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Sound Waves: Characteristics and Applications

NCERT Class 9 · Science Based on NCERT Class 9 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores the nature of sound as a form of energy produced by vibrating objects and propagated through a medium as longitudinal mechanical waves. It explains how sound travels through solids, liquids, and gases via compressions and rarefactions, and introduces key wave characteristics like wavelength, frequency, amplitude, and speed. The big idea a student must walk away with is that sound is a mechanical wave that requires a medium to travel, and its properties (frequency, amplitude, speed) determine how we perceive it (pitch, loudness) and how it can be applied in technologies like sonar and ultrasonography.
Key Concepts (28)
Sound
Sound is a form of energy that is produced by vibrating objects and requires a material medium for propagation.
e.g. The sound of a bell is produced by the vibration of the bell when struck.
Vibration
Vibration refers to the periodic to and fro motion (oscillations) of an object.
e.g. When a rubber band is plucked, it vibrates and produces sound.
Source of sound
The object that produces sound is called the 'source' of the sound.
e.g. A tuning fork is a source of sound when its prongs vibrate.
Vocal cords
Vocal cords are tightly stretched muscular flaps located inside the voice box or larynx in the throat, whose vibration produces sound in humans and some animals.
e.g. When you speak, you can feel vibrations in your throat due to vocal cord vibration.
Tuning fork
A tuning fork is a U-shaped metal bar with a stem, usually made of steel or aluminium, whose prongs vibrate when struck to produce sound.
e.g. Striking a tuning fork against a rubber pad makes it vibrate and produce a single-frequency sound.
Medium
The material through which sound propagates is called a medium. It can be a solid, liquid, or gas.
e.g. Sound travels through air (gas), water (liquid), and steel (solid).
Vacuum
A space where there is no medium (matter) is referred to as vacuum.
e.g. Outer space is a near vacuum where sound cannot propagate.
Compression
A compression is a region of air with higher density (compared to the average density) in a sound wave.
e.g. When a piston moves forward in a tube, it compresses the air, creating a compression.
Rarefaction
A rarefaction is a region of air with lower density (compared to the average density) in a sound wave.
e.g. When a piston moves backward in a tube, it creates a rarefaction.
Sound wave
A sound wave is a disturbance consisting of a series of alternating compressions and rarefactions propagating through a medium, without the actual flow of the particles of the medium.
e.g. The sound from a tuning fork travels as compressions and rarefactions through air.
Longitudinal wave
A longitudinal wave is a wave in which the particles of the medium vibrate back and forth parallel to the direction of wave propagation.
e.g. Sound waves in air are longitudinal waves.
Mechanical wave
A mechanical wave is a wave that requires a material medium for propagation.
e.g. Sound waves are mechanical waves because they need a medium like air, water, or steel.
Transverse wave
In a transverse wave, the particles vibrate in a direction perpendicular to the direction of wave propagation.
e.g. Light is a transverse wave, but it is not a mechanical wave as it can travel through vacuum.
Wavelength
The distance between two consecutive crests or two consecutive troughs is called the wavelength of a wave. It is represented by λ (Greek letter lambda) and its SI unit is the metre (m).
e.g. In a sound wave, the distance between two successive compressions is one wavelength.
Frequency
The number of density oscillations at a fixed point per unit time is the frequency of the sound wave. It is represented by ν (Greek letter nu) and its SI unit is hertz (Hz).
e.g. If a tuning fork vibrates 256 times per second, its frequency is 256 Hz.
Time period
The time taken for one complete density oscillation at a fixed point is defined as the time period of the wave. It is represented by T and its SI unit is second (s).
e.g. If a wave has a frequency of 5 Hz, its time period is 0.2 s.
Amplitude
The amplitude of a sound wave is the maximum change in the density of air in a compression (or a rarefaction) compared to the average density.
e.g. A louder sound has a larger amplitude, meaning a greater change in density.
Intensity of sound
The amount of sound energy passing through a unit area perpendicular to the direction of the propagation of sound wave in a unit time is called the intensity of sound.
e.g. As you move away from a speaker, the intensity of sound decreases because the same energy spreads over a larger area.
Speed of sound
The speed of sound is the distance which a point on a wave, such as a crest (or a trough), travels in unit time. It depends on the medium, temperature, and humidity.
e.g. The speed of sound in air at 22°C is about 344 m/s.
Pitch
Pitch is how frequency is perceived by humans. High pitch sounds have higher frequency, and low pitch sounds have lower frequency.
e.g. A whistle has a high pitch (high frequency), while thunder has a low pitch (low frequency).
Audible range
The audible range or the human hearing range is from 20 Hz to 20,000 Hz (20 kHz).
e.g. Sounds below 20 Hz (infrasound) and above 20 kHz (ultrasound) cannot be heard by humans.
Infrasonic waves
Sound waves with frequency below 20 Hz are called infrasonic waves.
e.g. Elephants can detect infrasound, and it is used to detect earthquakes and volcanic eruptions.
Ultrasonic waves
Sound waves with frequency above 20 kHz are called ultrasonic waves.
e.g. Bats use ultrasonic waves for echolocation, and doctors use them for ultrasonography.
Loudness
Loudness is how humans perceive the amplitude of a sound wave. Sounds with larger amplitude are heard louder. It is commonly measured in decibels (dB).
e.g. Normal conversation is about 60 dB, while a firecracker can exceed 100 dB.
Echo
An echo is the reflection of sound off a hard surface, heard after the original sound if the time gap is at least 0.1 s.
e.g. Shouting near a cliff produces an echo because the sound reflects back.
Reverberation
Reverberation is the persistence of sound after the source stops emitting sound, caused by multiple reflections from walls, arriving with a time difference less than 0.05 s.
e.g. In a large hall, sound may echo multiple times, creating a reverberation effect.
Echolocation
Echolocation is the ability to locate objects using reflected sound waves.
e.g. Bats emit ultrasonic waves and sense the echoes to locate prey in the dark.
Sonar
Sonar (sound navigation and ranging) is a technique where ultrasonic waves are sent into water and the reflected waves are analysed to determine the distance, direction, and speed of underwater objects.
e.g. Navies use sonar to detect submarines and shipwrecks.
Formulas & Equations (3)
Tν = 1
Reads as: Time period times frequency equals one
Variables: T = time period (s), ν = frequency (Hz)
Use when: When you need to find the time period from frequency or vice versa
⚠ Students often write T = 1/ν but forget that the product Tν equals 1, not T = ν.
v = λν
Reads as: Speed equals wavelength times frequency
Variables: v = speed of sound (m/s), λ = wavelength (m), ν = frequency (Hz)
Use when: When calculating speed, wavelength, or frequency of a sound wave
⚠ Students forget to convert units (e.g., kHz to Hz) before plugging into the formula.
distance = speed × time
Reads as: Distance equals speed times time
Variables: distance (m), speed (m/s), time (s)
Use when: When calculating distance to a reflecting surface in echo problems, or distance travelled by sound
⚠ In echo problems, students forget that the time given is for the sound to travel to the object and back, so they must divide by 2 to find the one-way distance.
How It Works
Production of sound by vibration
  1. An object (e.g., rubber band, tuning fork, vocal cords) is set into periodic to-and-fro motion (vibration).
  2. The vibrating object disturbs the surrounding medium (air, water, or solid).
  3. As long as the object vibrates, sound is produced; when vibration stops, sound stops.
💡 Sound is produced only when the object is vibrating — if the vibration stops, the sound stops immediately.
Propagation of sound through a medium
  1. A vibrating source (e.g., piston) pushes air particles forward, creating a region of higher density called a compression.
  2. The source then moves backward, creating a region of lower density called a rarefaction.
  3. These compressions and rarefactions travel outward through the medium as particles collide with neighbouring particles.
  4. The particles themselves do not travel with the wave; they only oscillate about their mean positions.
  5. The disturbance (sound wave) reaches the listener's ear, where it is perceived as sound.
💡 The particles of the medium do NOT travel with the wave — only the energy and disturbance propagate.
Reflection of sound and echo formation
  1. Sound waves strike a hard, smooth surface (e.g., cliff, wall).
  2. The sound waves bounce back following the laws of reflection (angle of incidence equals angle of reflection).
  3. If the reflected sound reaches the listener after a time gap of at least 0.1 s, it is heard as a distinct echo.
  4. The minimum distance to the reflecting surface for an echo to be heard is 17 m (using speed of sound = 340 m/s).
💡 The time gap must be at least 0.1 s for an echo to be heard; if less, the brain cannot separate the original and reflected sounds.
Echolocation by bats
  1. Bats emit short bursts of ultrasonic waves (frequency > 20 kHz).
  2. These waves travel through air and reflect off objects (e.g., prey, obstacles).
  3. Bats sense the reflected echoes and analyse the time delay and direction to determine the position, distance, and size of objects.
  4. This allows bats to navigate and hunt in complete darkness.
💡 Bats use ultrasonic waves, not audible sound, for echolocation because they have higher frequency and shorter wavelength, allowing better resolution.
Sonar operation
  1. A sonar device sends ultrasonic waves into water.
  2. The waves travel through water and reflect off underwater objects (e.g., submarines, shipwrecks).
  3. The reflected waves (echoes) are detected by the sonar receiver.
  4. By measuring the time taken for the signal to return, the distance to the object is calculated using distance = (speed × time)/2.
💡 The time measured is for the round trip, so the distance to the object is half of the total distance travelled by the sound.
Exam Traps — Don't Make These Mistakes
⚠ Confusing particles with the wave
✗ In a sound wave, the air particles move from the source to the listener.
→ The particles of the medium do not travel with the wave; they only oscillate about their mean positions. It is the disturbance (compressions and rarefactions) that propagates, not the particles themselves.
✓ In a sound wave, the particles of the medium vibrate about their mean positions, while the disturbance (energy) travels from the source to the listener.
⚠ Echo distance calculation error
✗ If sound takes 0.5 s to return, the distance to the wall is 340 m/s × 0.5 s = 170 m.
→ The time given (0.5 s) is for the sound to travel to the wall and back. The one-way distance is half of that: (340 × 0.5)/2 = 85 m.
✓ Distance to the wall = (speed of sound × time taken for echo to return) / 2.
⚠ Mixing up frequency and time period
✗ If frequency is 5 Hz, the time period is 5 seconds.
→ Time period T = 1/ν, so if ν = 5 Hz, T = 1/5 = 0.2 s, not 5 s.
✓ Time period T = 1 / frequency (ν). For ν = 5 Hz, T = 0.2 s.
⚠ Confusing amplitude with loudness
✗ Loudness is directly proportional to frequency.
→ Loudness is related to amplitude, not frequency. Higher amplitude means louder sound. Frequency determines pitch, not loudness.
✓ Loudness depends on the amplitude of the sound wave; larger amplitude produces a louder sound. Pitch depends on frequency.
⚠ Thinking sound can travel through vacuum
✗ Sound can travel through outer space because astronauts can hear each other.
→ Sound requires a material medium to propagate. Outer space is a near vacuum, so sound cannot travel. Astronauts use radio devices to communicate.
✓ Sound cannot travel through vacuum because it needs a medium (solid, liquid, or gas). In space, astronauts communicate using special devices that convert sound to radio waves.
⚠ Misidentifying infrasonic and ultrasonic ranges
✗ Infrasonic waves have frequency above 20 kHz.
→ Infrasonic waves are below 20 Hz, and ultrasonic waves are above 20 kHz. The terms are opposite.
✓ Infrasonic waves: frequency < 20 Hz. Ultrasonic waves: frequency > 20 kHz.
Compare & Contrast
Density of the medium
Compression
Rarefaction
A compression is a region of higher density (above average), while a rarefaction is a region of lower density (below average) in a sound wave.
Direction of particle vibration relative to wave propagation
Longitudinal wave
Transverse wave
In a longitudinal wave, particles vibrate parallel to the direction of wave propagation (e.g., sound). In a transverse wave, particles vibrate perpendicular to the direction of wave propagation (e.g., light, waves on a string).
Definition and relationship
Frequency
Time period
Frequency (ν) is the number of oscillations per unit time (Hz), while time period (T) is the time taken for one complete oscillation (s). They are inversely related: T = 1/ν.
Time gap and perception
Echo
Reverberation
An echo is a distinct reflected sound heard when the time gap between the original and reflected sound is at least 0.1 s. Reverberation is the persistence of sound due to multiple reflections arriving in less than 0.05 s, making the sound seem to linger.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks) on numericals or definitions, 1 Long Answer (5 marks) on wave propagation or echo calculation. Total: approximately 10-12 marks.
Most asked: Sound needs a medium to propagate (vacuum bell jar experiment) · Characteristics of sound waves: wavelength, frequency, time period, amplitude, speed · Relation v = λν and numerical problems · Reflection of sound, echo, and minimum distance for echo · Audible range, infrasonic and ultrasonic waves · Applications of ultrasound (sonar, echolocation, ultrasonography)
Don't skip: Numerical problems using v = λν and echo distance calculations (3-5 marks); Explain with a diagram how sound propagates through a medium via compressions and rarefactions (5 marks); Describe the vacuum bell jar experiment to show sound needs a medium (3 marks)
One-Line Revision (read aloud before the exam)
  1. Sound is a form of energy produced by vibrating objects.
  2. Vibration is the periodic to and fro motion of an object.
  3. Sound requires a material medium (solid, liquid, or gas) to propagate; it cannot travel through vacuum.
  4. In a sound wave, compressions are regions of high density and rarefactions are regions of low density.
  5. Sound waves are longitudinal mechanical waves where particles vibrate parallel to the direction of wave propagation.
  6. Wavelength (λ) is the distance between two consecutive compressions or rarefactions.
  7. Frequency (ν) is the number of oscillations per second, measured in hertz (Hz).
  8. Time period (T) is the time for one complete oscillation; T = 1/ν.
  9. Speed of sound v = λν, and it depends on the medium, temperature, and humidity.
  10. The audible range for humans is 20 Hz to 20 kHz.
  11. Infrasonic waves have frequency below 20 Hz; ultrasonic waves have frequency above 20 kHz.
  12. Loudness depends on amplitude; pitch depends on frequency.
  13. An echo is heard when the reflected sound reaches the listener after at least 0.1 s; minimum distance for echo is 17 m.
  14. Reverberation is the persistence of sound due to multiple reflections with time gap less than 0.05 s.
  15. Bats use echolocation by emitting ultrasonic waves and sensing their echoes.
  16. Sonar uses ultrasonic waves to detect underwater objects by measuring the time for echoes to return.
  17. The vacuum bell jar experiment proves that sound cannot travel through vacuum.
  18. In a sound wave, particles oscillate about their mean positions; the wave itself travels, not the particles.

MCQs — 10 Questions with Answers

Q1. What is the main cause of sound production in musical instruments?

  • A. Vibration of strings, membranes, or air columns ✓
  • B. Heat energy released from friction
  • C. Chemical reactions inside the instrument
  • D. Electromagnetic waves produced by materials

Answer: A — Sound is produced exclusively by vibrating objects that create periodic to-and-fro motion in a medium.

Q2. Which of the following is NOT a medium through which sound can travel?

  • A. Water
  • B. Steel metal
  • C. Vacuum in space ✓
  • D. Air

Answer: C — Vacuum contains no matter, so sound cannot propagate through it; sound requires a physical medium to travel.

Q3. In Activity 10.2, when a vibrating tuning fork touches water surface, waves form because:

  • A. Water molecules generate sound independently
  • B. The vibrating prongs transfer motion to water, creating surface waves ✓
  • C. Water temperature increases and creates waves
  • D. The tuning fork material reacts chemically with water

Answer: B — Vibrating prongs directly transfer their vibratory motion to water molecules, producing visible waves on the surface.

Q4. Ramesh places his ear against a wooden desk while his friend taps it on the other side. He hears the sound loudly. This demonstrates that:

  • A. Sound travels only through air, not solids
  • B. Sound can travel through solids and is often louder due to direct contact ✓
  • C. Wooden desks generate their own sound energy
  • D. Sound is absorbed by the wood and does not reach the ear

Answer: B — Sound propagates through solid materials (wood), and direct contact allows efficient transmission with less energy loss than through air.

Q5. Why can two astronauts NOT hear each other's voices or the sound of metal clanking during a spacewalk outside the spacecraft?

  • A. Sound waves are too weak to travel far in space
  • B. The astronauts' helmets block all sound waves
  • C. Space is a vacuum with no medium for sound to propagate ✓
  • D. Cosmic radiation absorbs all sound energy

Answer: C — Sound absolutely requires a material medium (solid, liquid, or gas) to propagate; the vacuum of space has no matter.

Q6. A rubber band is stretched across a box and plucked. As soon as the vibration of the rubber band stops, the sound also stops. This observation proves that:

  • A. Sound is produced only when objects vibrate continuously
  • B. The box absorbs sound energy after vibration stops
  • C. Sound production is directly linked to and dependent on vibration ✓
  • D. Air pressure changes stop the sound propagation

Answer: C — The immediate cessation of sound when vibration ends establishes the direct causal relationship between vibration and sound production.

Q7. In the bell jar experiment, as air is pumped out, the sound of the electric bell becomes progressively fainter. What does this prove?

  • A. Sound intensity decreases with temperature changes
  • B. The bell jar material absorbs sound waves
  • C. Sound requires a medium to travel from source to ear ✓
  • D. Electric bells work only in the presence of air pressure

Answer: C — As the medium (air) is removed, sound propagation fails progressively, directly proving that sound cannot travel without a medium.

Q8. Vocal cords in humans produce sound through which mechanism?

  • A. Chemical breakdown of air molecules in the throat
  • B. Vibration of muscular flaps located in the larynx ✓
  • C. Friction between the tongue and teeth
  • D. Resonance of the mouth cavity alone

Answer: B — Vocal cords are tightly stretched muscular flaps that vibrate as air passes through them, creating sound waves.

Q9. Two metal spoons are tapped together in air and then submerged in water and tapped again. The sound is heard in both cases. Which statement best explains this observation?

  • A. Sound is produced in water but not in air
  • B. Water conducts sound better than air, so only underwater sound is audible
  • C. Sound can propagate through both air and water as both are media for sound transmission ✓
  • D. The spoons vibrate differently in water compared to air

Answer: C — Both air and water are media that conduct sound; sound travels through water to the ear via air, proving both support propagation.

Q10. Grasshoppers and crickets produce sound by rubbing their wings or legs together. This method of sound production is based on which principle? [HOTS]

  • A. Chemical energy conversion to sound in the insect body
  • B. Mechanical vibration of body parts creating sound waves ✓
  • C. Electrical impulses triggering resonance in the wings
  • D. Heat generated by friction being converted directly to sound

Answer: B — Rubbing wings or legs causes mechanical vibration of these parts, which is the fundamental mechanism for all sound production in nature.

Flashcards

What is vibration?

Vibration is the periodic to and fro motion (oscillation) of an object that produces sound.

How is sound produced in humans?

Sound is produced when vocal cords (muscular flaps in the larynx) vibrate as air passes through them.

What is a tuning fork and what does it do?

A tuning fork is a U-shaped metal instrument with prongs that vibrate when struck, producing sound and waves.

Can sound travel through solids?

Yes, sound can travel through solids, liquids, and gases—any material is a medium for sound propagation.

What is a medium in the context of sound?

A medium is any material (solid, liquid, or gas) through which sound travels or propagates from source to listener.

What is a vacuum and can sound travel through it?

A vacuum is a space with no matter; sound cannot travel through a vacuum because it requires a medium.

What does the bell jar experiment demonstrate?

The bell jar experiment shows that as air is removed from the jar, sound becomes fainter, proving sound needs a medium.

Why can astronauts not hear metal clanking during a spacewalk?

Outside Earth, there is no air (vacuum); sound cannot propagate without a medium, so they cannot hear the sound.

How do bats locate their prey in darkness?

Bats use echolocation by producing sound waves that bounce off objects and return, helping them navigate and hunt.

What happens to sound when vibration stops?

Sound stops immediately because sound is produced only by vibrating objects; no vibration means no sound production.

Important Board Questions

Define vibration and explain how it is related to sound production. Give one example from your everyday life. [2 marks]

Define vibration as periodic to-and-fro motion. State that sound is produced only by vibrating objects. Use Activity 10.1 (rubber band) or vocal cords as example.

Explain why sound cannot be heard in a vacuum. Support your answer with reference to the bell jar experiment and the concept of a medium. [3 marks]

State that sound needs a medium (solid, liquid, or gas) to propagate. In the bell jar, as air is removed, sound becomes fainter until it stops—proving medium is essential. Define vacuum as a space with no matter.

Two astronauts are repairing a spacecraft during a spacewalk in space. They cannot hear each other's voices or the sound of metal clanking. Explain this observation using the concepts of sound production, propagation, and medium. Also, describe how they actually communicate during the spacewalk. [5 marks]

Sound requires vibration (produced) and a medium to propagate; space is a vacuum with no medium. Contrast with Earth where sound travels through air. Explain that astronauts use radio communication systems (electromagnetic waves) in spacesuits, which do not require a physical medium.

Assertion-Reason Questions for Sound Waves: Characteristics and Applications

Q1. Assertion (A):

Sound is produced when objects vibrate.

Reason (R):

Vibration is the periodic to and fro motion of an object, and sound energy results from these mechanical oscillations.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true. The reason correctly explains the assertion by defining vibration and connecting it to sound production. The plucking rubber band experiment and tuning fork observations both confirm this relationship.

Q2. Assertion (A):

In the vacuum bell jar experiment, the electric bell becomes inaudible as air is removed from the jar.

Reason (R):

Sound is a form of energy that can exist without any medium, but it loses intensity in low-pressure environments.

Answer: C — A is true but R is false — The assertion is true—the bell does become inaudible as air is removed. However, the reason is false because sound cannot exist or propagate without a medium; it doesn't merely lose intensity in vacuum. The reason misunderstands the fundamental principle that sound requires a medium for propagation.

Q3. Assertion (A):

Sound can propagate through solids, liquids, and gases.

Reason (R):

Because solids, liquids, and gases all contain matter that can vibrate and transmit mechanical waves.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the assertion is true. The desk knocking activity, submerged spoon activity, and daily experience with sound in air all support this. All three states of matter can serve as mediums for sound propagation.

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Case-Based Questions (CBQ) for Sound Waves: Characteristics and Applications

Case Study:

A research team conducted an underwater communication experiment. They submerged two hydrophones (underwater microphones) at different depths in a lake. When a vibrating tuning fork was struck 50 meters away from the hydrophones underwater, the sound was clearly detected. When the same tuning fork was struck in the air above the water surface 50 meters away, no sound was detected by the underwater hydrophones. The researchers concluded that sound travels differently through different media. They then repeated the experiment with the tuning fork struck at the water surface, where the water and air meet, and detected a clear signal underwater.

Q1. Why was sound from the tuning fork in air not detected by the underwater hydrophones?

  • A. Sound cannot travel through water at all
  • B. The distance of 50 meters was too far for sound to travel in air
  • C. Sound produced in air did not effectively transfer to the water medium at that distance ✓
  • D. The hydrophones were defective and could not receive any signals

Answer: C

Q2. What does the successful detection of sound when the tuning fork was struck at the water surface demonstrate?

  • A. Sound can only travel in water, not in air
  • B. Sound can travel through both air and water when there is direct contact between the media ✓
  • C. The water surface acts as a barrier that blocks all sound from air
  • D. The tuning fork produces different frequencies when at the water surface

Answer: B

Q3. Based on this experiment, which statement is most accurate about sound propagation?

  • A. Sound travels only through solids and liquids, not gases
  • B. Sound requires a continuous medium to propagate effectively ✓
  • C. Sound travels faster in air than in water
  • D. Sound cannot travel across boundaries between different media

Answer: B

Q4. If the researchers wanted to send a message from the air to underwater divers, what would be the most effective method based on this experiment?

  • A. Shout louder from above the water surface
  • B. Use the water surface only, without submerging any equipment
  • C. Place a vibrating source in direct contact with the water to allow sound to transmit into the water ✓
  • D. Increase the frequency of the sound source for better transmission

Answer: C

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