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Old 10th June 2004, 01:06 AM   #1
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Default Distance of port to back wall?

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

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Mitch
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Old 10th June 2004, 02:49 AM   #2
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What's the target SPL, and is the port flared?
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Old 10th June 2004, 04:39 AM   #3
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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.
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Old 10th June 2004, 08:30 AM   #4
Vikash is offline Vikash  United Kingdom
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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.
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Old 10th June 2004, 01:59 PM   #5
markp is offline markp  United States
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I leave three times the port size from the wall or it will continue to act as one large port of an unknown legth.
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Old 10th June 2004, 04:40 PM   #6
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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.
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Old 10th June 2004, 05:13 PM   #7
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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.
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Old 10th June 2004, 05:26 PM   #8
sreten is offline sreten  United Kingdom
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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.
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Old 10th June 2004, 07:00 PM   #9
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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.
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Old 10th June 2004, 07:51 PM   #10
sreten is offline sreten  United Kingdom
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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.

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