I would recommend that you download the FREE WinBox v.4.08 Excel Spread Sheet as well as the subsequent ubase628 Speaker Data Base File [although NONE of the three ubase files contain ANYTHING larger than 15-in]. Unzip into SAME Directory, start WINBOX and IMPORT the ubase628 file. You COULD also IMPORT the ubase19 file....but I do NOT recommend that you IMPORT the ubase410 file, since you would have to respond to hundreds of DUPLICATE menu boxes [NO, I don't KNOW if ubase410 is a subset of ubase628....but I suspect that it MIGHT be].
For other candidate Speakers, you would have to enter DRIVE UNIT PARAMETERS obtained from either the mfr's website or by searching for candidate Speaker Parameters.
WinBox will AUTOMATICALLY tell you whether a particular Speaker is best suited for CLOSED, VENTED [or BOTH???] types of Enclosures. In the CLOSED BOX Menu, a STANDARD Design will be displayed. To investigate Alternatives, enter User Data in the BLUE Entry Fields, info re. Damping Material and Leakage, as well as desired Qtc=0.707 [default, which will provide Critically Damped Transient Response, i.e. minimal Hang-time; or Reverberation]. Then Click on "Optimize Vb
for wanted Qt", so Spread Sheet will calculate the OPTIMUM Box Size that provides Qtc=0.707. Be sure to click on UPDATE; for Graphs and Frequency Response. Additional Performance Charts are provided in the CLOSED BOX Tab....as well. as in the COMPARE Tab.
In the VENTED Menu, a STANDARD Design will be displayed. To investigate Alternatives, enter User Data in the BLUE Entry Fields, incl info re. Damping Material and Leakage, as well as the desired amount of an Amplitude Hump you want just above the Rolloff Frequency [e.g. 1.0 up to say 3.0 dB]. Then Click on "Optimize Fb for wanted peak" [Fb is the BOX Tuning Frequency] so it will calculate the Box Design. Be sure to click on UPDATE for Graphs and Frequency Response. Additional Performance Charts are provided in the VENTED BOX Tab....as well. as in the COMPARE Tab.
I addressed PROS and CONS of CLOSED vs VENTED in an earlier post using a specific Peerless 12-in Woofer Example. CLOSED has advantages of fewer Rebound Transients, and typically can pump out MORE POWER well BELOW the Roll-Off Frequency due to better Xmax Control (Maximum Excursion of the Voice Coil)....so carefully look at the two Alternatives in the COMPARE Tab:
Speaker ticking noise
FYI: Those are NOT HUGE Boxes...hence the above recommendation to use SEPARATE Box for each 18-in Driver:
1 liter = 0.03532 cu ft
1 cu. ft. = 28.31 liters
PS: MULTIPLE Sub-Woofers should be in DIFFERENT locations so that they excite the various Room Modes DIFFERENTLY, which helps to smooth out the overall Low Frequency Frequency Response.