I'm a historian. I've struck on a curiosity about telecommunication technology in pre-modern times. I've read about a number of methods, and I'd like to ask some physics questions that got beyond my ken. This information is for my curiosity and creativity, not for any rigorous study, so I may be mentioning Wikipedia. Thank you for humoring me, and I hope I'm not asking too many questions :) I've now separated them into multiple threads by subject; this one ought to be about sound. To conform to PF custom, I'll be adding the other questions to threads after the first ones have been answered. Let anyone feel free to answer only those questions he prefers to. Sound My main curiosity is about acoustic mirrors (big concrete, dishes) and echoes. I've read about sound mirrors on several sites, many of those concerning devices built between the World Wars (and right before radar) with the idea of amplifying the noise of incoming planes (to forewarn of bombers). Cf. Japanese war tubas, as well: huge tuba-looking things aimed at the sky and attached to stethoscopes for monitoring. Question 1. Is there any acoustic property of concrete which makes concrete better than other types of common stone? Some pre-modern cultures had sorts of concrete, but it'd still be a technological limitation since they all had big stones?