b&c 21sw low tuned BR

Hi,

I wouldlike to build  low tuned BR box for one of the B&C 21 drivers.
Since I have little knowledge of box design, I better ask for some help.

21SW152 or 21SW115 would be better for low freq? (as LOW as possible)
Would use 4 ohm amp and a single drive cab.
I like the design in the linked video, looks solid, and might have pretty small vibrtions on the sides.
I know, must be heavy, but better to have a heavy and good one than a light and bad one.

Can anyone help me out with some info/plan(s), suggestions etc.?

https://www.youtube.com/watch?v=54Qh4X3IhKk

For home cinema?

I would use it for PA, but I love Detroit electro and other sub-bass heavy materials.

I know about SB-15121.

Any other suggestion? I would better DIY.

There's a user on here called crashpc I think, and he's crashpccz on YouTube who has done a good bit of testing 21s in small reflex cabs

+1 to checking exactly how low you need to get first.

Whole Body Vibration is a field of intense study - the benefits and issues caused by it are still under dispute, but there is a great paper that explored the thresholds and frequencies for inducing it in human listeners with subwoofers:
https://www.aes.org/e-lib/online/browse.cfm?elib=20383

The whole paper is worth a read, but you can se from the abstract that you 'only' need a linear sound pressure level of 107 dBZ at the listener from subwoofers to induce WBV. Also, the resonance of different body parts was assessed across all test subjects:


Note the x-axis scale is in 25 Hz divisions. The results clearly show that the majority of WBV is induced by frequencies between 60 Hz and 125 Hz, for all body parts. The peaks are resonances and sit at harmonics of the ~60 Hz initial peak.

If 'maximum perceived impact' is the goal, then you'd be better off focusing on designs that really hammer home the upper bass region.

Of course, I'm known for deploying systems with extended LF response. Part of that is based on an agreement I have with my good friend Jo Burton's research which indicates that people perceive a system that plays an octave lower as being louder and of better quality than one which 'only' reaches 30 Hz:
https://www.aes.org/e-lib/browse.cfm?elib=19179

However, I'm not going to lie to anyone and say that going for this is easy. The rule of thumb is that you need twice as much displacement and energy to achieve the same sound pressure level with an extra octave extension. The boxes need to be bigger - much bigger - and the diaphragms must be very controlled to prevent thermal runaway or cone damage from rocking modes.

The reality is that several well designed 18" reflex boxes with those modern drivers will likely have nearly as much 20 Hz output as a pair of 21" tuned super low, but with way more output in the more useful upper bass region. It's also more flexible as you can bring the number of boxes needed for the show.

If you feel that you want or need a dedicated infra box later, then look at adding a set of dual 21". But be warned, you'll spend a lot of cash on drivers, wood, amps and transport to keep up at concert levels... as an example, we have some custom dual 21" tapped horns that I designed for just this purpose. Similar to the TH221 from Danley, but DIY because it was hard to justify the equivalent cost of twelve more TH118XL just to hit these lower frequencies without proof that it won more business.

Four of them keeps up with twelve TH118XL playing everything 30 Hz up, which is impressive - except for the fact that once you put those four boxes in a van, there's no space or weight limit left for anything else. They're 250 kg each.

I've also worked with a 40" Powersoft M-Force box from Traction Sound as part of my time with the AUDINT sound art crew. That's a reflex box tuned to 16 Hz, the size of a small car, and it's loaded with cutting edge linear motor technology costing many thousands of £££. It's great, but it's not enough for a concert alone. Far from it. My tapped horns are much more efficient than a ported cabinet at those frequencies, so you'd need the same or more physical volume to achieve the same.



Stick with what you have, and design a big dual 18" reflex with an oversized, flared port in the middle. There's plenty of examples out there. Use a proper modern DSP amp with excursion, thermal and peak limiters to drive it to the fullest. Then revisit later.