Sound is produced when an object vibrates. A vibrating object moves repeatedly to and fro about its mean position. This repeated motion produces disturbances in the surrounding medium, which we hear as sound.

Vibration: The periodic to-and-fro motion or oscillation of an object about its mean position.

Consider a stretched rubber band fixed across an open cardboard box. When the band is plucked, it vibrates and produces sound. Once the vibrations stop, the sound also stops.

Examples of sound production

• Musical instruments: A stretched string vibrates when plucked. The membrane of a drum vibrates when struck.
• Wind instruments: In a flute or bansuri, the air column inside the hollow pipe vibrates.
• Human voice: Sound is produced by the vibration of vocal cords inside the voice box or larynx.
• Insects: Crickets and grasshoppers produce sound by rubbing certain body parts, such as wings or legs.
• Metal objects: Some metals produce a ringing sound when struck. This property is called sonority.

Tuning Fork

A tuning fork is a U-shaped metal instrument, usually made of steel or aluminium. Its two arms are called prongs or tines, and the lower part is called the stem.

When a prong is struck gently against a rubber pad, the prongs vibrate and produce sound. If a vibrating prong touches water, ripples appear on the water surface. This provides evidence that the prongs are vibrating.

After sound is produced, it must travel from its source to the listener. This movement of sound through a material substance is called propagation of sound.

The material through which sound travels is known as a medium. Sound can travel through all three common states of matter:

• Solids – such as wood, metal and a classroom desk.
• Liquids – such as water.
• Gases – such as air.

Sound travels through solids

If one person gently scratches or knocks on a desk, another person can hear the sound by placing an ear against the desk. The sound travels through the solid material.

Sound travels through liquids

When two metal spoons are tapped together while submerged in water, the sound can still be heard. This shows that sound can propagate through liquids.

Sound needs a medium to travel

A vacuum is a space where matter is absent. Sound cannot travel through a vacuum because there are no particles to transfer the disturbance.

Vacuum Bell Jar Experiment

An electric bell is placed inside a glass bell jar. Initially, the bell can be heard when it rings. When air is gradually removed using a vacuum pump, the sound becomes fainter. When most of the air is removed, the ringing bell can still be seen, but its sound is almost inaudible. When air is allowed back inside, the sound becomes audible again.

This also explains why astronauts cannot directly hear each other speaking during a spacewalk. Outer space is nearly a vacuum, so astronauts communicate using special devices fitted into their spacesuits.

Sound travels through a medium in the form of a wave. To understand this, imagine pushing and pulling one end of a stretched slinky. A disturbance moves along the slinky, but its individual coils do not travel all the way to the other end.

Compressions and Rarefactions

When a vibrating source moves forward, it pushes nearby air particles closer together. This creates a region of higher density called a compression.

When the source moves backwards, the nearby air particles spread farther apart. This creates a region of lower density called a rarefaction.

• Compression (C): A region where particles are closer together and density is higher than average.
• Rarefaction (R): A region where particles are farther apart and density is lower than average.

As the source continues vibrating, compressions and rarefactions are produced alternately. These disturbances travel through the medium.

Sound wave: A disturbance consisting of alternating compressions and rarefactions that propagates through a medium without the actual flow of the particles of the medium.

Longitudinal Waves

In a sound wave, particles of the medium vibrate back and forth parallel to the direction in which the wave travels. Such waves are called longitudinal waves.

Mechanical Waves

Waves that require a material medium for propagation are called mechanical waves. Sound is a mechanical wave because it cannot travel through a vacuum.

Longitudinal and Transverse Waves

Earthquakes produce seismic waves that can be longitudinal or transverse. Longitudinal seismic waves are detected first by seismographs.

Sound is a form of energy. A vibrating source transfers energy to the surrounding medium. As the sound wave travels, particles of the medium vibrate and collide with neighbouring particles, transferring energy from one region to another.

Activity: Moving Grains with Sound

Stretch a rubber sheet or balloon membrane over the opening of a container. Sprinkle a few grains of rice or salt on it. Produce a loud sound near the container without touching it.

The sound waves reach the sheet and make it vibrate. The grains move because of these vibrations. A louder sound can cause greater movement of the grains.

Microphone and Speaker

• Microphone: Converts sound energy into electrical energy. Sound waves make a thin membrane called a diaphragm vibrate, and these vibrations are converted into an electrical signal.
• Speaker: Converts electrical energy into sound energy. An electrical signal causes a cone or diaphragm to vibrate and produce sound.

As a sound wave travels, the density of the medium changes periodically. At a particular instant, density can be plotted against distance to represent the sound wave graphically.

• The horizontal axis represents distance.
• The vertical axis represents density of the medium.
• The average density is shown by a horizontal reference line.
• Regions above average density represent compressions.
• Regions below average density represent rarefactions.

Crest and Trough

In the graphical representation:

• Crest: The highest point of the curve, representing maximum density.
• Trough: The lowest point of the curve, representing minimum density.

Wavelength

The distance between two consecutive crests or two consecutive troughs is called the wavelength of a wave.

• Symbol: λ (lambda)
• SI unit: metre (m)

Frequency

Frequency is the number of complete density oscillations at a fixed point per unit time.

• Symbol: ν (nu)
• SI unit: hertz (Hz) or s⁻¹

Time Period

The time taken for one complete density oscillation at a fixed point is called the time period.

• Symbol: T
• SI unit: second (s)

A higher frequency means more oscillations per second and a shorter time period. A lower frequency means fewer oscillations per second and a longer time period.

Amplitude

The amplitude of a sound wave is the maximum change in the density of air in a compression or rarefaction compared with the average density.

A larger amplitude means a greater change in density and generally a greater amount of energy carried by the wave.

Intensity of Sound

The amount of sound energy passing through a unit area perpendicular to the direction of propagation of a sound wave in a unit time is called the intensity of sound.

As sound travels away from its source, its energy spreads over a larger area. Therefore, its intensity decreases with distance from the source.

Speed of Sound

The speed of sound describes how fast the sound wave travels through a medium.

Since frequency ν = 1/T, speed can also be written as:

Factors Affecting the Speed of Sound

• Sound travels fastest in solids, slower in liquids and slowest in gases.
• In air, speed increases when temperature increases.
• In air, speed also increases with increasing humidity.

For example, the speed of sound in dry air is about 331 m/s at 0°C and nearly 344 m/s at 22°C.

The physical properties of sound include frequency, wavelength, amplitude, time period and speed. However, the way humans experience sound is described using terms such as pitch and loudness.

Pitch

Pitch is the way humans perceive the frequency of sound.

• High-frequency sounds generally have a high pitch. Examples include whistles and sirens.
• Low-frequency sounds generally have a low pitch. Examples include thunder and the rumbling of aircraft.

Different people have different voices because their vocal cords and the shape of their throat, mouth and nasal cavities affect the sound produced. During adolescence, boys' vocal cords generally become longer and thicker, which can make their voices deeper.

Human Hearing Range

The approximate audible frequency range for humans is from 20 Hz to 20,000 Hz (20 kHz). The range varies between individuals and generally decreases with age.

Humans cannot normally hear infrasonic or ultrasonic waves. Dogs, cats, bats and dolphins can detect ultrasound, while elephants can detect infrasound.

Loudness

Loudness is the way humans perceive the amplitude of a sound wave.

• Greater amplitude is generally heard as louder sound.
• Smaller amplitude is generally heard as softer sound.
• Loudness decreases as we move farther from the source.

Intensity is a measurable physical quantity, whereas loudness depends on the listener's hearing ability. They are often used interchangeably in everyday language, but they are not exactly the same.

Measurement of Sound and Noise Pollution

Sound loudness is commonly expressed in decibels (dB). Normal conversation is approximately 60 dB, while very loud sounds such as firecrackers can exceed 100 dB.

Unwanted or harmful sound is called noise. Prolonged exposure to loud sound can affect sleep, health and hearing. Hearing loss can be tested using audiograms, and hearing aids may help people with hearing difficulties.

Tone, Musical Note and Timbre

• Tone: A sound of a single frequency, such as that produced by a tuning fork or whistling.
• Musical note: A combination of a fundamental frequency and higher frequencies called overtones.
• Timbre: The quality that makes sounds from different instruments distinguishable even when they play the same note at the same loudness.
• Octave: The interval between two notes in which one has twice the frequency of the other.

The tabla and mridangam have a black central patch on the drum membrane called syaahi. It changes the vibration of the membrane and helps produce a rich variety of sounds.

When sound waves strike a solid or liquid surface, they can bounce back. This phenomenon is known as reflection of sound.

Sound follows the same laws of reflection as light:

• The angle of incidence is equal to the angle of reflection.
• The incident sound, reflected sound and normal at the point of incidence lie in the same plane.

Echo

An echo is the repetition of a sound heard after it reflects from a distant surface. For example, when we shout near a mountain or cliff, our voice may return after a short interval.

The human ear can distinguish two sounds separately when the time interval between them is at least 0.1 second.

Minimum Distance for Hearing an Echo

Taking the speed of sound as 340 m/s:

Distance travelled in 0.1 s = 340 × 0.1 = 34 m.

This is the total distance travelled by sound to the reflecting surface and back.

Minimum distance of the reflecting surface = 34 ÷ 2 = 17 m.

Surfaces and Echoes

• Hard and smooth surfaces reflect sound effectively.
• Soft surfaces such as curtains absorb sound.
• Rough surfaces scatter sound in different directions.

Reverberation

When sound undergoes multiple reflections from the walls, ceiling and other surfaces of a large hall, the sound may continue to be heard even after the source stops producing it. This is called reverberation.

Reverberation occurs when reflected sounds persist with very small time differences, described in this chapter as less than 0.05 seconds.

Auditoriums are designed to control reverberation so that speech and music can be heard clearly. Sound-absorbing panels, curtains, upholstered chairs and soft porous materials help reduce unwanted reverberation.

Sound waves outside the normal human hearing range have several important uses in science, medicine and technology.

Infrasonic Waves

Sound waves with frequencies below 20 Hz are called infrasonic waves.

Applications include:

• Detecting natural events such as earthquakes and volcanic eruptions.
• Detecting severe storms, as these waves can travel long distances through air and the Earth.

Ultrasonic Waves

Sound waves with frequencies above 20 kHz are called ultrasonic waves.

Applications include:

• Ultrasonography: Imaging internal organs without surgery.
• Medical treatment: Breaking kidney stones into smaller pieces so that they can pass out of the body.
• Industrial cleaning: Cleaning delicate machine parts.
• Ultrasonic welding: Joining materials in industries.
• Defect detection: Finding defects inside metal blocks during construction and industrial testing.
• Object detection: Locating objects by analysing reflected sound waves.

Echolocation

Echolocation is the ability to locate objects using reflected sound waves.

Most bats produce short bursts of ultrasonic waves. These waves strike nearby objects and return as echoes. By sensing the reflected waves, bats can locate obstacles and prey even in darkness.

Dolphins, whales and some birds also use echolocation for navigation and hunting.

SONAR

SONAR stands for Sound Navigation and Ranging. It uses ultrasonic waves underwater to determine the distance, direction and speed of underwater objects.

Ultrasonic waves are sent into the water. When they strike an object, they are reflected back. The time taken by the signal to return is used to calculate the distance.

Sound and Technology

Sound is used in several modern technologies. Microphones capture sound and convert it into electrical signals, while speakers convert electrical signals back into sound. Ultrasonic sensors are also used in vehicle parking assistance systems to detect obstacles.

Sound and Scientific Research

Scientists use sound to study natural processes that cannot be observed easily. Sound sensors can help monitor aircraft and drones. Researchers also study sounds produced by organisms and changes in the environment.

Human Ear

When sound enters the ear, it causes the eardrum to vibrate. Tiny bones amplify these vibrations, and the cochlea converts them into electrical signals that travel to the brain. The brain interprets these signals as sound.