18" DIY cab

18s that go up to 5k?! I take it they aren't very sensitive at that kinda frequency?

 

If it does go to 5kHz it's with an unusably narrow dispersion angle. In any event I wouldn't use an 18 higher than 300 Hz. The specs aren't bad, but with no xmax and xlim there's no way to predict the displacement limited power.
If you're the least bit serious about doing your own cabs learn to use WinISD Alpha pro. They do have a forum as well for help.

 

Those drivers require a lot of volume. They look pretty good in about 12 cu ft tuned to 30 Hz, which is, IMO, too large, especially since you are looking at two of them. There are other 18"s that are more suited to smaller volumes. A 21" driver I'm looking at doesn't need as much volume as these 18"s.

 

What kind of bass/basses do you play? If you have multi string sub basses they'll come in handy but if you play a 4 string it'll be hard to get good clarity out of them. For sub woofers for a PA they'll work good.

 

That driver would be reasonably capable as a bass speaker in 120 liters with a 40 Hz port. The main benefit of a bigger box would be a smaller hump, if that is an issue. If the box has to be big, consider leaving some space on the baffle for a 10" midrange.

Assuming you find out what the Xmax rating is.

 

If you really want an 18, get a different driver. My RCF will do a nice flat 40Hz tune in 90L and has a 8mm Xmax.

fdeck and Bill's advice to cross it to something smaller should be followed. I'm using a 15 coax, mainly because it's incredibly efficient and I already own it, otherwise I'd use a 10NDA610 or JBL 2012 10" from 300 up.

 

I plugged the TS's into my spreadsheet, making a guess as to the inductance. (Ask them for it when you send the second mail). My technique may be non-mainstream, but I simply assume a 40 Hz port, and dial the box volume up and down until I see sufficient low end response and a tolerable amount of hump. If the hump is too tall, I consider the driver to be unsuitable. At 120 liters and 40 Hz tuning, the hump appears to be a couple dB.

I use this as a way to "triage" drivers for bass cab use. Of course a driver that I find to be suitable still has to go through other analysis, careful thought, and perhaps asking some pesky questions here on TB to make sure I am not doing anything dumb.

 

Get Unibox if you have Excel. Makes your triage automatic.
Also has an optional driver database, but beware, some of them are wrong, eg almost all the JBL's so check first.

 

True. My program has no inherent advantage over Unibox, except that my code is open, and I can add custom analyses such as my experimental waveform-based excursion and max-power graphs.

 

That's right. WinISD follows rules of thumb that are probably appropriate for designing hi-fi speakers around available drivers, but our needs are different. For a bass speaker dedicated to 4-string use, the port tuning simply has to be around 40 to 45 Hz. I discuss my technique in the little 12-inch design article at my web page.

 

What you are not seeing is relationship between the upper and lower impedance peaks.

You can tune any box to any frequency, but this does not make it the optimal tuning. For example, load a Delta 12LFa into WinISD. Flat is 3.973 cubic feet tuned at 43.6 Hz. Open a 2nd window with the same driver, but cut the box volume by 50% to 1.98 cubic feet, same tuning.

The small box appears to have a big rolloff. What is not seen (and not predicted by WinISD) is the big hump at the upper resonance point. This is what makes a boom box.

The opposite condition is Extended Bass Shelf (EBS). The box is 150~200% larger than optimum. This increases the magnitude of the lower impedance peak and extends response at the cost of overall lower output.

If you want to get the most output from a given box size, you can do it physically. Use a real time analyzer with a series of round port tubes. Substitute each port tube in your box, and take a sweep measurement. The result with the most meat down to your required low frequency is the winner.

 

The 12LF was just an example about changing tuning. I own a few of these, and am familiar with their performance.

I'm not really a fan of 18" drivers. IMO, when you get that large cabinet size, you are much better off with a horn. 12 to 13 cubic feet is a whole lot of cabinet for a single driver. I did build a Magnum 18LF for Chelsea Baker (www.aroarah.com) which she still uses under her D410-XLT. It fits her needs for minimum trailer space and a lift-up stand for her D410-XLT.

The Definimax is the replacement model, and performance is similar to the Magnum. Both are pricey drivers and very high quality. My measurements show the Eminence 18 cannot beat a 20" wide T39 bass horn with one BP102 10" driver. The 18 will take a lot more power than the BP102, so this might even it out somewhat.

 

I always include the proviso that an excessive hump is a reason for ruling out a particular driver / box combination. But your points are well taken.

 

What isn't visible in WinISD is the boominess of the hump. The more the box volume is reduced from optimal, the larger/nastier the upper resonance boom.

Think of Mr. Limpet and his thrum. Nasty and annoying resonance. One-note-wonder kinda stuff.