18" Sub Port Size Question

Archived from groups: rec.audio.tech (More info?)

"Joel Ramsey" <joelr@wcgusa.com> wrote in message
news:65afd655.0409200944.88e5af6@posting.google.com...
> I am buiding an 18" sub to add to my PA system for live music. I
> purchased the driver from an online vendor. Parts Express sells the
> same driver and has a recomended design produced by anonymous
> software. I'm assuming that the design is adequate.
>
> My question is regarding the port size. The suggested port diameter
> is 4.5" @ 2.7" long. The closest manufactured port I can find is 4"
> dia by 4" long (I can cut it to length). Standard PVC pipe steps up
> from 4" to 6".
>
> Would a 4" dia tube work OK or would it seriously affect performance?

2 x 4 inch ports should be OK for a single 18" driver.

> If I can use the 4" dia tube, would I need to shorten the length?

You would need to make them about an inch shorter. Just off the top of my
head.

Stephen

Archived from groups: rec.audio.tech (More info?)

joelr@wcgusa.com (Joel Ramsey) wrote in message news:<65afd655.0409200944.88e5af6@posting.google.com>...
> I am buiding an 18" sub to add to my PA system for live music. I
> purchased the driver from an online vendor. Parts Express sells the
> same driver and has a recomended design produced by anonymous
> software. I'm assuming that the design is adequate.
>
> My question is regarding the port size. The suggested port diameter
> is 4.5" @ 2.7" long. The closest manufactured port I can find is 4"
> dia by 4" long (I can cut it to length). Standard PVC pipe steps up
> from 4" to 6".
>
> Would a 4" dia tube work OK or would it seriously affect performance?

A 4" tube would work just fine. The difference in effective area
is only 20%.

> If I can use the 4" dia tube, would I need to shorten the length?

Yes, you'd have to shorten it by roughly 20%

> Should I use a 6" diameter tube and alter the length?

If you did, you'd have to make it over twice as long. If
you have the room for that, go for it.

Archived from groups: rec.audio.tech (More info?)

Parts Express sells tapered tubes as well.

"Joel Ramsey" <joelr@wcgusa.com> wrote in message
news:65afd655.0409200944.88e5af6@posting.google.com...
> I am buiding an 18" sub to add to my PA system for live music. I
> purchased the driver from an online vendor. Parts Express sells the
> same driver and has a recomended design produced by anonymous
> software. I'm assuming that the design is adequate.
>
> My question is regarding the port size. The suggested port diameter
> is 4.5" @ 2.7" long. The closest manufactured port I can find is 4"
> dia by 4" long (I can cut it to length). Standard PVC pipe steps up
> from 4" to 6".
>
> Would a 4" dia tube work OK or would it seriously affect performance?
>
> If I can use the 4" dia tube, would I need to shorten the length?
>
> Should I use a 6" diameter tube and alter the length?
>
> Am I concerned about something I shouldn't be concerned about?
>
> I've included some of the design and speaker parameters below. I
> would appreciated any insights.
>
>
>
> ---Box---
> Vb = 9.029 cu.ft
> V(total) = 9.141 cu.ft
> Fb = 35 Hz
> F3 = 36.34 Hz
> Fill = minimal
>
> --Vents--
> No. of Vents = 2
> Vent shape = round
> Vent ends = one flush
> Dv = 4.5 in (port diameter)
> Lv = 2.71 in (port length)
>
> --Driver Parameters--
> Fs = 27.4 Hz
> Qms = 5.59
> Vas = 14.9 cu.ft
> Xmax = 5.5 mm
> Qes = 0.3
> Re = 5.55 ohms
> Le = 1.57 mH
> Z = 8 ohms
> Pe = 600 watts