Any sound above 20 000 Hz is called ultrasound. It is too high pitched for humans to hear, but other animals (such as dogs, cats, dolphins and bats) can hear ultrasound. Ultrasound waves are ...

They sent a sound wave with a frequency of around 800 Hertz (roughly the high g of a soprano) through the first waveguide and measured how well it was transmitted to the second and third waveguides.

That is, for a signal with the frequency of 1 Hz, the proposed design can detect the minimum vibration of lunar geology is 10 −11 m. The location of the array is also carefully considered.