A police whistle exploits beating to produce a tone well below its actual frequencies

Evidence

A London police whistle makes just such use of beating of two nearby high-frequency tones. Suppose the whistle generates a 3000 Hz and a 3080 Hz tone. The combination beats at 80 Hz, well into our hearing range. The GCD of 1000 and 1080 is 40, which is the frequency of this combination, also well into our hearing range. The beating tone, however, is heard at 80 Hz, which is the frequency of the waveform pulses. The exact details of each pulse recur only at 40 Hz. — Eric J. Heller, Why You Hear What You Hear- An…, 2295

Categorization

• CANDY: The London police whistle creating an 80 Hz beat from two kHz-range tones is a delightful bar trivia fact.
• SHARE: Would delight anyone into acoustics or instrument design — the whistle as a deliberate beat-tone device.