Vergiliusma
Member
Subs rarely needs bracing. Frequencies are too low to be a potential problem.
It's not about what frequencies the sub is producing, but the fact that the internal pressure, the energy can excite panel resonances, so bracing is about breaking them up by shifting them up higher in frequency and damping their overall output.
Someone mentioned the Thor subwoofer. It's net internal volume is around 1.75 cubic ft. and is corner braced. As the enclosure gets larger and displacement goes up, bracing is just good practice to have your bases covered.
Even commercial manufacturers use bracing. It increases cost and shipping weight, so they wouldn't do it if it wasn't necessary.