port size v.s. number? help please!

[shpongley]

So basically Im trying to design a nice light, small 1x10 with the eminence bp102 around 18x14x13 outside dimensions. Im trying to fit in ports and wondering if more smaller ports is a bad idea? i know you want the largest diameter possible to limit noise, but (without talking about things i dont know) if tuning and all else remained constant, would say, 4x 2" ports of the correct length work vs. 1x 4" or 6"?

[DukeLeJeune]

In general to compare the airflow capability of ports, you look at the cross-sectional area; for example, two 4" diameter ports are the equivalent of a single 5.65" port.

However there is a "cheat" out there that enables you to get away with a smaller diameter (and therefore shorter, and therefore easier-to-fit-in-a-box) port than you otherwise could have, but it costs a few bucks more. The cheat is to use ports with flared ends, and the ones I use are made by Precision Sound Products and sold by Madisound and Parts Express and probably a few others. A single 4" diameter Precision Port would easily be sufficient for a single BP102. You need to take the flare into account when calculating the port length, fortunately the company has a good online calculator:

http://tess2.uspto.gov/bin/gate.exe?...009:j0m6rp.1.1

Actually I think even a single 3" Precision Port would work. I use two 3" Precision Ports on a woofer that moves more than twice as much air as the BP-102, and do so with the blessing of the Eminence engineer who designed the woofer. If you find that a 4" port won't fit into your box and give the desired tuning frequency, consider going with a 3" Precision Port. Don't glue the sections together; instead, use a few wraps of electrical tape or duct tape to join the sections together. This will give you the option of later changing the port length if you want to.

Best of luck to you.

[shpongley]

Thanks for the thorough answer! I actually saw the flared ports on parts-express and was thinking about a 4", but if I could get away with a 3" might as well try.

[DukeLeJeune]

You're welcome, shpongley. Parts Express has their own "house brand" flared ports for a few bucks less, but the Precision Ports are imho worth the difference because their flare is larger & more effective, which better serves the whole point of flaring (namely, to stave off the onset of turbulent airflow - chuffing - at the port ends).

[billfitzmaurice]

Originally Posted by DukeLeJeune

the whole point of flaring (namely, to stave off the onset of turbulent airflow - chuffing - at the port ends).

True, but flared ports do complicate the duct size calculation. It's no concern if you have the gear to measure the cab impedance to determine the box tuning, but can complicate matters if you don't. IME a single 4" duct is OK for BP102. Or two 3", or four 2". With longer throw drivers multiple smaller ports placed around the cab may have the advantage of more uniform pressure distribution inside the cab.