Triangular ports question

[macaroonie]

Hi all , my question is this , in some of the smaller JBL PA cabs they use triangular ports in the bottom corners. Would I be correct in assuming that these can be treated as if they are circular with the same cross sectional area.
I'm using WIN ISD

TIA

[Hoerninger]

I'd do so.
You can assume that the circular port is a bit too long.
The final tuning will be done with portlength - as usual.
____________
Peter

[cooky1257]

I thought that when using the cab walls as boundaries/edges to your port they are in 'effect' longer than the port length would indicate. There is a formula to take this into account but can't lay my hands on it.
Cooky

[spkrman57]

Are you changing the system from stock tuning?

Ron

[macaroonie]

No Ron , trying to tune from scratch. Usual arrangement , two triangles in the bottom corners of the baffle , 2206 woof with horn in a box about 600 x 400 x 375.
Win will give me a result for two circular ports of a given cross section , so my question is do I just transpose that area into a triangle. I suspect there is some change.

Thx for responses gents

[macaroonie]

Like this

[Ruediger]

I thought that when using the cab walls as boundaries/edges to your port they are in 'effect' longer than the port length would indicate. There is a formula to take this into account but can't lay my hands on it.
Cooky

The tube will act as if it were longer than it's physical length. You must add an end correction to each end of the tube.
For a free standing end of a tube (not flanged), add 0.613 x radius of tube
for a flanged end of a tube add 0.85 x radius of tube.

Ruediger

[cooky1257]

Thanks!

[1audiohack]

Here's another take on it, every time a triangular port shares a boundary, inside or out, the tuning will come down right about 1.3 to 1.5Hz. Also ports in close proximity to like enclosures will couple driving the tuning even lower. This is not an effect of mutual coupling of the drivers.

Triangular ports also have the highest resistivity per area of any shape save a rectangle with a high aspect ratio, ie, a slot.

Pictures and measurements posted on request.

[macaroonie]

The tube will act as if it were longer than it's physical length. You must add an end correction to each end of the tube.
For a free standing end of a tube (not flanged), add 0.613 x radius of tube
for a flanged end of a tube add 0.85 x radius of tube.

Ruediger

thanks for that but I am a little confused. When you say that the tube ( triangular ) will behave as if it is longer than it actually is , then why add even more length , thereby making it seem even longer etc etc.
Surely it will require a subtraction ?

[cooky1257]

Yep reduce.

[macaroonie]

Yep reduce.

Ah so. Cheers Cooky. I've spent the last couple of days mentally swooshing that one around.
In the real world about 5% less than sims would indicate for a tubular port

[Ruediger]

thanks for that but I am a little confused. When you say that the tube ( triangular ) will behave as if it is longer than it actually is , then why add even more length , thereby making it seem even longer etc etc.
Surely it will require a subtraction ?

Right, my statement was unclear.

To calculate the effective length, You must add the end corrections to the physical length.

Find out if the program which You use already includes the end corrections.

Ruediger

[subwoof]

hey there... put a pair of 3/4 plywood cleats long the side and bottom of the box where the ports will be. this way you can simply change the length of the "cover" to adjust the tuning.

sub

[macaroonie]

hey there... put a pair of 3/4 plywood cleats long the side and bottom of the box where the ports will be. this way you can simply change the length of the "cover" to adjust the tuning.

sub

Hi Mike , I don't quite get your meaning but I'm guessing a kind of sliding extra section to allow adjustment
Thanks nonetheless Mac