Sound: How We Hear, Communicate, and Enjoy Music

Sound is a part of our daily life.
We speak, listen, enjoy music, and hear warning signals because of sound.
In physics, sound is not just something we hear.
It is a form of energy produced by vibrations and travels as a wave through a medium.
Understanding sound helps us explain:

• How sound is produced
• How it travels
• Why some sounds are loud and some are soft
• How musical instruments work

Sound Waves

Production of Sound

Sound is produced when an object vibrates.
A vibrating object sets the surrounding particles of the medium into vibration.
These vibrations travel through the medium as sound waves.

Example:

When a tuning fork is struck, it vibrates and produces sound.

Need of a Medium

Sound requires a material medium to travel.
It can travel through:

• Solids
• Liquids
• Gases

Sound cannot travel in vacuum.

Example:

Sound cannot be heard in outer space where there is no air.

Nature of Sound Waves

Sound waves are longitudinal waves.
In longitudinal waves:

• Particles vibrate parallel to the direction of wave motion
• Energy travels forward, particles do not

Compressions and Rarefactions

Sound waves consist of:

• Compressions: regions of high pressure
• Rarefactions: regions of low pressure

These alternate regions move forward and carry sound energy.

Properties of Sound Waves

Sound waves are described using certain physical quantities.

Amplitude

Amplitude is the maximum displacement of particles from their mean position.

• Greater amplitude → louder sound
• Smaller amplitude → softer sound

Example:

Beating a drum harder produces louder sound.

Frequency

Frequency is the number of vibrations per second.

• Measured in hertz (Hz)
• Higher frequency → shriller sound
• Lower frequency → deeper sound

Example:

A whistle has higher frequency than a drum.

Time Period

Time period is the time taken for one complete vibration.
Frequency and time period are related:

• Higher frequency → smaller time period

Wavelength

Wavelength is the distance between two consecutive compressions or rarefactions.
Longer wavelength → lower frequency
Shorter wavelength → higher frequency

Velocity of Sound

Velocity of sound is the distance travelled by sound per second.

• Depends on nature of medium
• Fastest in solids
• Slowest in gases

Sound travels faster in water than in air.

Loudness and Pitch

Loudness

Loudness depends on amplitude of sound.

• Loudness is a physiological sensation
• Measured in decibel (dB)

Greater amplitude produces louder sound.

Pitch

Pitch depends on frequency of sound.

• High frequency → high pitch
• Low frequency → low pitch

Pitch helps us distinguish between a male and female voice.

Audible and Inaudible Sounds

Human ear can hear sounds of frequency between:

• 20 Hz to 20,000 Hz

Sounds below 20 Hz → infrasonic

Sounds above 20,000 Hz → ultrasonic

Uses of Ultrasonic Sound

• SONAR
• Medical imaging
• Detecting cracks in metals

Simple Musical Instruments

Musical instruments produce sound through vibrations.
They can be broadly classified based on how sound is produced.

String Instruments

In string instruments, sound is produced by vibrating strings.
Examples:

• Guitar
• Violin
• Sitar

Pitch depends on:

• Length of string
• Tension in string
• Thickness of string

Wind Instruments

In wind instruments, sound is produced by vibrating air column.
Examples:

• Flute
• Trumpet
• Harmonium

Pitch depends on:

• Length of air column
• Speed of air flow

Membrane Instruments

In membrane instruments, sound is produced by vibrating stretched membrane.
Examples:

• Drum
• Tabla

Louder sound is produced when membrane vibrates with greater amplitude.

Why Understanding Sound Is Important

This chapter helps explain:

• Communication systems
• Music and musical instruments
• Noise pollution
• Medical and industrial uses of sound

It also appears frequently in MCQs, assertion–reason, and conceptual questions.

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FAQs – SOUND (Exam-Focused)