Sound
Vibration that travels as a wave through a medium such as air, together with the sensation it produces in the ear. The range audible to humans is roughly 20 Hz to 20 kHz.
The supermassive black hole in the Perseus galaxy cluster sends sound waves through surrounding hot gas. Each oscillation takes about ten million years, far below human hearing. In 2022, NASA pitched it up about 57 octaves and released it as the viral 'sound of the universe.'
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A male cicada's abdomen is almost hollow. Instead of organs, it holds a resonating chamber and a tymbal — a stiff membrane buckled by muscle to make sound. The hollow belly amplifies it. The loud African cicada *Brevisana brevis* can reach 106 decibels.
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