Sound Notes

Sound is produced by objects that vibrate — a plucked rubber band, a ringing bell, a tuning fork, the stretched skin of a drum, and the vocal cords in our voice box (larynx). Stopping the vibration stops the sound.

Sound travels through a medium (solid, liquid, or gas). It cannot travel through a vacuum — a ringing bell inside a jar becomes silent as the air is pumped out.

The vibrating body sets the particles of the medium vibrating, and the disturbance passes from particle to particle until it reaches the ear. There, the eardrum vibrates and the signal is passed through the inner ear to the brain, which recognises it as sound.

In one complete back-and-forth vibration (one oscillation): the maximum displacement from the mean position is the amplitude, the time taken is the time period (T), and the number of oscillations per second is the frequency (f). Frequency is measured in hertz (Hz), and f = 1/T.

Loudness depends on amplitude — a larger amplitude means a louder sound. Loudness is measured in decibels (dB). Loudness also decreases as we move away from the source.

Pitch (shrillness) depends on frequency — a higher frequency means a higher pitch. A drum has a low pitch (low frequency); a whistle has a high pitch (high frequency). A child's or woman's voice is generally higher-pitched than a man's.

Humans can hear sounds of frequency roughly 20 Hz to 20,000 Hz. Frequencies above 20,000 Hz are ultrasound (heard by dogs, bats, dolphins); below 20 Hz is infrasound.

Pleasant sounds are called music; unpleasant, unwanted, and irregular sounds are called noise. Excessive or prolonged noise causes noise pollution, whose effects include lack of sleep, anxiety, high blood pressure, headache, and, over time, loss of hearing.

Noise can be reduced by planting trees along roads, keeping generators and loudspeakers away from homes, using silencing devices in machines and vehicles, and not using horns unnecessarily.