The sound-detection machine at the center of Roald Dahl's "The Sound Machine" is designed to pick up on sounds that exist outside of a human's audible range and transform the sounds into something audible to humans by pitching down the tone.
The sonic hearing range of the standard human ear is between 20 hertz and 20 kilohertz (20,000 Hz). The air vibrations that produce a 20 Hz sound create a very low bass rumble and have a long wavelength, while a 20 kHz sound would create a short-vibration wavelength that produces a high pitch. Most adults' high-frequency hearing deteriorates with age, meaning they are less sensitive to certain pitches that children can hear. This difference in hearing ability has led to the installation of anti-loitering devices in certain public spaces: the devices emit a high-pitched tone that irritates young people but is inaudible to adults.
Because of the physical differences in how ears are shaped, other animals can hear tones much higher-pitched than humans can. Bats, for instance, are attuned to hearing frequencies as high as 200 kHz; this allows them to echolocate by making screeching sounds that they can hear bouncing back from physical objects. But with this higher upper range, the lowest frequency bats can detect is 20 kHz—the upper range of humans. This means bats and humans experience the world on completely separate sound spectrums.