The driver of choice this time around was the B&C 18TBX100-8, a very popular 18" driver that's used by quite a few locak DJs, either in DIY boxes or as replacement drivers in commercial pro audio equipment. The Parts Express website suggests using these in a 104 litre (net) cabinet that results in an F3 of 50 Hz, but that seems a bit high for a subwoofer using an 18" driver. B&C's own notes suggest an "extended bass shelf" alignment of 200 litre (net) turned to 34 Hz that extends the response down to 34 Hz, which is a good step in the right direction. However pass-band efficiency clocks in around 96dB/1W/1M or less, which is slightly low for a pro audio subwoofer, and there's also the question of how much port compression this design is going to suffer from at high power levels. So, after a week or two of fiddling around with HornResp and considering a few designs, and updating my horn design spreadsheet with a few extra tweaks, I once again settled on a tapped-horn alignment, this one with a net volume of 405 liters and a predicted Fb of 33 Hz and a predicted efficiency of 100+ dB/1W/1M in its pass-band. Yes, it's a large box, but smaller than most horns based on 18" drivers, and remember this is going in a club, not in the back of a car, so there's quite a bit of space available.
To assist with coming up with the best dimensions for this design, I modified the spreadsheet I used to help with the design of POC#3 and including a few new features, including the ability to incorporate "cone compensation" into the design. What "cone compensation" is is basically reducing the cross-sectional area of the tapped-horn in the vicinity of the driver's cone to compensate for the volume of air contained within the cone itself. While some have said that this wouldn't make that much difference in the output of the tapped-horn, including "cone compensation" in this particular type of fold also results in an increase in the horn's path length, which in turn drops the tapped-horn's Fb. Lower Fb in the same box size - that's usually a good thing! This particular design includes 5000 cm^3 of "cone compensation" at the S2 point in the tapped-horn layout.
Anyway, here are the HornResp parameters for the design:
The predicted performance is as follows:
The required panel dimensions as predicted by my spreadsheet are as follows:
Not included in the above list are the panels required to brace the enclosure properly. And this big baby definitely needs to be braced properly!
The required layout is as follows: