I seem to recall Jamo and Thiel using concrete in some of their products.
Concrete Audio Speakers...Speaker Enclosures made out of Concrete ?..
Could concrete speaker enclosures give us the most realistic sound ?
https://www.concrete-audio.com//en/#lautsprecher
Not connected with this company in any way
https://www.concrete-audio.com//en/#lautsprecher
Not connected with this company in any way