Re: Closing an open back cab?
Three things:
I would be concerned with heat build up if you closed that cab. Even with a fan, if the box is sealed, all you are doing is blowing hot air around. You need some sort of vent.
MDF is an acceptable cabinet for building enclosures. MDF is great because as a material it’s a little more lifeless than Baltic Birch. In other words, a speaker made of MDF relies more heavily on the voice of the speaker than a cabinet of Baltic Birch. Birch cabs do sometimes sound more “alive” because the birch simply adds more coloration to the tone. Both are fine in terms of strength. MDF is actually fairly dense and survives facial impacts well. It’s the end grain that you have to be cautious of. Because of its durability, stiffness, and rather sterile (by itself) sound, MDF is the industry standard in car audio enclosure construction. It’s simply great for playback. But for guitar cabinets, I’d build with Baltic Birch before I would build with MDF. I would never consider low grade plywoods, OSB, or particle board of any type.
Closing an open back guitar cab isn’t as big of a deal as people may make it out to be. In open back (free air/infinite baffle) applications, cabinets do not resonate as well as sealed enclosures. The sound is a little more open but does not have the fullness of a sealed enclosure. In sealed enclosures, as the volume of air within the cabinet decreases, the speaker’s power handling increases, the bass response drops, but the sound emitted has more “punch” especially in the lower frequency ranges. As the interior volume increases, the speaker’s power handling decreases, the bass response increases, but the speaker begins to lose the punchiness. BUT, given the design of guitar loudspeakers (they don’t move much air), their materials (which aren’t as stiff as woofers), and the volume of air we are talking about inside a guitar cabinet, much of what I just talked about doesn’t apply much to guitar cabinets. Simply stated, your average cabinet sealed guitar cabinet has so much volume per driver that the box’s properties approach a free air situation. The tonal variations you hear are more resonance inside the cab and a more directional sound.
Scott is right… there are programs that exist for the design of enclosures for audio playback, but they are typically more geared towards subwoofer enclosures, where the physical dimensions of an enclosure have a far greater impact on what you hear than guitar loudspeaker enclosures.
