Sound is one of the most important ways we communicate and experience the world. Whether it is the song of a bird, the ringing of a bell, music from an instrument or the voice of a friend, sound surrounds us constantly. But what exactly is sound? In physics, sound is a form of energy that produces the sensation of hearing, and it is produced when an object vibrates.
When you pluck a guitar string, strike a drum, or clap your hands, the object moves back and forth rapidly — this to and fro motion is called vibration. The vibrating object sets the air around it into motion, and this disturbance travels through the air as a wave to reach our ears. In this chapter we study how sound is produced, how it travels, its properties such as frequency and amplitude, and how we hear it.
All sounds are produced by vibrating objects. A vibrating object moves to and fro about its mean position, and this motion is called vibration or oscillation. The object pushes and pulls the air particles around it, setting up disturbances that travel as sound.
Examples of vibration producing sound: - A stretched guitar string vibrates when plucked and produces sound. - A drum skin vibrates when beaten and produces sound. - The vocal cords in our throat vibrate when we speak. - A tuning fork vibrates when struck and produces a musical note. - Even the whistle and the flute produce sound because the air column inside them vibrates.
If the vibration stops, the sound stops — as when you touch a ringing bell and hold it, the sound dies out. This shows directly that vibration is the source of sound.
Sound is a mechanical wave and needs a material medium — a solid, liquid or gas — to travel. It cannot travel through a vacuum. This was demonstrated by a classic experiment: when an electric bell is placed inside a bell jar from which air is gradually removed, the sound of the bell gradually fades and finally stops, even though the bell keeps ringing and can be seen striking. This proves that air is necessary for sound to travel.
Sound travels through all three states of matter: - Through solids: Sound travels fastest and clearly, e.g., through a railway track. - Through liquids: Whales and fish communicate through water. - Through gases: Sound travels through air to our ears.
The speed of sound is highest in solids, lower in liquids and lowest in gases.
When a vibrating object disturbs the medium, the disturbance travels as a wave. In a sound wave, the particles of the medium vibrate parallel to the direction of wave travel, so sound waves are longitudinal waves. As the wave passes:
The compressions (higher pressure) and rarefactions (lower pressure) travel through the medium, carrying the sound energy from the source to the listener. Although the particles vibrate about their positions, they do not travel with the wave; only the disturbance moves forward.
Sound waves are described by their properties:
The speed of sound in air at 20°C is about 343 metres per second (m/s). Speed depends on the medium and temperature, not on frequency or amplitude.
The human ear can hear sounds with frequencies between about 20 Hz and 20,000 Hz (20 kHz). This range is called the audible range of sound for humans.
Bats, dolphins and whales use ultrasound to locate objects by listening to the echoes of the sounds they produce — a technique called echolocation.
Sounds are of two kinds based on how pleasant they are:
Noise is measured in decibels; sounds above about 80 dB can be harmful. Prolonged exposure to loud noise can damage our ears and cause hearing loss.
The presence of excessive or unwanted sounds in the environment is called noise pollution. Sources of noise pollution include traffic, factories, aircraft, loudspeakers and construction work. The effects of noise pollution on health include:
Measures to reduce noise pollution: - Use horns and loudspeakers at low volume and only when necessary. - Place noise-producing industries away from residential areas. - Use silencers in vehicles and keep machines well maintained. - Grow trees along roads — green vegetation absorbs sound. - Ensure the use of earplugs or earmuffs in very noisy workplaces.
The human ear consists of three parts:
The two most important ideas of this chapter are that sound is produced by vibration and that it requires a medium to travel. Activities confirm these: a plucked string vibrates and produces sound, and the bell in a bell jar becomes silent as air is removed. Sound travels as compressions and rarefactions, has frequency (pitch) and amplitude (loudness), and our ears detect it by converting vibrations into nerve signals. Understanding these basics helps us control sound, reduce noise pollution and use sound technology like ultrasound.
| Property | Definition | Unit | Effect on Sound |
|---|---|---|---|
| Frequency | Number of oscillations per second | Hertz (Hz) | Higher frequency = higher pitch |
| Amplitude | Maximum displacement from mean position | Metre | Larger amplitude = louder sound |
| Speed | Distance sound travels per second | m/s | Depends on medium (343 m/s in air at 20°C) |
| Type | Frequency | Who Hears / Produces |
|---|---|---|
| Infrasonic | Below 20 Hz | Elephants, whales; earthquakes |
| Audible | 20 Hz – 20,000 Hz | Humans |
| Ultrasonic | Above 20,000 Hz | Bats, dogs (up to 50 kHz), dolphins |
| Feature | Musical Sound | Noise |
|---|---|---|
| Vibrations | Regular, periodic | Irregular, non-periodic |
| Effect | Pleasant | Unpleasant |
| Examples | Flute, sitar, violin | Traffic, machinery, crackers |
Sound is a form of energy produced by vibrating objects and travels as a longitudinal wave of compressions and rarefactions, requiring a material medium — it cannot pass through a vacuum. Its character is described by frequency, which gives the pitch, and amplitude, which gives the loudness, while the human ear is tuned to hear frequencies from about 20 Hz to 20 kHz, with infrasonic and ultrasonic sounds beyond that range. Understanding sound also reveals the problem of noise pollution, whose harmful effects on health can be controlled by civic measures and personal care. From the vibrations of a plucked string to the workings of the inner ear, the study of sound connects physics with the everyday joy of hearing music, speech and the natural world.