Port Length Question

[MonkeyMike]

Long time reader... first time poster...

Lets assume I calculated a port length of 30 inches (just a nice round number for discussion purposes). And in my port length calculations, I used an end correction factor for a port with two flared ends.

When I go and actually build this port, do I make the straight length 30 inches and add the flares onto the ends? Or do I consider some portion of the port radius as part of the straight length measurement?

The only resource I could locate was the following:

http://www.subwoofer-builder.com/port-flares.htm

On that page, the author states that 1/2 of the port radius should be considered in the measurement of the port straight length.

~mike

[Dennis H]

On that page, the author states that 1/2 of the port radius should be considered in the measurement of the port straight length.

Yeah, that sounds about right as a rule of thumb. If you're using Precision Ports, their online calculator says to make the whole assembly 1" longer than the straight pipe calc. Example:

20" straight pipe

or

19" straight
flares on each end with 1" radius
effective length 19 + 1/2 + 1/2 = 20"
total assembly length 21"

http://www.psp-inc.com/

[Eric S]

30" is a fairly long port. I remember coming across a note a while ago that indicated you should try to keep the port length under 5x to 6x the port diameter. Otherwise, you run the risk of creating port resonance that you don't want.

[MonkeyMike]

30" is a fairly long port. I remember coming across a note a while ago that indicated you should try to keep the port length under 5x to 6x the port diameter. Otherwise, you run the risk of creating port resonance that you don't want.

for the rl-p15 subs that everyone seems to be building (myself included), a 31" port is the calculated length for a 6" port (265 liter effective volume, 15 hz tune, with both port ends flared).

~mike

[Eric S]

Sounds like you're in good shape, Mike! Just wanted to point out some limits...

Eric

[JonP]

Heh... I was going to say that's long... but a 6" is a pretty large port!

And, that's right there at 5X the diameter...

IIRC, the "organ pipe resonance" has to do with the length, and not so much with the diameter... though it and the box volume probably have a small influence. There are calculators (Unibox for example) that will calculate the resonance for you. If it's too low, and your crossover is too high... you might get some noise problems...

I tried putting a nearly 30" port in a 3 cu box (4 4" elbows, not much pipe between them) it was close to 200Hz resonant, and I think it made the otherwise nice results kind of mushy and boomy. All those elbows probably didn't help with basic port turbulence noise, either. Converted it back to sealed.

Sub was a RS 12", more for music not HT, so higher cross, more picky listening, etc... In a bigger, lower tuned and crossed monster, the effects would be less noticable.