8 Basic Diagrams On How To Measure Port Length...

[Texas Tech]

Well a while back i made these diagrams as i was speaking to someone (PowerNAudio, probably one of the nicest guys ive ever spoken with) who was teaching me how to measure the port length correctly, so you know what length to put into the tuning equation to get the correct tuning frequency for your port. I think he checked some of the diagrams but not all of them. So i am posting them to see what you guys say, so i know if they are correct or not.I will label the pictures with letters, so it is easier for you guys to say Diagram A is incorrect....yadda .....Some of them may be correct and others may not be... and made some people as well as myself will learn from this.

a)

c)

d)

e)

f)

g)

h)

So have at it!

[Texas Tech]

wait i have to change something it will be 2 minutes, so dont comment till then cuz something might have changed

-EDIT-Alright everything looks the way i want it to.

And a thanks goes out to PowerNAudio for teaching me, any of these diagrams that are correct is because of him.

[LSN2meBoom916]

OK-

I only looked at the 1st couple and I'm curious about the "end correction" calculation?

Maybe you could post how you arrived at the 2" end correction?

This pretty technical so it may be that either you or I have it a little off but...

I read it- 1/2 H (Highet of port not width...?)-see red highlight below in JL quote...

Thus-I think it REALLY depends on the cross sectional area (Sd) of the vent/port to determin end correction.

How "tall is the vent in the 1st 2 pics? That "Height" would affect Sd of the vent for sure.

Here's some sites that suggest port design info pretty well IMO and the 1st one discusses end correction.

http://www.diysubwoofers.org/misc/portcal.htm

http://mobile.jlaudio.com/support_pages.php?page_id=165

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* EFFECTIVE PORT LENGTH:

Quoted from http://www.jlaudio.com/tutorials/ports/index.html: The effective length of the port is found by adding an end correction factor. An end correction factor is necesary because more often than not, one wall of the port is also one wall of the enclosure and this wall extends beyond the end of the port thus effectively adding length to the port (remember, the driver can't "see" the length of the port, it can only go by what it "feels" is going on). To calculate end correction factor, simply add one-half of h to the physical length you calculated above (L1 + L2).

(from http://www.2k4civic.com/portcalculator.html )

Unless I missed something.

Mind you, it's almost 2am and I'm damn tired so I VERY WELL have just missed it...

From BassBox 6 Pro Owners manual:

-"What is end correction? It's an internal parameter that compensates for the fact that a vent usually appears to be slightly longer than it really is. This is because the air immediately outside each end of the vent will vibrate along with the air inside the vent."

.......

-"The end correction of a ROUND vent is is equal to the internal diameter (Dv) of the vent. For example, a vent with an inside diameter of 2 incheswill appear to be 2 inches longer than its actual length (Lv)"

-Pg 238.

That's all I have off the bat-lemme know if that helps.

[Texas Tech]

the h in jl audios diagram is the width in my diagrams. imagine jl audios diagram flipped so that the side with the measurements of the L1 and L2 are facing the ground. So for diagram a), the h according to jl audios website would be 4 in.

[LSN2meBoom916]

Ok- but isn't it a fraction/percentage of the CROSS SECTION of the vent?

Only in the ex of a round port was it the diam of the vent, but with a rectangular port/vent, wouldn't it require more than one # or measurement like your vent is 4"x however tall it is and you'd take the cross section of that?