Close your eyes for a moment and listen. The hum of a fan, the chirping of birds, a conversation in the next room, the rumble of traffic outside — we live immersed in a world of sound. Yet how does sound actually travel from a source to our ears? Why does the pitch of an ambulance siren change as it passes you? Why does your voice echo in an empty hall but not in a furnished room? Why can bats navigate in pitch darkness? All of these questions are answered by the physics of sound waves.
Sound is a mechanical, longitudinal wave — it requires a material medium (solid, liquid, or gas) to travel, and the particles of the medium vibrate back and forth in the same direction as the wave travels, creating alternating regions of compression (high pressure) and rarefaction (low pressure). Unlike light, sound cannot travel through vacuum: there is no sound in space.
For CBSE Class 9 Physics Chapter 12 (Sound), this is a crucial topic that appears in every board examination and forms the foundation for understanding acoustics, music, medical imaging, and sonar technology. This guide covers all subtopics: production and propagation of sound, wave parameters (frequency, wavelength, amplitude, time period, speed), characteristics of sound (pitch, loudness, timbre), speed of sound in different media, reflection of sound (echo, reverberation), range of hearing, infrasonic and ultrasonic waves, SONAR, and medical uses of ultrasound.
- 1. Production and Propagation of Sound
- 2. Wave Parameters — Frequency, Wavelength, Amplitude, Speed
- 3. Characteristics of Sound — Pitch, Loudness, Timbre
- 4. Speed of Sound — Values and Factors
- 5. Reflection of Sound — Laws and Applications
- 6. Echo and Reverberation
- 7. Range of Hearing — Infrasonic, Audible, Ultrasonic
- 8. SONAR — Sound Navigation and Ranging
- 9. Applications of Ultrasound
- 10. Solved Numerical Problems
- 11. Frequently Asked Questions (FAQ)
1. Production and Propagation of Sound
Examples of vibrating sources:
• Vocal cords: When you speak, air from the lungs makes the vocal cords vibrate, producing sound.
• Tuning fork: When struck, the prongs vibrate at a specific frequency, producing a pure tone.
• Guitar/sitar string: Plucking makes the string vibrate, producing sound.
• Drum skin: Hitting the drum skin makes it vibrate, producing a deep sound.
• Bell: Striking a bell makes the metal vibrate at its natural frequency.
Propagation (How sound travels):
Sound is a mechanical longitudinal wave. Here is how it propagates through air:
(1) When an object vibrates, it pushes the adjacent air molecules forward, creating a compression (region of high pressure, high density).
(2) As the object moves back, the air molecules spread apart, creating a rarefaction (region of low pressure, low density).
(3) These alternating compressions and rarefactions travel outward in all directions as a sound wave.
(4) The air molecules themselves do NOT travel with the wave — they only vibrate about their mean positions. It is the disturbance (the pattern of compressions and rarefactions) that moves.
Sound requires a medium: Sound cannot travel through vacuum (no particles to vibrate). Astronauts in space cannot speak to each other without radio communication — there is no medium for sound in the vacuum of space.
2. Wave Parameters
Also: T = 1/f (Time period in seconds, where f is in Hz)
And: v = λ/T (Speed = Wavelength / Time period)
In air at 20°C: v ≈ 343 m/s ≈ 340 m/s (use 340 m/s in CBSE problems unless specified)
3. Characteristics of Sound
The same physical parameters (frequency, amplitude, wavelength) are perceived by humans as subjective qualities of sound. There are three perceptual characteristics of sound that you must distinguish clearly:
4. Speed of Sound — Values and Factors Affecting It
| Medium | Speed of Sound | Relative Speed |
|---|---|---|
| Air (0°C) | 331 m/s | Slow (reference) |
| Air (20°C) | 343 m/s ≈ 340 m/s | Slow |
| Water (25°C) | 1,498 m/s ≈ 1500 m/s | ~4.4× faster than air |
| Sea water | ~1,520 m/s | ~4.5× faster than air |
| Steel / Iron | ~5,100–5,900 m/s | ~15–17× faster than air |
| Glass | ~4,500–5,300 m/s | ~13–15× faster than air |
| Wood (Oak) | ~3,800 m/s | ~11× faster than air |
| Vacuum | 0 m/s | Sound cannot travel |
2. Density of medium: Sound travels faster in denser media (liquids and solids) than in gases, because molecules are closer together and collide more frequently, transmitting the vibration faster.
3. Elasticity of medium: Sound travels faster in more elastic media (like steel) because elastic restoring forces are stronger, transmitting the disturbance more quickly.
4. Humidity: Moist (humid) air has slightly lower density than dry air (water vapour is lighter than nitrogen/oxygen). So sound travels slightly faster in humid air than in dry air at the same temperature.
Speed does NOT depend on: Frequency, amplitude, or wavelength of the sound wave (all frequencies of sound travel at the same speed in a given medium — no dispersion for sound in air).
5. Reflection of Sound — Laws and Applications
First Law: The angle of incidence (angle between incident sound ray and normal to the surface) equals the angle of reflection (angle between reflected sound ray and normal): ∠i = ∠r.
Second Law: The incident sound ray, the reflected sound ray, and the normal to the reflecting surface at the point of incidence all lie in the same plane.
Conditions for good reflection:
• The reflecting surface should be large compared to the wavelength of sound.
• Hard, smooth surfaces (concrete, marble, plaster, metal) reflect sound well.
• Soft, porous surfaces (curtains, carpets, foam) absorb sound and give poor reflection.
Why do we hear echoes from hard walls and hills but not from trees? Hard, large surfaces reflect sound efficiently. Trees and shrubs are soft, irregular, and porous — they absorb and scatter sound rather than reflecting it clearly.
6. Echo and Reverberation
• The reflected sound must reach your ears at least 0.1 second (1/10th of a second) after the direct sound. This is because the persistence of hearing (the time the ear retains a sound impression) is about 0.1 s.
• For sound to travel to the reflecting surface and back in 0.1 s:
Total distance = v × t = 340 × 0.1 = 34 m
Distance to reflecting surface = 34/2 = 17 m
So the reflecting obstacle must be at least 17 metres away for an echo to be heard.
Reverberation: When a sound is produced in a large enclosed space (concert hall, auditorium, cathedral), multiple reflections arrive at the listener’s ears in very rapid succession (less than 0.1 s apart). These overlapping reflections are not heard as separate echoes — instead, the original sound appears to be prolonged. This effect is called reverberation.
How architects control reverberation: Concert halls are designed with optimal reverberation time (typically 1.5–2 seconds for music, shorter for speech). Too little reverberation (over-damped) sounds “dead”; too much sounds muddled. Soft furnishings (seats, curtains, carpets), sound-absorbing panels, and curved reflective surfaces are used to fine-tune reverberation.
7. Range of Hearing — Infrasonic, Audible, Ultrasonic
Animals that use infrasound:
• Elephants: communicate over 10+ km using infrasonic calls (< 20 Hz) through the ground and air.
• Whales: blue and fin whales produce infrasonic calls (10–40 Hz) for long-distance communication across ocean basins.
• Before earthquakes, some animals (dogs, cats, elephants) detect infrasonic P-waves and flee before the destructive S-waves arrive — an early warning system.
Within the audible range:
• Low frequencies (20–250 Hz): bass sounds — rumbling, drums, bass guitar
• Mid frequencies (250–4000 Hz): speech, most music instruments
• High frequencies (4000–20,000 Hz): treble sounds — whistle, flute, piccolo
Note: Range of hearing decreases with age. Elderly people often cannot hear above 8,000–12,000 Hz.
Animals that use ultrasound:
• Bats: emit ultrasonic pulses (50,000–100,000 Hz) and detect echoes to navigate and hunt insects in complete darkness (echolocation).
• Dolphins: use ultrasound for echolocation and communication underwater.
• Dogs: can hear up to 65,000 Hz (dog whistles use ultrasound).
• Moths: some species can detect bat ultrasound to evade predation.
8. SONAR — Sound Navigation and Ranging
Working principle:
(1) A SONAR device (transducer) emits a pulse of ultrasonic waves downward (or in the desired direction) into the water.
(2) The ultrasonic pulse travels through water, strikes a solid object (the ocean floor, a submarine, a school of fish, a wreck), and is reflected back as an echo.
(3) The transducer detects the returning echo and measures the time delay (t) between emission and reception.
(4) Distance = Speed × Time/2 = v × t/2 (divide by 2 because the sound travels to the object AND back).
Why ultrasound (not audible sound) for SONAR?
• Ultrasound has shorter wavelength → better resolution and more precise detection of small objects.
• Ultrasound can be directed in narrow beams (like a torch beam) for precise directional measurement.
• Ultrasound travels farther in water before being absorbed (compared to visible light, which is absorbed within metres in seawater).
Applications of SONAR:
• Submarine detection: Naval vessels use SONAR to detect enemy submarines.
• Ocean depth measurement (echo sounding): Oceanographic ships map the ocean floor using SONAR.
• Fish detection: Fishing boats use SONAR to locate large schools of fish.
• Underwater archaeology: Locating sunken ships and underwater ruins.
• Navigation: Mapping underwater obstacles and navigating submarines.
• Geological surveys: Mapping underwater geological formations and oil deposits.
9. Applications of Ultrasound
10. Solved Numerical Problems
Time = Total distance / Speed = 680 / 340
Range: 500 Hz is between 20 Hz and 20,000 Hz.
Since sound travels to wall and back: Minimum distance to wall = 34/2 = 17 m
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11. Frequently Asked Questions (FAQ)
Sound is a form of energy produced by vibrating objects. It is a mechanical, longitudinal wave that travels through a material medium (solid, liquid, or gas).
Production: Sound is produced when an object vibrates (moves rapidly back and forth). Examples: vibrating vocal cords, guitar strings, drum skin, tuning fork prongs.
Propagation: The vibrating object pushes adjacent air molecules, creating a compression (high pressure). When it moves back, it creates a rarefaction (low pressure). This pattern of alternating compressions and rarefactions travels outward as a sound wave. The air molecules themselves don’t travel — only the disturbance pattern moves.
Why sound cannot travel in vacuum: Sound needs a material medium (particles to vibrate). In vacuum, there are no particles, so no sound can propagate. This is why astronauts use radios in space and why outer space is completely silent.
Pitch: Pitch is the perceptual quality that distinguishes a high note from a low note. Pitch depends on the frequency of the sound wave: higher frequency = higher pitch. A flute produces high-pitch sound (high frequency); a bass drum produces low-pitch sound (low frequency). Female voices are generally higher-pitched than male voices. Unit of frequency: Hertz (Hz).
Loudness: Loudness is the perceptual quality that distinguishes a loud sound from a soft (quiet) sound. Loudness depends on the amplitude of the sound wave: greater amplitude = greater loudness. A shout has much greater amplitude than a whisper. Loudness is measured in Decibels (dB). Threshold of hearing: 0 dB; normal conversation: 60 dB; pain threshold: 120 dB; jet engine: 140 dB.
Summary: Pitch = determined by frequency (f). Loudness = determined by amplitude (A) — specifically loudness ∝ A². Two sounds can have the same pitch but different loudness (same f, different A), or same loudness but different pitch (same A, different f).
Echo: An echo is a reflected sound that is heard distinctly after the original sound has stopped. For a sound to be heard as a separate echo, the reflected sound must reach the listener at least 0.1 seconds after the original sound (because the persistence of sound in the human ear is about 0.1 s — the ear retains the impression of a sound for 0.1 s after it stops).
Minimum distance calculation:
For the reflected sound to reach the ear at least 0.1 s after the original:
Total distance (to reflecting surface + back) = v × t = 340 × 0.1 = 34 m
Distance to reflecting surface = 34/2 = 17 m
Conditions for a good echo:
(1) The reflecting surface must be large (compared to the wavelength of sound).
(2) The reflecting surface must be hard and smooth (stone wall, cliff, building).
(3) The distance from the source to the reflector must be at least 17 m.
(4) The original sound must be sharp and brief (not a continuous sound).
SONAR (Sound Navigation And Ranging) is a system that uses ultrasonic waves to detect, locate, and measure the distance of objects underwater.
Working principle:
(1) A transducer emits a pulse of ultrasonic waves (high frequency, short wavelength) into the water.
(2) The wave travels through water until it strikes a solid object (ocean floor, submarine, rock, school of fish).
(3) The wave reflects back (echo) and is detected by the same transducer.
(4) The time delay (t) between emission and reception is measured.
(5) Depth or distance: d = v × t / 2 (dividing by 2 since sound travels there and back).
Why ultrasound? Ultrasound has short wavelength, giving better resolution and the ability to be directed as a narrow beam for precise measurements.
Applications:
(1) Detecting submarines by naval vessels.
(2) Measuring ocean depth (echo sounding) — creating ocean floor maps.
(3) Locating schools of fish for fishing boats.
(4) Detecting underwater obstacles for submarine navigation.
(5) Geological surveys of underwater oil and mineral deposits.
(6) Locating sunken ships and archaeological underwater sites.
Range of hearing for humans: 20 Hz to 20,000 Hz (20 kHz). Sounds within this frequency range are called audible sounds. The range decreases with age — elderly people often cannot hear above 10,000–12,000 Hz.
Infrasonic sound (infrasound): Sound with frequency less than 20 Hz. Humans cannot hear infrasound but may feel it as vibrations.
• Animals: elephants communicate over distances of 10+ km using infrasonic calls. Some animals sense infrasound from earthquakes and flee before disaster strikes.
• Natural sources: earthquakes, volcanic eruptions, ocean waves, atmospheric turbulence.
Ultrasonic sound (ultrasound): Sound with frequency greater than 20,000 Hz (20 kHz). Humans cannot hear ultrasound.
• Animals: bats (up to 100 kHz), dolphins, dogs (up to 65 kHz).
• Applications: medical ultrasonography, SONAR, industrial cleaning, crack detection in metals, lithotripsy (kidney stone removal), ultrasonic welding, humidifiers.
Summary:
• Infrasonic: f < 20 Hz (below human hearing)
• Audible: 20 Hz ≤ f ≤ 20,000 Hz (human hearing range)
• Ultrasonic: f > 20,000 Hz (above human hearing)
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