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In terms of physics, all mechanical systems have a natural resonance frequency. That is a frequency where the system will maximally absorb energy, when stimulated with that given frequency. If you look a simple system, like a transducer, it is reflected by the resonance peak you see in the response. It is easier to see if you look at the impedance curve, which shows a rising impedance at the resonance.
Panels in subwoofers are the same. There are multiple resonant frequencies, for the various parts of the enclosure based upon the physical properties of the panels. Those properties are length, mass, and stiffness of the panel. Maybe an easier analogy is that of a guitar string. The frequency a guitar string will play (its natural resonance frequency) is based upon the length of the string, the tension (stiffness), and its mass. A lighter, shorter or tighter string all translate to a higher natural frequency. All you have to do to convince yourself is play a guitar string, and tighten it, play the thinner (lower mass) strings and compare them to the thicker (higher mass) strings. If you play a string and put your finger down on part of the fret (making it shorter), you get a higher note.
All of this, directly translates to panels used in loudspeakers and subwoofers. You use braces, to shorten the panel, and raise the natural resonance of that panel. Why raise it? Because you don't want the device that you are designing, to play at a frequency that will stimulate the natural frequency of the materials that make it up. That has the effect of storing energy, and releasing it in ways that do not coincide with accuracy. So we add braces to INCREASE the resonant frequency of a given panel. What other things can we do to achieve the same thing? We can use lower mass, which is counter to what most people are thinking about when building a sub but lower mass materials, will have a higher natural resonance. We can also use stiffer panels, by choosing a different materials that is stiffer than MDF.
The one factor I've left out is damping. Damping is anything that reduces the amplitude of the oscillations in a system. It is the ability to stop the oscillations quicker. A mechanical system that most people understand and can relate to is the shock absorbers in your car. The system has a spring, and a shock absorber that moves a piston through a cylinder filled with oil to create a frictional loss. This system damps the spring oscillation. Take the shocks off a car, and it will bounce happily down the road in and uncontrolled manner. The damping nature of the shocks create a frictional loss, that damps the oscillation. That is the same thing that happens when you add damping to any system, acoustical or mechanical.
MDF has good damping properties, even though it has high mass. Adding thicker panels, actually lowers the resonance frequency, but it increases the damping. Using braces, to effectively shorten the panel raises the resonance frequency.
Make sense?
