Sound comes from vibrations
Every sound is produced by something vibrating: moving rapidly to and fro.
Pluck a stretched rubber band and you see it vibrate and hear a sound.
Touch a ringing school bell and you feel it vibrating. Hold it tightly and the sound stops.
In our throat, the vocal cords vibrate when air from the lungs passes between them. This produces our voice.
In musical instruments, different parts vibrate:
Instrument | What vibrates |
|---|---|
Sitar, guitar, veena | stretched strings |
Tabla, dholak | stretched membrane |
Flute, shehnai | the column of air inside |
Manjira, ghatam | the body of the instrument |
Sound needs a medium
Sound travels through solids, liquids and gases. It cannot travel through a vacuum (empty space).
If a ringing electric bell is placed in a glass jar and the air is slowly pumped out, the sound becomes fainter and fainter, even though you can still see the hammer striking the bell. Without air there is nothing to carry the vibrations.
This is why astronauts on the Moon, which has no air, talk to each other by radio.
Sound travels fastest in solids, more slowly in liquids, and slowest in gases. Putting your ear to a table lets you hear a scratch at the other end clearly.
How we hear
The outer ear collects sound and funnels it into the ear canal. At its end is the eardrum, a thin stretched membrane. Sound vibrations make the eardrum vibrate. These vibrations are passed on through the tiny bones of the middle ear to the inner ear, which sends signals to the brain.
Never put sharp objects into your ear. They can damage the eardrum.
Describing a vibration
Amplitude: how far the object moves from its central (rest) position.
Time period: the time taken for one complete oscillation.
Frequency: the number of oscillations per second, measured in hertz (Hz). A frequency of 500 Hz means 500 vibrations per second.
Loudness and pitch
Property | Depends on | Effect |
|---|---|---|
Loudness | amplitude | larger amplitude means a louder sound |
Pitch | frequency | higher frequency means a higher (shriller) pitch |
Loudness is measured in decibels (dB). A whisper is around 30 dB and normal conversation around 60 dB. Sounds above about 80 dB can be painful and damaging with long exposure.
A woman's or child's voice usually has a higher pitch than a man's voice, because their vocal cords are shorter and vibrate at a higher frequency.
What can we hear?
Human ears can hear frequencies from about 20 Hz to 20 000 Hz. This is the audible range.
Sounds below 20 Hz are infrasonic. Elephants and whales can use them.
Sounds above 20 000 Hz are ultrasonic. Dogs, bats and dolphins can hear them. Ultrasound is used in hospitals to take images of organs inside the body.
Noise and noise pollution
Pleasant sounds are called music. Unpleasant, unwanted sounds are noise. Too much noise is noise pollution. Its sources include vehicle horns, loudspeakers, firecrackers, construction work and factories.
Noise pollution can cause lack of sleep, high blood pressure, anxiety and even permanent hearing damage.
Ways to reduce it:
keep TVs and music at low volume and avoid unnecessary honking,
fit silencers to vehicles and machines,
plant trees along roads and around buildings,
set up airports and noisy factories away from homes.