‘Ultrasound and infrasound’ explains that humans can hear sounds with frequencies in the range of 20 Hz to around 17 kHz, but there is a wide ultrasound soundscape beyond this range. Bats, for example, are able to navigate and hunt using echolocation, generating ultrasounds and timing the delays until they hear the echoes, which inform them of the distances to nearby objects. Ultrasound can also be used medically to help repair damaged muscles, scan foetuses, or treat inoperable tumours. At extremely high frequencies, sounds behave like particles called phonons. Infrasound occurs at lower frequencies and can travel far further than audible sound, through sea, ground, or air.