Sources and receivers of sound
Every body that produces sound vibrates: a ruler projecting from a desk edge, a stretched string, a loudspeaker cone, vocal cords. A repeating motion is called an oscillation (vibration). The number of oscillations in 1 second is the frequency (ν); its unit is the hertz (Hz): 1 Hz = 1 oscillation per second. The human ear hears roughly from 16 Hz to 20 000 Hz; sound below 16 Hz is infrasound, above 20 000 Hz ultrasound. Many animals hear differently: dogs, cats and bats also hear ultrasound, and dolphins perceive very high frequencies (above 100 kHz). An instrument that produces a sound of an exact frequency is a tuning fork: a two-pronged metal device invented in 1711 by the English musician John Shore; its sound is amplified by a resonator (a wooden box); instruments such as the rubab, dutar and tanbur also have a resonator under the strings. Sound receivers: the human ear (the eardrum vibrates and the signal goes to the brain) and the microphone, which converts sound vibrations into electrical ones (Greek mikros — small, phone — sound). Listening to loud music for a long time reduces hearing — protect your ears.
Experiment: let a ruler project over a desk edge and set it vibrating with a long and a short free end — with the short end a sound is heard (higher frequency). With a tuning fork, touch it to water and watch the splashing. Safety: do not make very loud sounds near ears; do not strike a tuning fork on hard objects, use a rubber mallet; do not flick the ruler at fingers.