Anyone care to explain end correction?

[Triticum Agricolam]

It says that the effective length of the port is longer than the physical length by half of the width of the port. So, I would have to subtract half the port width in order to make my effective length accurate. No? I would prefer someone actually explain why one way is the correct way rather than just tell me to read something I already read that totally contradicts what someone else showed me. I created this thread to actually learn, not so someone can just tell me how to do something without explaining it to me.

And I take it you don't want to answer my other question? You expressed that you had other concerns, but won't elaborate...

Yeah, the JL paper does say to add one half of the width of the port to get your effective length, its a simple way to add is some error correction and is better than nothing, but it not the best IMHO.

As you have found out, there are multiple ways to figure end correction and port length. Its not that one way is correct and the others are incorrect, its more that they are all estimations and some are more sophisticated than others.

Personally I don't use the "add half the port width" way to calculate end correction, I use the info that Krakin listed combined with the JL formula and my own personal experience and I usually do pretty well. The best thing you could probably do is a little trial and error. Make a box with the port extra long and then figure out where its tuned to and cut down the port until you get where you want. Then you can compare what you actually end up with with what the formulas say you should be. As long as you have to tools and the time MDF is cheap compared to the experience you will gain.

[user18436572]

It says that the effective length of the port is longer than the physical length by half of the width of the port. So, I would have to subtract half the port width in order to make my effective length accurate. No? I would prefer someone actually explain why one way is the correct way rather than just tell me to read something I already read that totally contradicts what someone else showed me. I created this thread to actually learn, not so someone can just tell me how to do something without explaining it to me.

And I take it you don't want to answer my other question? You expressed that you had other concerns, but won't elaborate...

Yeah, the JL paper does say to add one half of the width of the port to get your effective length, its a simple way to add is some error correction and is better than nothing, but it not the best IMHO.

As you have found out, there are multiple ways to figure end correction and port length. Its not that one way is correct and the others are incorrect, its more that they are all estimations and some are more sophisticated than others.

Personally I don't use the "add half the port width" way to calculate end correction, I use the info that Krakin listed combined with the JL formula and my own personal experience and I usually do pretty well. The best thing you could probably do is a little trial and error. Make a box with the port extra long and then figure out where its tuned to and cut down the port until you get where you want. Then you can compare what you actually end up with with what the formulas say you should be. As long as you have to tools and the time MDF is cheap compared to the experience you will gain.

I'm sure this is more than likely a really dumb question, but how exactly would I test for the actual tuning frequency? Kinda hard to cut down the port on a vent. lol.

[Joe X]

You can measure tuning plotting an impedance curve, you will need a few things to do the measurements, there will be an impedance drop near the calculated tuning and that's your actual tuning frequency.

[Triticum Agricolam]

I'm sure this is more than likely a really dumb question, but how exactly would I test for the actual tuning frequency? Kinda hard to cut down the port on a vent. lol.

Testing and trimming isn't that tough to do, you just have to plan ahead during construction. Here is a picture I stole from JoeX to demonstrate:

If you wanted to test and trim with this design you would build like normal, gluing everything together except for panel A. Panel A you would screw in place and then you would screw the top/side (the panel opposite panel B that isn't shown in the picture) to seal up the box. You have to make clean, accurate cuts and use lots of screws but it will seal up well enough. Worst case scenario you get out the duct tape. Then you can test to see where you tuning is and you can still open everything back up and trim down panel A as needed.

Its a time consuming process, but it can be done.

[user18436572]

You can measure tuning plotting an impedance curve, you will need a few things to do the measurements, there will be an impedance drop near the calculated tuning and that's your actual tuning frequency.

DMM, Clamp, and some tones? lol. Simple enough.

So, Joe....Triticum seems to feel the first equation using the different values for K yields the most accurate results in his testing. Are you trying to lean me toward the JL formula, because you feel it is more accurate in your own testing? Or because you feel it is a safer bet for a n00b designer like myself due to the fact that it should typically result in a longer physical port length?

Triticum: I'll have to do that when I have a lot more spare time and money! lol.

[Joe X]

Triticum Agricolam states he uses the K values to determine effective length and plug that into the JL equation, I say neither method will zero in actual tuning in every case and the differences are negligible for daily applications in most cases. Thinking about port area, cabin gain, etc will have a deeper effect on your box outcome.

[user18436572]

Triticum Agricolam states he uses the K values to determine effective length and plug that into the JL equation, I say neither method will zero in actual tuning in every case and the differences are negligible for daily applications in most cases. Thinking about port area, cabin gain, etc will have a deeper effect on your box outcome.

Ahh. Well, since you brought that up, actually...

1. I have no idea how to calculate cabin gain or resonant frequency.

2. I have read that port area should be between 12-16in²/ft³ but I have also read that piston displacement plays more of a role in determining this, which totally makes more sense to me. I also remember reading a thread somewhere a while back where someone mentioned using smaller port area for lower tuning to help control the woofer better?

Teach me more! I know there is much more to know and learn here.

Oh, and please elaborate on what other concerns you had about things I had said. I am trying to learn everything I can here. Enclosure design is my weak spot in car audio, and no doubt the most important aspect. Help a brother out.

[Triticum Agricolam]

Ahh. Well, since you brought that up, actually...

1. I have no idea how to calculate cabin gain or resonant frequency.

2. I have read that port area should be between 12-16in²/ft³ but I have also read that piston displacement plays more of a role in determining this, which totally makes more sense to me. I also remember reading a thread somewhere a while back where someone mentioned using smaller port area for lower tuning to help control the woofer better?

Teach me more! I know there is much more to know and learn here.

Oh, and please elaborate on what other concerns you had about things I had said. I am trying to learn everything I can here. Enclosure design is my weak spot in car audio, and no doubt the most important aspect. Help a brother out.

There really is no way to calculate cabin gain, you can measure it if you have the right equipment, that's the only way to know for sure. Using tools like WinISD you can roughly approximate it, but that is as good as it is going to get.

The "port area should be 12-16 sq in/cu ft" rule is one that a lot of us wish would just go away and die. Its better than nothing and it can be an OK starting point but it really doesn't take nearly enough information into consideration. The better way to calculate port area is based on driver radiating area, xmax, and desired tuning. There are online calculators to help you do this or you can go straight to the Small's formula.

To get better cone control tuning lower (while leaving port size the same) can certainly help with that, depending on the situation, but decreasing port area is not the right way to do it. If you decrease your port area enough in an effort to get cone control what you will eventually have is effectively a leaky sealed box. It will give you cone control, but it will kill your output.