the cabinet at about 29 Hz. I would have to strain my brain to figure out what size insert would achieve that size, or just put smaller tubes inside lengths of pool noodles maybe.

OK, I did the math (on-line circle area calculator) and it looks like a 4.1" port is about a 13.2 square inch circle. A 3.2" port is about 8.04" square inches. So to reduce the 4.1" port to 3.2" means losing about 5.16 square inches, which is about the size of a 2.56" inch tube. So maybe some 2" ABS pipe wrapped with friction tape ...
 
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Thanks. Interesting. I wonder what the minimum recommended area for my two ports would be.

At this point, I'm not considering making any irrevocable changes; just maybe putting some removable "gap-fillers" (solid, smaller pipes) into the ports to lower the tuning.
 
Zonker92 said:
Thanks. Interesting. I wonder what the minimum recommended area for my two ports would be.

At this point, I'm not considering making any irrevocable changes; just maybe putting some removable "gap-fillers" (solid, smaller pipes) into the ports to lower the tuning.
Click to expand...

That is the approach I would take. No reason a port has to be round. A 1" square piece of wood would reduce the area by one square inch, etc. Some tacky putty would hold it in place as a temporary trial.
 
Adding things in the port with sharp corners will just exacerbate the windage (official name for port noise) issue, which is already likely the limiting factor. The velocity of air flow in the ports is already being increased by cutting the port area, so adding other things that can cause noise isn't that great an idea.

I think his idea of inserting tape-wrapped tubes to knock down the diameter will give him a better idea of how far he can push this re-tune.

I couldn't put my hands on a simple tool for calculating min. port cross-section, but my recollection is that some of the programs we used back in the day had criteria for that. He'll find out pretty quickly with the inserts, though.

John
 
How about an entirely different way to go. Instead of closing a port and shortening the other, how about just using both ports, but lengthening them a bit?

Use 4" PVC/ABS cut to length and lathe it down so that it just fits within the existing ports (a sleeve-job if you will).
 
OK, that would be about 12.5-inch tubes, but then their rear openings would be only about an inch from the damping in the cabinet back.

Maybe right angles would work, though. Hmm ...
 
Be aware that tuning ports/ducts are just as effective when mounted outside the box ( as inside ) .

This makes varying the tuning ( for test purposes ) a piece of cake .

:)
 
Huh. That is very good information; thanks!

And I just had a brainwave. My L250s are tuned with a rear port of about 4" in diameter and about 5" long, and obviously in so designing them, Greg Timbers didn't think that port noise was a problem.

On the 4430s, I'm building some outboard charge-coupled crossovers, so I'll be removing the terminal plates in back, leaving a 4.25 x 5.25" hole in the back, and running the cables for the drivers through that back opening somehow.

So I could just block the front ports and mount a 4" port (it looks like it should be about 4.5" long) inside a blanking plate that I would place where the factory terminal plate used to be, with the driver cables going through a hole in the plate next to the port. That would convert the speakers into rear-ported cabinets. I could adjust the length to hit 29 Hz.

If I blocked off the front port from their back ends, to hide the plugs from view, I guess that would the reduce the cabinet volume? Or maybe just stuff them with a few inches of a 4" pool noodle painted black.
 
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Do what? I'm not gonna cut a single hole or make a single non-reversible change: Just (maybe; this is all theoretical right now) pull the crossover and screw a ported plate in its place, using the existing screw holes. And from the front, the only visible difference would be port plugs, and I'd make them almost invisible by making them stain or flat black and recessing them in the openings. They'd be just shadows to the eye.

Yeah, my whole ethic on a project like this is: Do no harm and change nothing permanently. Build little plug-in modules using all existing holes.

I hate hacking stuff up. I'm obsessive about making all changes 100% reversible. The way I see it, it's like swapping wheels or intakes on a car, where everything is bolt on, bolt off.

Another project I'm considering is building stands for them. And again, I plan not to put a single hole or cut into the speakers. All fully reversible. This is how I have fun. :D
 
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