Mike Lenehan of Lenehan Audio in Australia, uses differential bracing including 6mm and 4mm sprung steel plates inside his ultra high end loudspeakers. Connected electrically to the ground terminal, as a ground plane. (I suspect also acts much like a Faraday Cage as well.)
He also uses depending on the outright high end models, cast iron rod as well as silicone filled rigid copper tube bracing (due to it's high Young's modulus). Which is why he uses very pure copper screws to secure the drivers in his all of his two way speakers. To help alleviated ringing of the spiders on the drivers against the baffle.
I have even heard a pair of his standmount speakers with an external crossover, and the cabinets were lined with 4mm solid copper plate.
Curious recording studios in LA and NYC learning of his achievements have enquired and purchased his nearfield monitors to satisfy their curiosity after being informed; immediately ordering a second pair for another studio room after verifying the reviews.
In a sea of well advertised audio equipment, his speakers are a well hid pot of audiophile gold. I worked for Mike and still own two pairs of his speakers, after giving my brother an older model, and selling my first pair to help pay for them.
I have built them myself, and I would welcome anyone who may be interested why they out perform speakers costing more than 10 times their cost. Real science, why it's superior and not salesman hyperbpole.
The benefits of metals in an enclosure are real, so long as you can also account for the limitations. Even metals have a resonant frequency, attenuating the ringing of metal is also why it benefits being laminated to another material.
