My favorite part of the Amazon “Rings of Power” series so far is when Disa is explaining how Dwarves understand the rocks around them. She sings to the rocks, and the way they resonate to her song tells her about the composition inside them, so she knows where to dig.

This resonates with me, too, because I worked in ultrasonic materials-characterization at NASA back in my salad days. That’s exactly how we did it. The speed of sound in rock is around 6,000 m/s and in gold it’s about 3,200 m/s, so this would definitely work. Wherever her song hit a vein of gold, the change in the speed of propagation means sound waves would refract and reflect back to the singer.

It also tells us something about Dwarves that we didn’t know before. Some background: The speed of a wave is equal to the wavelength times the frequency. The smallest feature you can identify with sound waves is about a wavelength in size. If Disa is a soprano, she can sing a note that’s about 1100 Hertz, which means she can’t hear anything in the rock smaller than 5.4 meters across, which is about 18 feet. That’s not very useful; she must be able to sing higher than that. You could do a lot with notes 10 times that high. A seam of ore is frequently a foot wide.  Maybe she can sing 10-20 kHz. That’s the top end of what humans can hear, but it’s well within a dog’s range. Dwarf women and dogs might get along well.

Bats are the secret

Domesticated animal

Real precision work will take more, though. The head of a rock-hammer is about 2 cm across. It would be more useful to find things that size. To hear something that small, she’ll need to sing about 300 kilohertz. That’s too high even for bats, who max out at 200 kHz, but there are clever things you can do to improve resolution if you have more than one transmitter. Disa is certainly doing something like that, since she’s talking about a song, not just a note. She could be accompanied by a bat chorus. Just like humans keep chickens in a coop near the house, Dwarves might keep a cave of bats near their own excavation to help with surveying.

Conclusions
  1. We don’t know anything about female Dwarves because not only are they rarely seen, they’re impossible to hear.
  2. People who compose film music for Dwarves that is dominated by bass notes have it all backwards.