How thick should the front baffle of speakers be?

The price of 1/4" aluminum cost about $28 per square foot, price on buymetal.com

I would think that aluminum would sound better due to the simple fact that it is roughly 3 X stronger than wood. I would also think that equal parts of aluminum would be much heavier. 3 X the wood to equal the strength of aluminum would still be lighter than the aluminum. I dont think aluminum is used because it is cheaper I think it is used because it is much stronger and less material is required.

Cabinet resonances can interfere with the output of the drivers in a bad way but they’re also unavoidable. All loudspeaker cabinets will resonate irregardless of the material but some less than others eg open baffles, transmission lines and some materials will have different resonances than others. Some designers prefer the sound of ply resonance to that of MDF but it’s largely down to application. It’s more of a question of exactly where you want to place those pesky resonances.

It’s very important issue because where these resonance issues appear when a speaker is operating at full or near full throttle can make or break a particular design. Some just totally fall apart.

It should be noted once and for all that a super rigid cabinet design is not the answer because as the OP points out, that usually leaves the internal box pressures with nowhere to go but back out through the cone as well as moving the resonances up into a more noticeable frequency range.

Midrange resonances, even mild ones are a sonic disaster because that’s where voices are and that’s where our ears are the most sensitive!

So thicker cabinets are not without compromise.

So as often is the case with loudspeaker design it’s a question of choosing where to place / balance / hide those inevitable design compromises. Some designs are cleverer than others at doing this, no doubt.

Harbeth employ a lossy cabinet design which supposedly dumps these resonances below the hearing threshold, but that’s just one approach amongst many. They seek the cleanest midrange performance possible but it also has its compromises as they freely acknowledge their speakers may not be the best for Heavy Rock or Metal etc.

Horses for courses as usual.

Uh, that’s one of the main benefits of isolating speakers - to remove the cabinet resonances. The other main benefit is to eliminate mechanical feedback via the floor. Problem solved! 🤗