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