Originally Posted by
Shinobiwan 
First off thanks for the kind words.
To answer your question, yes extensive bracing is immediately noticeable. Its not always a large difference depending on what your comparing it to but it is always noticeable over a lesser braced boxed.
The most apparent effect is that the sound moves away from the speakers and associated boxes. This is natural because a greater percentage of the sound energy being radiated into the room is coming from the drive units. The cabinets have less sonic signature, don't add anything objectionable and don't radiate sound into the room alongside the drivers.
One of the reasons why small bookshelves sound great and do the imaging and soundstage thing well is because their cabinet are less resonant than an averagely built floorstander.
Whilst constructing, my rule of thumb is sub-divide the space within an enclosure as if your building a small standmount. So I normally work with a brace-to-brace spacing of no more than 20cm and normally 15cm. If you combine this with a matrix bracing format as well as laminated MDF wall construction of 9mm + 18mm MDF then you end up with something that overcomes many of the resonance issues associated with enclosures. You can go further with this too by adding mass loading to the internal walls.
To illustrate what your dealing with here are some accelerometer tests taken of MDF in various states of bracing and treatment. BTW an accelerometer is a small measuring device that attaches to a surface and measures seismic vibration within the material.
Here is 18mm MDF unbraced:
You can clearly see the rather hefty broad Q resonance at 190hz and the narrower Q of the 600hz one, both of these really extend in time smearing the original signal wreaking havoc on midrange clarity and giving a cupped and nasal character to vocals whilst slowing upper bass making it sound boxy and compressed. The good news is that sub builders have a nice material as there's not a whole lot going on down low.
Now take a look at 18mm MDF with 15cm brace-to-brace spacing:
Notice how the resonances have shifted up in frequency by over an octave or double the frequency. Not only that but the initial level the resonance and the following decay has improved too. This is all natural since your stiffening the whole box structure with the bracing. The trick here is to raise or lower those resonances out of the operating range of the driver that sits in the box. For my project show here these need to moved out past 1.5Khz where the midrange crossed to the tweeter and/or lowered significantly.
Finally take a look at 18mm MDF with the bracing shown above but also with mass loading added by way of using heavy lead sheet attached to all the internal walls. This is what I use in my midrange enclosure:
A remarkable improvement over just 18mm MDF and doesn't get much better than this unless you start looking into using concrete, multi material laminations or composites:
So its clear that for the most important mid frequency range you should take things a step further and build up a multilayer material approach such as I did for this project with 9+18mm MDF, 2mm lead, 2mm bitumen, 10mm acoustic foam. The subjective and objective differences these details make over an untreated and poorly braced cabinet are pretty massive with the latter sounding very much like a loudspeaker reproducing music and the former being much more natural and having realistic timbre and tonality that can fool you into thinking your listing to the real thing at times. Basically every facet of the sound improves to some degree when the box construction is right, there's not such thing as overbuilt here.
Hope this helps.
