How thick should the front baffle of speakers be?

simonmoon, thank you for being the voice of reality here. Cheers.
The reason they used plywood back in the old days was that they had not developed MDF yet which is a far better inexpensive material to make loudspeakers from because of it's damping characteristics and higher mass. It is much stiffer than plywood. The old speakers were indeed musical instruments and colored beyond belief. Just listen to any Bozak B302A a speaker that was lucky in that it's colorations were euphoric as all get out. But some of the old Altecs were honky old boom boxes. Klipsch did a good job of controlling it. Higher mass and thicker stiffer walls are always an advantage however the speaker manufacturers have to balance that against shipping costs as for most it is a competitive market. People who buy Magico M7s could give a hoot.You have to pay a rigger to get the things into the house not to mention pray that your floor will support them.

You don't get doppler effects off baffle flex the baffle exists to hold transducer to support mid-bass frequency and to limit vibration levels. An over small speaker will require BSC- baffle step correction in its network to compensate if baffles undersize it is why small speakers sound thin. Also aluminum is cheaper by the ton than plywood and it is also lighter why it is used so much over ply it is not better sounding just cheaper to build ship etc.

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.