Speaker cab question

[I'm a beginner!]

Hello again everybody

I just received a new speaker and figured I might as well go ahead and build that extension cab that I've been wanting to do for a while...
Got some very nice 18mm birch ply from a chap that had it "left over" so I saved him a trip to the tip...

I know that speaker cabs for guitar don't have to be anything fancy, BUT...

Something in my mind is telling me that once, a long time ago, somebody mentioned that they can be any shape "but not perfectly square". I think the reason given was that a square cab would "suffer from "standing waves"".

Can somebody shed some light on whether that is true? Because I really can't remember where I heard / read it and I haven't found an answer through the Google.

What would the effect of these "standing waves" be on the sound? Or, for that matter the cab itself... will the woodworms explode?

[eschertron]

Standing waves are waves at the frequency reinforced by parallel reflective surfaces (acoustically reflective, that is). The first modal reinforcement (and the strongest?) will be for a frequency that has a wave length of 2X the distance between the parallel surfaces. So for a box that's 1 meter from side to side (yes, big I know, but serves to illustrate) the wave length is 2m of the lowest frequency reinforced. That's going to be *about* 170Hz. If the height and the width of the cab are both 1m, then both sets of cab sides will reinforce the same freq. Acoustic engineers often plan for rooms (or in this case, speaker cabs) that have dimensions related by irrational ratios, to spread the modal reflections out as much as possible. The non-parallel engineering is sometimes used, although it tends to suffer from the fact that the solutions to finding the resonant frequencies become very complex, and for a hobbyist (or a guitar cab manufacturer) it's not considered worthwhile.

[edit] long answer short, the standing waves will add to the 'boominess' of the speaker at the resonant freqs. Sometimes a cab is designed to emphasize frequencies that are lacking in the speakers response, to help even out the sound. Reinforcing frequencies that are already over-represented will make the speaker even louder at those freqs. [/edit]

You haven't mentioned if you are planning to build a closed- or open- backed cab. That will add another set of parallel surfaces, as well as add a resonance effect not related to linear dimension, but to volumetric dimension. The simplest way to get a cab that sounds good is find an existing design for a cab that sounds good to you, and do something like it. If you want the visual novelty of a cab that's not 'square', then go for it. I hope your woodworking skills are up to the challenge.

[I'm a beginner!]

Hmmm... alot to take in!

I was going to have the cabinet back split approximately two thirds of the way up so the top third of the back can be removed for an open-ish sound, or removed completely for a fully open sound. It's only going to be a 1x12, although thanks to me purchasing another "unnecessary" (according to her of the many shoes) amp I will soon be having to make a 2x12 as well...

[eschertron]

"unnecessary" (according to her of the many shoes)

Hey! I know her!

Yup. A couple of removable semi-closed-back panels will have a much greater impact on the sound of the cabinet than about anything else, given the choice of construction materials. 3/4" birch ply will be pretty awesome!

In contrast to the math involved with standing waves and size ratios for resonance peaks, materials science is more like voodoo (to me, at least). Have a look at baffle board construction techniques to see if a thinner ply, or another type of wood entirely, might be in order based on what adjectives you find appealing for how the baffle influences the sound of the cab. Some baffle materials will be considered 'warmer', or 'brighter', or more or less 'neutral' than others.

[Tooboob]

Here's a few things to think about.
1. There are many manufacturers with broad lines of 12" guitar speakers. The differences between them are quite audible. Even if you theoretically "nail" the cabinet, you still have to go on a speaker hunt.
2. Closed back "acoustic suspension" speakers for HiFi generally have some sort of damping material in them - fiberglass insulation or something more or less exotic. Speakers for guitar generally don't, except for some of them.
3. A guy named Thiel, probably leveraging previous research, came up with some mathematics for the problem. You can design a nice ported speaker if you have the speaker data sheet and the equations. You'll wind up with a pretty nice HiFi speaker, and it will probably sound a bit lifeless when driven by your guitar amp, unless you're a clean freak like me.
4. you'll find that the HiFi ideal speaker doesn't rock your boat (or it will), and getting it all wrong probably won't sound too good either. You'll be fighting harsh or muted highs, and boomy or muted lows. Some speakers (generally the higher power ones) are less dependant on the enclosure design, and some take on more of the character of the cab.

So the first advice, to copy something you like, is good advice.

I add that it's good to provide as much provision for tuning as possible. Port size and location make a big difference, and you'll have a way to play with that. You can also raise the speaker off the floor, lean it back, put it against the wall, etc. Some insulation lining at least the back changes things considerably. Changing speakers is easier if you front mount them, but that rubber ring on the speaker matters, and it's for rear mounting. When you do the 2x12, parallel versus series connection makes a diference. Front ports work differently than rear ports. As mentioned, changing speaker models makes a big difference too.

If there wasn't voodoo and taste involved, all speaker cabinets would be the same.