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-   -   Distance of port to back wall? (http://www.diyaudio.com/forums/multi-way/35746-distance-port-back-wall.html)

mitch808 10th June 2004 02:06 AM

Distance of port to back wall?
 
Is ~3" an adequate distance from the back wall of my sub box to the port?

TIA

Mitch

454Casull 10th June 2004 03:49 AM

What's the target SPL, and is the port flared?

mitch808 10th June 2004 05:39 AM

I didn't plan on using a flared port, although I gather it would help immensly.

But I'm shooting for about a F3 of 19Hz, at around 109db.

Vikash 10th June 2004 09:30 AM

As a general rule, I would leave at least one port dia. distance away from the back wall. If you port dia. is 4", leave at least 4" from the wall.

markp 10th June 2004 02:59 PM

I leave three times the port size from the wall or it will continue to act as one large port of an unknown legth.

454Casull 10th June 2004 05:40 PM

Quote:

Originally posted by mitch808
I didn't plan on using a flared port, although I gather it would help immensly.

But I'm shooting for about a F3 of 19Hz, at around 109db.

I would use more distance then, just to be on the safe side.

roddyama 10th June 2004 06:13 PM

In a quick search, a number of manufacturers suggest placement similar to Vikash at about one port diameter. I must admit that, intuitively, this seems wrong to me. Particularly considering the air velocity for 109db @ 19Hz in a 3" port will approach 30ft/sec. (this is also an argument that velocity and not acceleration is more directly responsible for SPL, but that's a different discussion.)

I think I would consider a larger port if possible.

I would be very interested to hear an explenation on how you can place the port so close to a wall and not adversely affect its output.

sreten 10th June 2004 06:26 PM

The critical distance is when the the CSA of the port
circumference to the wall = the CSA of the port.

This occurs when the spacing is 1/4 of the port diameter.

So I agree with Vikash, and have to disagree with MarkP.

And to answer Roddyama's query with a spacing of 1 port
diameter away from the wall the acoustic impedance change
at the exit of the port is only slightly affected.

Note that there is a huge change in air velocity at the exit.

:) sreten.

roddyama 10th June 2004 08:00 PM

Quote:

Originally posted by sreten
And to answer Roddyama's query with a spacing of 1 port
diameter away from the wall the acoustic impedance change
at the exit of the port is only slightly affected.

Note that there is a huge change in air velocity at the exit.

:) sreten.

Yes, I can see the where the air velocity would change dramatically at the port exit (essentially a half space). What it seems you should get is the loading on the port of a conical horn that is expanding at the rate of 2pi X distance from the ports center.

sreten 10th June 2004 08:51 PM

Quote:

Originally posted by roddyama

Yes, I can see the where the air velocity would change dramatically at the port exit (essentially a half space). What it seems you should get is the loading on the port of a conical horn that is expanding at the rate of 2pi X distance from the ports center.

.

Well its only 2pi for half effective baffle width, after that its 4pi,
given free space. For close to wall its ~ 2pi above baffle width.

:) sreten.


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