Regarding the Jern cast-iron loudspeakers:
The castings are what’s known as "grey iron" which is incredibly inert, they are probably the most inert material that loudspeaker enclosures have ever been made from (and one of the heaviest, so we’re not going to see "normal sized" speakers with grey iron enclosures anytime soon). And THEN the enclosure has this rounded shape like two spheres of different sizes merging, which is GEOMETRICALLY incredibly rigid and ACOUSTICALLY very close to ideal (which just about nothing else approaches). None of this would matter if they didn’t sound excellent, and they DO. As a big speaker manufacturer and proponent I VERY SELDOM get excited about small speakers but I was extremely impressed by the Jerns which is why I stuck around to learn more about them when I first encountered them years ago.
No affiliation, just mad respect. The Jern is a speaker which keeps me away from trying to build a competing small speaker because I don't think I could get as many things right as it does.
Duke