PA subwoofer in a car

The mission is simple:
A friend of mine wants a car sub that could be also used on an a occasional outdoor event every now and then. He can fit up to 210L box in his car so I guess an efficient PA woofer would be ideal since he could get lots of SPL on like 300W amp (while using it as a car sub, for outdoor use he would probably use a different amp)
So far Beyma 18LEX1600Nd seems like a great woofer that can go really loud and quite low.
My questions are:
Is this a good idea?
Is that woofer okay or is there something that would work better for this use?
In simulation with 210L box with BR tuned to around 30hz I got -3dB at 32Hz and the membrane didn’t hit Xmax even at 1600W at 23hz which is absolute overkill anyway so I guess no need for second BR.

I haven't used the driver so this advice may be a bit limited..

When you said in your 1st post your friend had 210L available, was that for the whole box?
If so, you'd be best to model at ~70% of that for Vb, to account for the walls of the box, the port, driver, bracing, handles and a recessed baffle to allow a protective grille over the driver if it is going to be moved out of the car and used with an audience anywhere near it.

Tuning frequency is of course a trade off between extension and efficiency - the optimum frequency will be influenced by what type of program material will be played. If mostly mainstream pop/rock etc, a 30Hz tune may be lower than needed to reproduce most of that content, and thus gives up a bit of level at 40Hz and higher where it may be more useful.
If your friend is into genres that do have a lot of lower bass content, then ignore the above sentence ;)
Thank you for useful info! The 210L volume is with all the overhead (walls, parts, etc) subtracted, so it’s the actual usable volume inside the box.
About the tuning - it will be used mostly used for electronic music so I wanted to make it able to go down to 30hz inside the car and 35hz outside. Outside usage will be mostly on some small events with friends (up to like 20-30 people).
Two more questions: how does tuning affect the efficiency? I know that tuning higher gives a slight bump in higher bass response - is that what you meant?
How thick should the walls be? Would 30mm MDF be enough for such a big box? Or should we also put some structural supports?
Cool, so all good for the box size and tuning target.

Re the Efficiency, it's only a dB or 2, but if you really need to be as loud as possible it's worth thinking about.
Here's 3 versions of your project with different tunings - 30, 33 & 36Hz. Higher tunings give a bit more level not just at the exact tuning frequency, but also over an octave or so above it - which is quite a lot of the range in which we use subwoofers.

18LEX1600Nd FR.png

However, there's a little bit more to it.
Here's the excursion plots for these 3 tunings, at the same drive level:

18LEX1600Nd Excursion.png

As we tune higher, the excursion above the tuning frequency reduces. That means we could drive the higher tuned versions harder than the low tuned ones. Of course, that does require using a High Pass Filter just below the tuning frequency to prevent the uncontrolled excursion down there from damaging the driver (& causing ridiculous amounts of distortion).

So, if we increase the drive level so that each one is at 14mm excursion, the maximum SPL becomes this:

18LEX1600Nd SPL.png

Re the box construction, 30mm MDF is going to be very heavy.
The normal construction for PA subs is 18mm Baltic Birch plywood. That is both lighter and more rigid than MDF, and is usually made with waterproof adhesives so is better at surviving any rain when outside. You'll probably paint the cab to help with that too, but MDF does tend to fall apart if it absorbs much moisture.
Even using BB ply, you'll definitely want bracing inside. There will be lots of threads on subwoofer bracing here, but the main goals are 1: to split large panels into smaller ones so that any resonance they have is pushed up out of the frequency range of the sub and 2: tie together opposite sides of the box so the internal pressure does not cause it to "balloon".


EDIT PS - just realised that I've got an older version of the specs for the driver in WinISD, in case you were wondering why my curves might be a little different from yours. The relative differences between tunings will be similar though.
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FWIW, if you're looking to build a box that will both work in-car and as a PA subwoofer, pay special attention to impedance. Car audio amps are usually rated for 4 ohm and lower loads, so I'd suggest looking at solutions that present that type of load to the car audio amp.

1. a dual driver box with two 4 ohm PA drivers can be wired as a 1 x 2 ohm load for car audio use, or 1 x 8 or 2 x 4 ohm load for PA amps.
2. a dual-driver box with two 8 ohm drivers can be wired as a 1 x 4 ohm load for car audio use or 2 x 8 ohm load for PA amps

A 300W amp (rated into 4 ohms) can swing about 35V into that 4 ohm load - very far from 87.5 V
Re tuning - thanks for explaining, I guess I’ll go with a single adjustable 160mm port so I can adjust the tune if needed.
Re impedance - I’m well aware about 8R being problematic in a car for exactly those reasons mentioned but I had a hard time searching for a speaker that would be 4R and be as good or better than this one for the application. But I’m not locked on buying this one so if you know about a better one under 500€, any suggestions would be appreciated.
Re power - Most of its life the sub will be in a car with under 500W of power and on the very few outside events I doubt we’ll ever go over 1KW so those were the numbers I used during simulations.
Re material - Thank you big time for explaining this, definitely going to switch to bb plywood and brace it inside.
Re tuning - thanks for explaining, I guess I’ll go with a single adjustable 160mm port so I can adjust the tune if needed.
You should also be able to achieve this variable tuning with a trifurcated shelf vent. Just block up one of the sections of the vent to tune the box lower, and this can be done with a simple panel than can be unfolded or slid into place.
Re tuning - is there any disadvantage when tuning with variable length plastic port?
Re speaker - 18SW100 doesn’t seem to simulate all that nicely in a 210L box, which makes sense when looking at its VAS parameter, maybe I’m doing something wrong (so all corrections/explanations are appreciated) but doesn’t that mean that I’d have to put it into a smaller box and lose a bunch of efficiency because of it?
What car do you use?!
Cars have "room gain" cause of their small volume, often starting at pretty high frequencies (50-60Hz). So you would get linear SPL dow to 20Hz when you put a CLOSED box in your car tuned around this frequency!
For PA use I would for sure use a portet enclosure - you need that gain outside. With your available volume, to fit in a car and to get maximum output I would think about 2 15" drivers, there are plenty which work in relatively small volumes.

So I would build a reflex housing where I can close the ports easily. Tailored for your car size.
The tuning port will be adjustable without disassembling anything. I will also measure the actual frequency and impedance responses during tuning to make two “presets” for the port length to get best tune both in and out of the car.
I think that (other than maybe the driver?) I’ve got it (hopefully) finally figured out. Thanks for all the input, y’all were very helpful and I appreciate it a lot :)
I guess I’ll slowly start putting the final parts list together for the friend to order.

Of course I’m still open for suggestions or ideas on how to improve the project.
They only would have perfect 12dB slope when being stiff and sealed - most cars aren't. (Cars for competition get stiffed and sealed btw)
Closed subs are way easier to EQ as reflex and you don't have to care about lowest frequencies as much. But of course you will have rsonances with both of them, after all it's always the same "room" ;-)