OK, ported boxes are a pain in the butt to design!

Jstas

I can do a sealed box easily! This tuning stuff is a drag when you are going for big pressure!


The box I'm putting together from this thread:

http://www.polkaudio.com/forums/showthread.php?t=67143

for a pair of Memphis M3124D's has been a thorn in my side.

Currently the box they are in is a 6+ cubic foot beast with a gigantic rectangular vent. The box is tuned for about 47 Hz or so they say. I honestly think it's too high. After plotting out response on a graph, the "hump" in response that the guys in that other thread say is there doesn't seem to be there or at least not nearly as pronounced as they say it is.

I've already established that the resonance measurement of the car is off. How much, I don't know. Maybe we'll find out after I get this box put together.

The box is 35 inches by 17.5 inches by 16.125 inches. I think it might actually be a tad wide for the space between the shock towers but we'll see. The owner of the car can't seem to get off his dead butt and get me measurements.

Anyhow, I was going to build a dual chamber box but, I could not keep port velocity below 30 f/s without a common chamber box. Consequently, I have three 4 inch ports to keep the velocity down and the port tubes in as straight a line as possible in the depth of the box. Both ends will be flared so my port velocity of 23.2 f/s should not be that big of an issue because the flared ends manage air flow better.

The box is tuned at the resonant frequency of the sub because I can't trust the car measurements. It's about 30-35% smaller than the old box so moving it around should be easier too. I'm going to mount the 3 ports in a vertical line between the subs. That way air flow isn't obstructed by the sides of the trunk compartment. That should help port velocity and cabin loading too.



A couple things I learned about "tuning". You aren't tuning a resonance like so many people think. You are managing air flow. When you tune to a resonance, you are making it so that the air pocket behind the woofer has enough space so that the timing of the backwave moves the air in the port in sync with the woofer cone so it compliments the woofer instead of fighting it. Granted, this is in reality what everyone is doing, I just don't think that as many people really understand it as much as they say they do.

Air flow is crucial. It's how the cabin gets pressurized. If your box/ports are too big, they will not have enough velocity to effectively load the cabin to it's maximum. If the cabin is not loaded to its maximum, you can tune to the resonance of the car all you want, it will just fight you until you hit the maximum SPL you subs are capable of producing. For that, you need to build the enclosure properly.

Enclosure construction is not a "wham, bam, thank you ma'am, slap it together any old way and call it a day" kind of proposition. Especially if you are tuning the enclosure to increase output. It is pretty much an exact science and requires accurate measurements from the company that designed and built the driver. Numbers matter.

SPL is volume. There is no difference. Someone can try and tell you until they are blue in the face that SPL and Volume are not the same. There are several definitions of volume, one of which is esoteric but for the most part volume can be described as amplitude, dynamics, loudness and SPL. If someone tells you that Volume is not the same as SPL ask them how they measure SPL. If they say anything other than dB (decibels) they are wrong. Then ask them how they measure Volume and if they say dB...I rest my case.

Audio is physics. Electronics and the electricity that runs the electronics is also governed by physics. Numbers describe EVERYTHING in physics. If audio, electronics and electricity are based in or described by physics then physics and the numbers used to describe physics can describe anything in audio. Any audible manifestation or even inaudible manifestation, unless it's just in one's own head, can be described with numbers. Specs don't describe everything nor tell you how something will sound because how the something interacts with the environment it is installed in is where those manifestations come from. But that environmental interaction can be described with numbers. Most people who tell you that it can't just don't really fully understand what is going on with those manifestations.


So I dunno if this box is going to produce more favorable results. We'll see I suppose. I do know that the box I built is about 4.009 cu. ft. which is in line with what the subs should be performing at their best in. The response hump is pretty wide too and goes above a -3 dB slope around 36 Hz and stays high all the way to about 52 Hz where it falls off below -3 dB again. The old box seems to have a drastic spike that doesn't rise as high as this box. I tuned the port for this box to be around 40-42 Hz, right smack in the middle of that peak. I think the car's resonance is closer to that too than the 47 Hz they measured it at. I got a bunch of towels and stuff that I can rubber band up and stuff in places to see how the dampening will change the car too. Maybe I'll get some more out of it, maybe I won't. All I really want is 2-3 dB more because that will put him over the top in his class and no one will be able to touch him.

exalted512

I'm interested in your results, I think your numbers might be shy of his though...hopefully I'm wrong, we shall see.
-Cody

Deadof_knight

I have found desing and planning are not futile but it gets ya close and then when it happens its like YEA I built this !!!! or uggg I built this !

I built a box for an 18 of mine (4.25cuft sealed ) didnt like how it sounded so i turned it into a single chamber box for 2-12s ported SHAZZAM that baby rocks ......go figure !

Jstas

exalted512 wrote: »

I'm interested in your results, I think your numbers might be shy of his though...hopefully I'm wrong, we shall see.
-Cody

Why do you think that?

exalted512

Generically speaking, lower tuning (especially that low) and a smaller box yield unfavorable results.
-Cody

Jstas

Well, I'm thinkin' he's too high on his resonance of the car. I'm only about 5-7 Hz lower than where he is "tuned" with the other box. But I think that 6 cubic feet is way too much box for his subs.

exalted512

Let us know how it turns out!
-Cody

Jstas

I've been messin' around with this Eminence Designer software. I really like it!

But I found a way to get exact numbers off of the graphs.

Currently the box is gigantic and tuned for 47 Hz.

The box I designed is only about 30% smaller. It's about 4.2 cubic feet from the 5.98 cubic feet. The ports are 3, four inch ports with flared ends on both ends. The power level is about 1100 watts. After looking at the normalized and custom response curves, I have about a 10 Hz plateau where the response is -1.8 dB. The only time it gets higher than that is around 500 Hz where it reaches +2.2 but that has to be a resonance of some kind.

But that plateau goes from 39.9 Hz to 49.6 Hz. The customize response graph which incorporates power levels says that the subs will be operating at 122 dB from 36.6 Hz to 67.8 Hz.

With the flared ports and essentially what amounts to a 10x4 inch hole in the box, this thing is looking to at least match the numbers the previous box has with a smaller, lighter package.

Got the materials last night, gonna cut it up tonight and get it all ready to get glued and screwed. Just waiting on the parts from Parts Express and then we can go BOOM to the tune of 145-155 dB on 1100 watts of power.

Jstas

Box is screwed and glue! Waitin' for the glue to set up. Then it's cut some speaker holes and caulk the whole thing. Wait for that to set up and start mounting ports and speakers.

Then we go BOOM!

dorokusai

Don't blow it up John, that would be counterproductive.

Jstas

Who said anything about blowing it up?

Jstas

All the holes are cut and every joint is caulked. Waiting for that to cure then I trim up the port tubes and mount them.

Jstas

The box is done!

One thing I am not real happy about is that there is only about an inch and a half of space between the internal mouth of the port and the back of the box. I don't think it's going to hurt performance too much but if it does, I'm gonna tear them out and put half inch rings under the mounting rings. That should bring it up to a bit over 2 inches with the gasketing material. That should be enough space and it won't extend past the subs so it won't look silly either.

The flared ports were a **** to get mounted though. I was hoping some rubberize gasket material would be good enough but it didn't work out and I ended up RTV'ing the ports in. It came out sloppier than I wanted it to but I can trim the extra goobers off with a razor blade once they are cured. I might have to reseal them too though. The holes had to be a bit wider than the base of the flares to accommodate the flare properly. Needless to say, because of the taper, very little stability from the port tube in the hole. They shifted around on me when I was trying to screw them down. The ports were a cinch to assemble too. Hope they don't have to come out because the PVC cement has them secured together pretty tight. But overall, I'm quite pleased with the box. It don't necessarily have to be pretty and it's not but it straight and clean though.


Oh and after messing around with the software some more, I found a way to factor in resonance of the car. With this box tuned the way it is, there is a nice 2dB hump between 40 Hz and 50Hz which is supposedly where the car's resonance is. I think that with the old box, the size made it real peaky and they weren't hitting the full performance of the subs because the performance peak was off slightly from the resonance peak of the car. With the smaller box, it tightens things up a bit widens the response to cover more range.

Jstas

Well, we burped the previous box at 47 Hz and it hit 142.3 dB

Then we stuffed my box in but found out that I built it too tall because I was given wrong measurements. So we shoe horned it in. Went through a series of tests with the highest being about 43 Hz. So we went for that it topped out at 143.6 dB.

But like I said, the ports are pretty close to the back wall of the box so I'm gonna cut some spacers and pull them out 2 inches or so. That will give it over 3 inches clearance and that should be enough space to push air better. I think I can hit the 145-146 dB goal.

exalted512

Congrats! YGPM.
-Cody

Jstas

Reported.

Die spammer scum!