Physical port length vs acoustic length

[dbsupra 69]

This is for a project I'm working on.  I've only ever done a slot port once that didn't physically bend but that was on a really small enclosure.

 

This port is physical length of 12.5" and 6" width.  Is this acoustic length right?  I'm used to a slot port having a second leg and just adding 1/2 of the port width.  For this enclosure, if the acoustic length is right on the pic then it will make nearly a 10hz change in tuning.  Kind of a big deal.

 

Thanks!

 

[shizzzon 461]

awaiting results...

[shizzzon 461]

Nobody knows?

 

Here is the question reworded-

 

The depth of the box is 18.5" and the port is 6" wide.  The depth of the port wall is 12.5" (leaving a 6" gap from the back wall.

 

The question is this-

 

Is the acoustical length shown above correct in that it is 22.25" long or do you not count anything past the back of the wall regardless of the port area size inside the box?

 

I cant answer this because it's our debate.

[cobra93 47]

The effective port length includes the back wall and half the width of the port on both ends of the port.

It's 22.25" of effective port length, 25.25 if you add 1/2 the width of the port at the outside termination.

 

It's like getting lucky to have it work out like that, or good planning.

Edited August 6, 2013 by cobra93

[shizzzon 461]

the top part of the picture, past the 3\4" mark, where it says 3" is taking into account half the port right there which puts it at 22.25" + external baffle thickness.

[REVpuggie 23]

I always avoid a port length like this ...

[swift 501]

the top part of the picture, past the 3\4" mark, where it says 3" is taking into account half the port right there which puts it at 22.25" + external baffle thickness.

If that 3in is part of the port wall also or the L portion then yes 22.25. If the port wall is straight with no L then the length is 19.25 and that 3in should not be counted.

Edited August 7, 2013 by swift

[onebadmonte 235]

build the box, add the woofer, and test with the woofer tester or the dayton audio DATS thingie and you'll have your answer.

[dbsupra 69]

Its a straight port no L. I didn't believe him. I would have thought the length would only extend 3" past the physical port. I have never built this way before and I usually trust what sean says but I wanted to see if he was in left field on this one.

I ended up building an L since I'm confident how that works. I do have a DATS and usually test after building. Generally its pretty close to what I calculated. But this making a 10hz difference I went with what I was familiar with. This is going to be a comp box with a tuneable port.

[shizzzon 461]

We had to physically add an L because the Torres calculator is using improper calculations to determine port tuning, port length and port displacement.

 

I proved it by designing a straight port in that software with plenty of space in the rear of the enclosure and it calculated port displacement to be 0.8cuft larger for no reason whatsoever.

 

So, we had to use winisd since winisd bases measurements on NET only versus Torres bases everything on gross which is not ideal.

 

 

And swift- the 3" shown before the port is a general acoustical rule which states when using a slot port, add half the width of the port to the acoustical length.

 

The 3" was shown in the pic to help in calculating acoustical length.

 

But again, we eventually had to add more physical port length due to inaccuracies in Torres.

 

Whether or not the NEW tuning is right or wrong via DATS, the fact that Torres doesn't know how to calculate port displacement alone is a reason to discontinue use of the program altogether.