acoustical stuffing / sound dampening subwoofer cabinet REL BRITANNIA B1 subwoofer

Having researched a lot about subs before I built mine, I found that acoustic stuffing in a subwoofer is not as critical as you might think. The cabinet may have been left empty by design of REL. Adding acoustic stuffing can change/reduce the volume inside the cabinet and affect the subwoofer frequency response (if you add too much).

If you want to improve things on your subwoofer (and you’re not afraid to get your hands dirty), I would apply as Dynamat Xtreme to as much of the inner walls as possible. In a subwoofer like this, cabinet resonance is more critical than internal standing waves. You can see that it doesn’t really have much internal bracing to resist flex/vibration, so reducing the resonance of the cabinet walls will be an improvement.

If you really want to add stuffing, then first apply the Dynamat. Then use spray glue to coat the inner walls one at a time. Then immediate apply a 1-2" layer of acoustic stuffing to the wall you are working on. A good product is Acousta-Stuf Polyfill from Parts Express. It is not fiberglass, so you can handle it with bare hands. It is easy to pull apart for any thickness. The spray glue is available from any hardware store or Home Depot. I have used products such as 3M Super 77 or Loktite spray adhesive.

I do not have any acoustic stuffing in my vented subs and they sound great.  Instead, I built very extensive bracing internally, so that there is not one section of wall greater than 8" that is not braced.

There could be several subwoofer drivers that may work well in this cabinet.  The most important T/S parameter to look at is VAS.  I would avoid car subwoofers because they are designed to operate in a smaller box than normal subs.  Your box is 4+ cubic feet (or about 120-130 liters).  So look for a subwoofer that has a VAS larger than 4 cu.ft or 130 liters.

The other part is the port tuning.  It's a 4.75" port and not that long.  Depending on the subwoofer, it could cause a slight peak just before the frequency response rolls off (probably in the 25-35hz area).  It really depends on how the driver rolls off the lower frequencies and how the port comes in to compensate for those rolled-off frequencies.  It may be difficult to get something exactly, but you could get a very good performer.  Madisound has Morel and Scan-Speak subs that are nice.  I wouldn't worry about trying to get the super expensive massive subs, like RE Audio or Stereo Integrity.  Those are for the nutsos.  JL Audio makes a very nice product, but watch for the VAS being too small.

Take out your current subwoofer and make sure of the exact diameter of the hole.  Subwoofer sizes can vary slightly, even though they are marketed at 12".

Madisound has dimension drawing on all drivers.  You could also check out Parts Express.

Depending on cabinet volume and tuning frequency, some drivers may want a port that is 25-35" long (based on 4-3/4 diameter).  That's what I really mean.  A shorter port means the tuning will be at a higher frequency, which could mean a bump/increase at something like 35hz where the port and the driver are working together.  This may not be really bad at all.  It's up to you, but the goal would normally be to engineer a flat response with a controlled roll-of.

Do you have a copy of Bass Box Pro?

https://www.parts-express.com/bassbox-6-pro-software-cd-rom--500-923

It might be good just to buy at $129.  Then try to measure/calculate the actual interior volume of your cabinet.  Then select a driver at random (say ScanSpeak 30W/4558T) and then play with the Fb port tuning frequency until you get a 4.75" port close to the 17-18" length.  Then do a frequency response graph to see how it looks.  I did a few, and some may have a very slight bump at about 40hz.

I haven't used other software, but if you are serious about building your own subwoofers, Bass Box Pro is really the best.  You don't have to buy anything.  It's up to you if you are really picky about how a sub performs.

You could just buy a SanSpeak or Morel subwoofer.  Then do dynamat damping on the interior walls, and then spray-glue some Acousta-Stuf Polyfill on the inside walls.  Like timlub said, the stuffing will damping some of the peak that happens right before frequency roll-off.  This might help work with some of these other subwoofer drivers.

If you're still looking for possible drivers, the Rythmik Audio appears to be very nice:

http://www.rythmikaudio.com/DS1200driver.html

The specs say the DS1200 works fine in a 3-4 square foot vented cabinet.  I don't know the pricing, but you can contact them.  They are in Texas.

You could look at the Scanspeak 30W/4558:

https://www.madisoundspeakerstore.com/approx-12-subwoofers/scanspeak-discovery-30w/4558t-12-subwoofer/

It has very good XMAX.  It's an aluminum cone and the moving mass is less (less weight).  It has a lower resonance frequency than the AD12-300 (17hz vs. 24.6hz of the AD12).  It has a higher VAS than the AD12.  It is more expensive, but it's a very nice sub.

Couple other options:

https://www.parts-express.com/peerless-by-tymphany-835017-12-aluminum-cone-xxls-subwoofer--264-1114

https://www.parts-express.com/eminence-lab-12c-high-power-12-subwoofer-speaker-driver-4-ohm--290-668

Neither one has as good of XMAX as Scanspeak, but the Eminence has a higher power rating.  I would not get too attached to a high power rating.  I have some SonicCraft designed 12" woofers that are supposed to be 150 watts RMS, but I have driven them very loud with no problem.  You just don't want constant high power bass.  For home theater, this is just fine as loud bass is infrequent.

Faital Pro makes some good stuff.  However, this woofer would not be a good choice for your REL box.  For one thing, the resonance frequency is very high at 45hz - making this a general woofer instead of a subwoofer.  Subwoofer drivers typically have a resonance frequency between 17hz and 25hz.  The second thing is the VAS on the Faital is only 1.06 cubic feet.  This woofer would not perform well at all in your 3.5 cubic foot REL B1 box.  I modeled the Faital in BassBox Pro and it really wants a 0.7 cubic foot box for vented, and it still would not give you deep bass like a normal subwoofer.  The Faital stuff is designed to have very high efficiency for pro audio sound - to be very loud for live sound concerts.

Both the Scanspeak and the Peerless would be highly superior choices.  I don't know about the Eminence, but I suspect it would be fine  The eminence is not in the BassBox library and I didn't have time to enter in all the T/S parameters.