Its worth remembering that guitar rigs are set up with different goals than home stereo systems, but the same techniques apply.

Open-back cabinets are designed to create a path from the back of the speaker to the front face of the cabinet that is just the right length to cause the soundwaves from the front and the back to add to each other. This is usually more effective at lower frequencies, but there is sometimes noticeable effect at higher frequencies as well.

Its a bit of a crap-shoot because the wavelength gets shorter as the frequency increases. This means that the same path-length that caused the two wavefronts to add up at one frequency, may cause some cancellation at another frequency.

Closed-back cabinets do not make direct use of the sound waves coming off the back of the speaker cone. This means that at some frequencies, they're less efficient than open-back cabs. It also means they are less likely to suffer from cancellation effects.

Another difference is the way the two cabinets 'feel' to a speaker.

An open back cab applies very little 'load' on a speaker, because there is no restriction on the movement of the cone. This translates into a 'loose' sound.

A closed cabinet loads or restrains the movement of the speaker cone because of the increased air pressure in the cab as the cone moves in, and the reduction of cabinet air pressure as the cone moves out. This tends to dampen cone movement, resulting in a 'tight' or 'punchy' sound. The closed-back cab can be tuned to either enhance or reduce these effects.

Adding a bass-reflex port is one way of tuning the cab. The port may be simply a hole cut in the front baffle of the cab, or it may be a tube. Its another way of providing a path for the soundwaves from the rear of the speaker to add to the front soundwaves, while retaining some of the loading effects of the closed-back cab. It tends to result in more sharply defined frequency response characteristics, and sometimes that's just what the designer wants.

Okay, so what does this have to do with your question? You now have just enough knowledge to give yourself permission to get in there and get your hands dirty.

Start by cutting some 3/4" plywood to the right size to seal up the back of your cab. You can install it by screwing some 1x1's around the inside of the cab opening, giving you something to screw the new back to. [u]Don't do anything that you cannot undo later!![/u] You may not like the results, and you don't want to trash an expensive piece of gear. Use plenty of screws, no more than 3 or 4 inches apart. The cab needs to be airtight around the seams.

Now, you get to play! Start out [u]gently!![/u] You will have changed the loading on the speakers dramatically, and they may not be happy about it.

If it sounds too 'tight', you might try sticking some fibreglass insulation to the insides of the cab. Long drywall screws, stuck through pieces of cardboard a few inches square, work well for this.

Be [u]very[/u] carefull with the fibreglass... gloves, goggles, and heavy, full-coverage clothing is important. Getting glass fibres in your skin itches like mad for a long time. Getting glass fibres in your eyes can blind you.

If the sound is still too 'loose', you will have to reduce the cabinet volume. Fitting pieces of plywood at 45 degrees across each corner of the inside of the cab, from front to back, is a good approach. Go overboard at first. Reduce the cab volume as much as possible. Then, if it sounds like you've gone too far, you can trim the pieces a bit smaller, and re-install them.

Experiment with it. This is a way of finding your sound. If you decide that the original setup sounded better than any of your versions, you can remove all of your mods, and there's no harm done. If you hit on a magic combination that you really like, you can tidy things up, glue in the 1x1's, put some Tolex on your new cabinet back panel, and rock on!

[Edited by Lordathestrings on 10-11-2001 at 10:02 PM]