Distance of port to back wall?

[mitch808]

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

TIA

Mitch

 

[454Casull]

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

 

[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.

 

[Vikash]

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]

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]

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]

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]

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]

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]

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.