Quick question about port placement

[owdi]

I'm building a 4th order bandpass enclosure and need a 4" wide, 6.5" long circular port (according to WinISD). This means my port needs to have an effective length of 9.4" after end correction is taken into account.

My problem is that the 4" diameter port I do have is only 3.75" long, with a .25" flare on one end. I have heard that placing the end of a port close to the back wall will extend the effective length of the port by up to the diameter of the port. This would be nearly perfect, giving me an effective length of 9.7". Since my port is 4" in diameter, how far should it be placed from the back wall to get the longest possible effective port length? 2"? 1"?

Dan

[johninCR]

It seems like your numbers are off, so double check your figures and I don't think there's any way you are going to get 3.7" inches to turn into 9.7 effective, although I do tell my wife it's possible all the time. Even flush against an interior corner of the cab, I think you only get 1/2 of the diameter in additional effective length.

Just go out and buy the correct length of 4" ID PVC and place the port correctly.

[owdi]

EDIT: in my previous post I said the inner port length is 3.75", that was wrong.. it is actually 3.25".

johninCR - From reading the WinISD help files on end correction my 3.25" long port has an effective length of 6.2". I got this by adding:

3.25" (inner length of port)
.42 x 4" = 1.68" (flared end correction)
.3 x 4" = 1.2" (free end correction)

With my current plans, the free end will be 1.5" away from the back wall. That is less than half the diameter of the port away from the wall, which will extend the acoustical length of the port by........

....any guesses?

Dan

[BillFitzmaurice]

Don't use the room as a duct, make the port the right length to begin with. Use an elbow in the box if you have to. Then you can use the distance to the wall for final tweeking of the bass response. Too short a duct limits what you can do.

[johninCR]

I always like to double check that kind of thing against the formula, because I'm never sure exactly what has been considered by the program. For example, I believe you are referring to flanged and free end corrections, not a flare correction. The should be already considered by the program.
Check out http://www.diysubwoofers.org/portcal.htm

The length adjustments related to flared ends have to do with the radius of the flare, so you are going to get little help with that 1/4" roundover.

[owdi]

Originally posted by BillFitzmaurice
Don't use the room as a duct, make the port the right length to begin with. Use an elbow in the box if you have to. Then you can use the distance to the wall for final tweeking of the bass response. Too short a duct limits what you can do.

Not possible without going to a smaller port. I already have the 4" port, and don't want to wait however long it would take for partsexpress.com to ship me a 3" port.

Well, the box is cut, so all that is left is to screw it together and test it. If things work out I'll make a diy website about this project Don't want to give any more details right now.... he he, I know I'm breaking some rules here.

Dan

[johninCR]

Why wait on Parts Express. Just go to the hardware store and get a piece of 4" PVC or make a wooden port which should look better protruding from the outside anyway.

[owdi]

Looks like it worked. I just took in-car measurement of my bandpass enclosure and putting the end of the port close to the back wall appears to have lowered the tuning from the worse case scenario of 105hz, to a much nicer 90hz. I got about 3" for free

The part of the enclosure with the port is 10" x 17" x 5.5". The back of the 4" diameter/4" long port is about 2" from the back wall, mounted in the exact center of the 10"x17" panel. My two 6.5" speakers are only about 2" away from the port... which is probably what is causing super-nasty resonance in the 700hz - 1200hz area (measured several 24db spikes). Damn... I was hoping to not need a crossover.

Dan

[BillFitzmaurice]

The cause of your resonant peaks are most likely 1/2 wavelength reflections. The 5.5" dimension is 1/2 wavelength at 1200 Hz, while 10 inches is 1/2 wavelength at 680 Hz. You need to have the entire interior of the enclosure lined with an inch of high density polyester batting to damp them out.

[owdi]

Originally posted by BillFitzmaurice
The cause of your resonant peaks are most likely 1/2 wavelength reflections. The 5.5" dimension is 1/2 wavelength at 1200 Hz, while 10 inches is 1/2 wavelength at 680 Hz. You need to have the entire interior of the enclosure lined with an inch of high density polyester batting to damp them out.

Hmm... I tried to place my speakers in such a way as to minimize these. There are two speakers in the ported chamber, each located at opposite corners. Also, I thought dips, not peaks, happen at 1/2 wavelengths, since the reflection is essentially 180 out of phase.

Dan