20,000 hertz

"'I believe,' he said, speaking more slowly now, 'that there is a whole world of sounds about us all the time that we cannot hear. It is possible that up there in those high-pitched inaudible regions there is a new exciting music being made, with subtle harmonies and fierce grinding discords, a music so powerful that it would drive us mad if only our ears were tuned to hear the sound of it."

[Roald Dahl - The Sound Machine - 1949]