Class 9 Science: Sound Waves: Characteristics and Applications — Chapter Question Set

(a) vibrates

(d) vibration

(d) the vocal cords

(c) larynx (voice box)

(d) grasshopper

(a) stem

(c) the prong is vibrating

(c) medium

(c) solids, liquids and gases

(c) becomes fainter and almost inaudible

(c) space is a near vacuum and sound needs a medium

(d) parallel to the direction of propagation

(d) longitudinal waves

(a) longitudinal mechanical wave

(d) light

(d) compression

(b) rarefaction

(c) energy

(d) sound energy to electrical energy

(a) electrical energy to sound energy

(d) crest

(a) minimum density

(a) two consecutive crests or troughs

(b) metre

(c) hertz

(b) ν = 1/T

(a) 5 Hz

(c) 0.2 s

(b) density compared to the average density

(c) more energy

(d) intensity

(c) v = λ × ν

(b) solids

(d) 340 m s⁻¹

(c) 4–5 times faster

(c) increases

(d) increases

(d) 20 Hz to 20,000 Hz

(d) infrasonic waves

(c) ultrasonic waves

(c) elephant

(d) bat

(d) ultrasonic waves and echoes

(c) sound navigation and ranging

(d) 0.1 s

(c) 17 m

(a) reverberation

(a) less than 0.05 s

(d) 17.2 m

(c) 1700 m away

(a) 85 m

(d) 688.5 m

(d) breaking kidney stones into small pieces

(c) ultrasonic waves

(d) decibels

vibration

medium

mechanical

wavelength

hertz (Hz)

λ × ν (wavelength × frequency)

solids

infrasonic

0.1

reverberation

echolocation

pitch

crest

amplitude

ultrasonography

False

False (they are longitudinal)

False (only energy travels; particles oscillate)

True

True

True

True

False (it increases)

True

False (they detect infrasonic waves)

True

True

False (it uses ultrasonic waves)

True

True

(a)

(c)

(a)

(a)

(a)

(d)

(d)

(a)

The Moon has no atmosphere — it is a near vacuum — and sound requires a material medium (solid, liquid or gas) to propagate. Since there are no particles to carry the vibrations, the sound of an explosion on the Moon can never reach our ears.

Sound in musical instruments is produced by (i) vibrating strings, as in a sitar or guitar, (ii) vibrating air columns, as in a bansuri (flute), and (iii) vibrating membranes, as in a tabla. Any two of these, with their examples, answer the question.

When the prong of the struck tuning fork touches the water surface, waves and splashing are seen on the water. This proves that the prongs of the tuning fork are vibrating, and since sound is always produced by vibrating bodies, it demonstrates the vibrational nature of sound.

The time period of a wave is the time taken by it to complete one full oscillation. Here, T = total time ÷ number of oscillations = 2 s ÷ 100 = 0.02 s, so the time period of the source is 0.02 seconds.

A frequency of 512 Hz means that the tuning fork completes 512 oscillations every second. Therefore, in one minute (60 s), it completes 512 × 60 = 30,720 oscillations.

The wavelength is found using the relation λ = v/ν, where v is the speed of sound and ν is the frequency. Substituting the values, λ = 344/172 = 2 m, so the wavelength of the sound wave is 2 metres.

Intensity is an objective, measurable physical quantity, defined as the sound energy passing through a unit area per unit time. Loudness, in contrast, is the subjective perception of that sound by the listener and depends on the hearing ability and sensitivity of the individual's ear.

A sound wave is called a mechanical wave because it requires a material medium (solid, liquid or gas) for its propagation. The particles of the medium vibrate about their fixed positions and collide with neighbouring particles, thereby transferring the disturbance from the source to the listener.

SONAR stands for Sound Navigation and Ranging. It sends ultrasonic waves into water and analyses the reflected waves to find the distance, direction and speed of underwater objects. One use is detecting submarines and shipwrecks or measuring the depth of the ocean.

(i) Ultrasonography uses ultrasonic waves to image internal organs without surgery, which is why it is safely used to examine unborn babies. (ii) Ultrasonic waves are also used to break kidney stones into small pieces that can pass out of the body naturally, avoiding an operation.

Curved ceilings reflect sound waves and spread them uniformly towards the audience seated in all parts of the hall. This improves the audibility of speech and music so that every listener hears the performance clearly.

Reverberation is the persistence of sound in an enclosed space due to multiple successive reflections from its walls and ceiling. It is reduced in auditoriums by using sound-absorbing materials such as panels, curtains, upholstered chairs and other soft, porous surfaces.

The distance is estimated using distance = speed × time. Substituting the values, distance = 340 × 3 = 1020 m, so the lightning struck approximately 1020 metres (about 1 km) away.

A louder sound has a larger amplitude, which means the wave carries more energy. Because of this greater energy, the wave can spread over a larger area before its intensity falls to zero, so it travels a longer distance before dying out.

The laws of reflection of sound state that (i) the direction of incidence and the direction of reflection make equal angles with the normal at the point of incidence, and (ii) the incident sound, the reflected sound and the normal at the point of incidence all lie in the same plane. These laws are identical to the laws of reflection of light.

The frequency is found using ν = v/λ = 340/0.01 = 34,000 Hz. The time period is T = 1/ν = 1/34,000 ≈ 2.94 × 10⁻⁵ s. So the wave has a frequency of 34,000 Hz and a time period of about 2.94 × 10⁻⁵ seconds.

Sound travels faster and with less energy loss in solids than in air because the particles of a solid are closely packed and transfer the disturbance efficiently. When the ear is pressed against the desk, more sound energy reaches the ear through the dense medium, making the sound appear louder.

On a humid day, the air contains more water vapour, which is lighter than dry air, and the speed of sound increases with humidity. The disturbance therefore travels more efficiently, allowing sound to be heard over a longer distance than on a dry day.

Sound reaches the two ears at slightly different times because of the distance between them. The brain compares which ear receives the sound first and uses the tiny time gap between the two signals to pinpoint the direction of the sound source.

The total distance travelled by the pulse (down and back) = v × t = 1500 × 4 = 6000 m. Since this is a round trip, the depth = 6000/2 = 3000 m, so the ocean is 3000 metres deep at that point.

Introduction: Sound is produced whenever an object vibrates, that is, moves to and fro about its mean position.
Key Points:
1. Vibration creates sound — A stretched rubber band or a struck metal object vibrates and produces sound.
2. Tuning fork activity — A tuning fork struck on a soft rubber pad is heard when brought near the ear.
3. Water surface test — Touching a vibrating prong to water makes waves, proving the prongs vibrate.
Conclusion: Sound always originates from a vibrating object, called the source of the sound.

Introduction: A sound wave is a series of alternating high and low density regions travelling through a medium.
Key Points:
1. Compression formation — A forward-moving piston pushes nearby air particles together, raising density locally.
2. Rarefaction formation — A backward-moving piston lets air spread out, creating a lower density region.
3. Alternating pattern travels — Colliding particles pass the disturbance on, while particles merely oscillate about mean positions.
Conclusion: The travelling series of compressions and rarefactions constitutes the sound wave.

Introduction: The bell jar experiment demonstrates that sound needs a material medium to propagate.
Key Points:
1. Set-up — An electric bell is switched on inside a sealed glass bell jar.
2. Removing air — As a vacuum pump sucks out air, the sound becomes fainter and almost disappears.
3. Conclusion from observation — When air returns, the sound is heard again; sound cannot travel in vacuum.
Conclusion: Sound propagation is impossible without a medium, proving sound is a mechanical wave.

Introduction: Sound is a form of energy that can move objects even without direct contact.
Key Points:
1. Grain activity — Grains are sprinkled on a rubber sheet stretched over a container.
2. Producing loud sound — A loud sound made nearby makes the sheet vibrate and the grains jump.
3. Energy transfer — The sound wave transfers energy to the sheet, showing sound carries energy.
Conclusion: Sound propagates energy through the medium's vibrating particles and their collisions.

Introduction: Three quantities describe the basic characteristics of any sound wave.
Key Points:
1. Wavelength (λ) — Distance between two consecutive crests or two consecutive troughs; SI unit is metre.
2. Frequency (ν) — Number of density oscillations per unit time at a fixed point; SI unit is hertz.
3. Time period (T) — Time taken for one complete oscillation; frequency is the reciprocal, ν = 1/T.
Conclusion: These quantities are connected by v = λν, giving the speed of the wave.

Introduction: The amplitude of a sound wave determines how much energy the wave carries.
Key Points:
1. Amplitude definition — Maximum change in density in a compression or rarefaction from the average density.
2. Amplitude and energy — A wave of larger amplitude carries more energy than one of smaller amplitude.
3. Intensity and distance — Energy spread over a larger area with distance, so intensity decreases away from the source.
Conclusion: Striking a source harder gives larger amplitude, more energy and hence higher intensity.

Introduction: The speed of sound depends on the medium through which it travels.
Key Points:
1. Order in media — Sound travels fastest in solids, slower in liquids and slowest in gases.
2. Typical values — Approximately 5000 m s⁻¹ in steel, 1500 m s⁻¹ in water and 340 m s⁻¹ in air.
3. Air factors — Speed in air increases with temperature and with humidity of the air.
Conclusion: Closer packing of particles in denser media allows faster transfer of the disturbance.

Introduction: Pitch and loudness describe how humans subjectively perceive sound.
Key Points:
1. Pitch and frequency — Higher frequency sounds are perceived as high pitch, like a whistle or siren.
2. Loudness and amplitude — Larger amplitude is perceived as louder sound, smaller amplitude as softer.
3. Subjective nature — Both depend on the listener's hearing ability, unlike measurable physical properties.
Conclusion: Pitch relates to frequency, loudness relates to amplitude; together with timbre they characterise perception.

Introduction: Humans can hear only a limited range of frequencies.
Key Points:
1. Audible range — From 20 Hz to 20,000 Hz; it varies with age and person.
2. Infrasonic waves — Frequencies below 20 Hz; elephants can detect them.
3. Ultrasonic waves — Frequencies above 20 kHz; dogs, bats and dolphins detect them.
Conclusion: Sounds outside 20 Hz to 20 kHz exist but are inaudible to human beings.

Introduction: An echo is the reflection of sound that we hear separately after the original sound.
Key Points:
1. Condition for echo — The time gap between original and reflected sound must be at least 0.1 s.
2. Distance calculation — Sound travels 340 × 0.1 = 34 m in 0.1 s from source to reflector and back.
3. Minimum distance — Half of 34 m, that is about 17 m, is the minimum reflecting distance.
Conclusion: Reflecting surfaces must be at least 17 m away for a distinct echo to be heard.

Introduction: Ultrasonic waves, being inaudible and highly directional, have many practical applications.
Key Points:
1. Medical uses — Ultrasonography images internal organs; ultrasound breaks kidney stones into pieces.
2. Industrial uses — Cleaning delicate machine parts, welding, and detecting defects inside metal blocks.
3. Detection uses — Locating objects by reflected ultrasonic waves, as in sonar for underwater objects.
Conclusion: Ultrasonic waves help detect, measure and treat where ordinary methods fail.

Introduction: Echolocation is locating objects using reflected sound waves.
Key Points:
1. Bats' method — Bats emit short bursts of ultrasonic waves which reflect off obstacles and prey.
2. Sensing echoes — From returning echoes bats judge position, distance and shape in darkness.
3. Human adaptation (sonar) — Sonar sends ultrasonic waves into water; reflected waves reveal distance, direction and speed.
Conclusion: Dolphins, whales and some birds also echolocate; humans copy the principle in sonar.

Introduction: Reverberation is the persistence of sound after the source stops, due to multiple reflections.
Key Points:
1. Cause — Reflected sounds arrive within about 0.05 s and overlap with the original sound.
2. Undesirable effect — Excessive reverberation garbles speech and music in large halls.
3. Control measures — Sound-absorbing panels, upholstered chairs, curtains and porous surfaces reduce it.
Conclusion: Well-designed halls balance reverberation so audiences everywhere hear clear sound.

Introduction: The ear converts sound energy into electrical signals that the brain interprets.
Key Points:
1. Eardrum vibration — Incoming sound makes the thin eardrum membrane vibrate.
2. Amplification — Tiny bones in the middle ear amplify these vibrations.
3. Signal conversion — The cochlea converts vibrations into electrical signals that rush to the brain.
Conclusion: The brain perceives these signals as sound; two ears help locate its direction.

Introduction: The graph of a sound wave directly gives the wavelength, from which other quantities follow.
Key Points:
1. Wavelength — Distance between consecutive crests, λ = 3.44 m.
2. Frequency — ν = v/λ = 344/3.44 = 100 Hz.
3. Time period — T = 1/ν = 1/100 = 0.01 s.
Conclusion: The wave has wavelength 3.44 m, frequency 100 Hz and time period 0.01 s.

(a) As air is pumped out, fewer and fewer air particles remain to carry the disturbance, so less sound energy reaches the ear and the bell sounds fainter.
(b) If air is let back in, the sound gradually becomes as loud as before, since the medium needed to propagate it is restored.
(c) The experiment concludes that sound needs a material medium — it is a mechanical wave and cannot propagate through a vacuum.
(d) This principle applies in outer space, which is a vacuum; astronauts therefore use special radio devices fitted into their spacesuits to communicate.

(a) The flash is seen first because light travels enormously faster than sound — light reaches the eye almost instantly while sound takes several seconds to arrive.
(b) Distance = speed of sound × time delay = 340 × 5 = 1700 m, which is about 1.7 km.
(c) The strike would be farther away, because distance is directly proportional to the time delay — doubling the delay doubles the distance.
(d) The speed of light is about 300,000 km s⁻¹, so it covers 1700 m in roughly five-millionths of a second, making its travel time negligible compared with that of sound.

(a) An echo is a sound that is reflected from a surface and heard separately from the original sound.
(b) Distance = (v × t)/2 = (340 × 0.5)/2 = 85 m, since the sound travels to the wall and back in 0.5 s.
(c) In the small bedroom, the reflected sound returns in less than 0.1 s, so the brain cannot separate it from the original sound, and no echo is heard.
(d) The reflected sound must return after a minimum gap of about 0.1 s for the human ear to perceive it as a distinct echo.

(a) Ultrasonic waves are highly directional and can locate objects precisely by reflection, whereas ordinary audible sound spreads out and cannot pinpoint targets.
(b) The recorded time includes the pulse travelling to the submarine and back again, so it must be halved to get the one-way distance.
(c) Distance = (1530 × 0.90)/2 = 1377/2 = 688.5 m, so the submarine is about 688.5 metres away.
(d) The same principle is used to measure ocean depth and locate shipwrecks; bats also use echolocation to navigate and find prey.

(a) The 40 kHz waves lie above the upper audible limit of about 20 kHz for humans, so they are ultrasonic and cannot be heard by the driver.
(b) Time = 2d/v = (2 × 1.2)/345 ≈ 0.007 s, which is the round-trip time of the wave.
(c) The sensor measures the echo time and computes the distance using distance = (speed × time)/2, giving the distance to the obstacle.
(d) A bat uses exactly this principle — it emits ultrasonic waves and locates obstacles and prey from their echoes.

(a) Time through the steel fence = distance ÷ speed = 340/5000 = 0.068 s.
(b) Time through air = 340/340 = 1.0 s.
(c) The time difference = 1.0 − 0.068 = 0.932 s.
(d) Yes, since the gap of 0.932 s is much larger than the minimum 0.1 s needed, she hears the two sounds clearly as separate signals.

(a) Noise is unwanted or unpleasant sound that causes discomfort or disturbance to the listener.
(b) Loudness is commonly measured in the unit called the decibel (dB).
(c) Prolonged exposure to loud sound can cause hearing loss and adversely affects health, sleep and overall hearing ability.
(d) A hearing aid helps such people; it contains a microphone, an amplifier and a speaker that together make sounds louder for the ear.

(a) The depiction is wrong because no sound of the explosion could actually be heard in space — only the flash of light would be seen.
(b) Outer space is a near vacuum, and sound is a mechanical wave that needs a material medium to travel, so it cannot propagate through space.
(c) Astronauts communicate during spacewalks through special radio devices fitted into their spacesuits, which transmit signals through space.
(d) Electromagnetic waves such as light and radio waves can travel through space because they do not require any medium.

(a) Pitch is directly related to frequency — the higher the frequency of a sound, the higher is its perceived pitch.
(b) Each musical note sounds different because each note has its own characteristic frequency, which our ear perceives as a distinct pitch.
(c) The interval between two notes where one has exactly double the frequency of the other is called an octave.
(d) A tuning fork produces nearly single-frequency sound, which is why it is used as a standard reference; whistling is another example.

(a) Wave B has the greater wavelength, since its crests are farther apart — the distance between two successive crests is larger.
(b) Wave B has the greater amplitude because of its higher peaks; larger amplitude means the wave carries more energy and sounds louder.
(c) Wave A will sound higher pitched, because its shorter wavelength means a higher frequency (v = λ × ν), and higher frequency is heard as higher pitch.
(d) The formula connecting the three quantities is v = λ × ν, where v is the speed, λ the wavelength and ν the frequency of the wave.

Introduction: Sound is a form of energy produced by vibrating objects and carried through media by travelling disturbances.
Key Points:
1. Production by vibration — Every sound originates from a vibrating object like a string, membrane or air column.
2. Human voice — Vocal cords in the larynx vibrate; tongue, lips and nasal cavity shape it into speech.
3. Propagation mechanism — Particles collide with neighbours and pass the disturbance onward without travelling themselves.
4. Compressions and rarefactions — Alternating high and low density regions move away from the vibrating source.
5. Medium requirement — Sound travels through solids, liquids and gases but never through vacuum.
Conclusion: Sound is a disturbance of the medium's particles, started by a vibration and carried as energy.

Introduction: A sound wave is described by wavelength, frequency, time period and amplitude, all seen on a density–distance graph.
Key Points:
1. Wavelength (λ) — Distance between two consecutive crests or troughs; measured in metres.
2. Frequency (ν) — Number of density oscillations per second at a fixed point; measured in hertz.
3. Time period (T) — Time for one complete oscillation; related to frequency by ν = 1/T.
4. Amplitude — Maximum change in density from the average; determines the energy carried.
5. Graph features — Crests mark maximum density (compressions) and troughs mark minimum density (rarefactions).
Conclusion: These quantities together determine the wave's speed (v = λν), pitch and loudness.

Introduction: The speed of sound depends on the medium through which it propagates and on air conditions.
Key Points:
1. Dependence on medium — Sound travels fastest in solids, slower in liquids and slowest in gases.
2. Numerical comparison — About 5000 m s⁻¹ in steel, 1500 m s⁻¹ in water, 340 m s⁻¹ in air.
3. Effect of temperature — Speed in air rises from about 331 m s⁻¹ at 0 °C to 344 m s⁻¹ at 22 °C.
4. Effect of humidity — More moisture in air increases the speed of sound.
5. Independence from frequency — In most media speed depends only on the medium, not on the source frequency.
Conclusion: Particles packed more closely and energetic transfer conditions make sound travel faster in some media.

Introduction: Echo and reverberation both arise from the reflection of sound from surfaces.
Key Points:
1. Echo formation — Sound reflects off a hard, distant surface and returns to the listener separately.
2. Condition for echo — The reflected sound must arrive at least 0.1 s after the original.
3. Minimum distance — At 340 m s⁻¹ the reflector must be at least 17 m away from the listener.
4. Reverberation — Multiple reflections arriving within 0.05 s make sound persist after the source stops.
5. Control of reverberation — Sound-absorbing panels, curtains and upholstered furniture reduce it in halls.
Conclusion: Smooth hard surfaces strengthen echoes while soft porous surfaces absorb sound and clear it.

Introduction: Ultrasonic waves are sound waves with frequencies above 20 kHz, beyond human hearing.
Key Points:
1. Medical imaging — Ultrasonography images internal organs of the body without surgery.
2. Treatment — Ultrasound breaks kidney stones into small pieces that pass out of the body.
3. Industry — Used for cleaning delicate machine parts, welding and detecting defects inside metal blocks.
4. Navigation — Sonar uses reflected ultrasonic waves to find underwater objects and ocean depth.
5. Nature's users — Bats, dolphins and whales use ultrasonic echolocation to hunt and navigate.
Conclusion: Though inaudible to us, ultrasonic waves serve medicine, industry and navigation powerfully.

Introduction: SONAR (Sound Navigation And Ranging) locates underwater objects using reflected ultrasonic waves.
Key Points:
1. Transmission — A sonar device sends an ultrasonic pulse into seawater from a ship.
2. Reflection — The pulse strikes an object like a submarine or the seabed and reflects back.
3. Detection and calculation — The echo time t is measured; distance = (v × t)/2 for speed v.
4. Applications — Determines distance, direction and speed of submarines, shipwrecks and ocean depth.
5. Example — Echo returns in 0.90 s at 1530 m s⁻¹; distance = (1530 × 0.90)/2 = 688.5 m.
Conclusion: Sonar adapts bat echolocation to human underwater exploration and defence.

Introduction: The human ear converts sound vibrations into electrical signals interpreted by the brain.
Key Points:
1. Eardrum — Sound entering the ear makes this thin membrane vibrate.
2. Amplification by bones — Tiny middle-ear bones quickly amplify the eardrum's vibrations.
3. Cochlea — Converts the amplified vibrations into electrical signals that rush to the brain.
4. Perception — The brain perceives these electrical signals as sound.
5. Direction sensing — Two ears let the brain compare arrival times and locate the sound's origin.
Conclusion: This delicate chain from eardrum to cochlea makes our remarkable sense of hearing possible.

Introduction: Human perception of sound involves subjective quantities pitch and loudness, distinct from measured physical quantities.
Key Points:
1. Pitch perception — Higher frequencies are heard as shrill, high-pitch sounds like sirens.
2. Loudness perception — Larger amplitudes are heard as louder sounds, smaller ones as softer.
3. Loudness and distance — Loudness decreases as we move away from the source.
4. Noise pollution — Unwanted, harmful sound; exposure above limits affects health, sleep and hearing.
5. Protection — Hearing loss is tested with audiograms; hearing aids with microphone, amplifier and speaker help.
Conclusion: Loudness is measured in decibels; normal conversation is about 60 dB and loud sounds exceed 100 dB.

Introduction: Mechanical waves are classified by the direction of particle vibration relative to propagation.
Key Points:
1. Longitudinal waves — Particles vibrate parallel to the direction of wave propagation.
2. Sound is longitudinal — Air particles vibrate back and forth along the direction the disturbance travels.
3. Transverse waves — Particles vibrate perpendicular to the direction of propagation.
4. Examples — Light is transverse; seismic waves may be longitudinal or transverse.
5. Common feature — Both types are mechanical if they need a medium; light needs none.
Conclusion: The piston-and-tube model shows sound clearly as a longitudinal mechanical wave.

Introduction: The wave equation v = λν links speed, wavelength and frequency in any medium.
Key Points:
1. Given data — Wavelength λ = 3.44 m; speed in air v = 344 m s⁻¹.
2. Frequency in air — ν = v/λ = 344/3.44 = 100 Hz.
3. Time period — T = 1/ν = 1/100 = 0.01 s.
4. Wavelength in water — λ' = v'/ν = 1500/100 = 15 m.
5. Interpretation — The same frequency gives a longer wavelength in water due to higher speed.
Conclusion: Frequency depends on the source, while wavelength and speed depend on the medium.