Sound

Note: This chapter was removed from the NCERT curriculum in the 2022 rationalization. Retained here as sound, SONAR, infrasound/ultrasound detection, and noise pollution are directly relevant to GS3 topics on submarine technology, disaster early warning, and environmental pollution.

Sound is far more than what we hear — it is the basis of submarine warfare (SONAR), nuclear test detection (CTBTO infrasound arrays), medical imaging (ultrasound/sonography), and earthquake early warning. UPSC GS3 tests submarine detection technology, India's naval power, nuclear non-proliferation (CTBT), and noise pollution policy. All trace back to the physics of this chapter.

PART 1 — Quick Reference Tables

Sound Spectrum — Frequency Ranges

Speed of Sound in Different Media

Note: Speed of sound increases with temperature (faster in warm air); and increases from gas → liquid → solid (denser, more elastic medium = faster sound)

Noise Levels — Decibel Reference Scale

PART 2 — Detailed Notes

1. Production and Propagation of Sound

Sound is produced by vibrating objects. A vibrating tuning fork compresses and rarefies the air around it alternately — creating a longitudinal wave (compressions and rarefactions travel in the same direction as the wave propagation).

Sound requires a medium — it cannot travel through vacuum. On the Moon (no atmosphere), astronauts cannot communicate by voice — they use radio waves (electromagnetic waves, which do NOT require a medium). This is the fundamental difference between sound and light.

Sound wave properties:

• Wavelength (λ): Distance between two consecutive compressions (or rarefactions)
• Frequency (f): Number of vibrations per second; unit: Hertz (Hz)
• Amplitude (A): Maximum displacement of particles from their rest position; determines loudness
• Wave speed: v = f × λ; speed depends on medium and temperature, NOT on frequency or amplitude

2. Characteristics of Sound

Loudness (Volume):

• Determined by amplitude of vibration
• Measured in decibels (dB) — a logarithmic scale (10 dB increase = 10× the sound intensity)
• Sustained exposure to 85 dB+ → hearing damage (cochlear hair cell destruction — irreversible)

Pitch:

• Determined by frequency — higher frequency = higher pitch
• A child's voice has higher frequency (pitch) than an adult male voice
• Musical instruments are tuned by adjusting the frequency of vibration (tension of strings, length of air column)

Quality (Timbre):

• What distinguishes a guitar note from a piano note at the same pitch and loudness
• Determined by the waveform shape — the mix of fundamental frequency and overtones (harmonics)

3. Reflection of Sound — Echo and Reverberation

Reflection of sound follows the same laws as reflection of light: angle of incidence = angle of reflection.

Echo: A reflected sound that is heard distinctly separately from the original sound. For an echo to be heard, the reflecting surface must be at least 17.2 metres away (so that reflected sound reaches the ear at least 0.1 seconds after the original — the persistence of sound in human ear is ~0.1 s).

Reverberation: Multiple reflections in rapid succession in an enclosed space → prolonged, lingering sound. Used beneficially in concert halls (adds richness to music — controlled reverberation); problematic in conference halls (makes speech unclear). Managed by acoustic panels (sound-absorbing materials on walls and ceilings).

4. SONAR — Sound Navigation And Ranging

SONAR uses the principle of echo — emitting a sound pulse and timing the return of the reflection to determine the distance of an object.

Distance = (Speed of sound in water × Time for echo to return) / 2

(Divide by 2 because the sound travels to the object and back — twice the distance)

5. Infrasound — Below Human Hearing, Above Human Detection

Infrasound (< 20 Hz) travels very long distances (thousands of kilometres) with little attenuation because low-frequency waves are not easily absorbed by the atmosphere.

Natural sources of infrasound:

• Earthquakes (P-waves are infrasound range)
• Volcanic eruptions (Krakatoa 1883 — infrasound heard 5,000 km away)
• Meteorite impacts
• Ocean waves and surf
• Severe thunderstorms, tornadoes

Animals that use infrasound:

• Elephants communicate over tens of kilometres using infrasound rumbles (20–30 Hz) — through the ground as well as air
• Blue whales communicate across ocean basins using infrasound (10–40 Hz)
• Some scientists believe birds and migrating animals use infrasound for navigation (sensing topographic infrasound patterns)

6. Ultrasound — Applications

Ultrasound (> 20 kHz) is used widely because of its ability to penetrate materials and reflect from boundaries.

Medical applications:

• Sonography (Ultrasonography): 2–18 MHz sound waves; safe (no ionizing radiation); images soft tissues; used for abdominal imaging, pregnancy monitoring, cardiac (echocardiography), guided biopsies
• Lithotripsy: Focused ultrasound to break kidney stones (calculi) without surgery — Extracorporeal Shock Wave Lithotripsy (ESWL)
• Physiotherapy: Low-intensity ultrasound promotes tissue healing

7. Musical Instruments — Vibration Physics

Musical instruments produce sound through controlled vibration:

• Stringed instruments (chordophones): Vibrating string (sitar, veena, violin, guitar, piano). Frequency depends on: string tension (higher tension = higher pitch), string length (shorter = higher pitch), string mass/thickness (thinner = higher pitch). Frets on sitar/guitar change effective string length.

• Wind instruments (aerophones): Vibrating air column (flute, bansuri, shehnai, trumpet, oboe). Frequency depends on length of air column (open/closed holes change effective length). India's classical instruments — the bansuri (bamboo flute) and shehnai (double-reed aerophone) — are UNESCO recognized as part of intangible cultural heritage.

• Percussion instruments (membranophones/idiophones): Vibrating membrane (tabla, dholak, mridangam) or solid object (ghatam, kanjira). Tabla's complex tonal quality comes from the special paste (syahi — iron filings and flour) on the drumhead that creates multiple overtones; studied by physicists as a uniquely sophisticated percussion design.

8. Noise Pollution

Unwanted sound is noise pollution — a serious environmental and occupational health hazard.

Health effects of noise pollution:

• Hearing damage (NIHL — Noise-Induced Hearing Loss): Permanent at 85+ dB sustained; cochlear hair cells destroyed; irreversible
• Cardiovascular effects: Chronic noise exposure linked to hypertension, heart disease (via stress hormone activation — cortisol, adrenaline)
• Psychological effects: Sleep disturbance, anxiety, impaired concentration, reduced productivity
• Wildlife impact: Disrupts bird communication and navigation; affects marine mammals (SONAR controversy — naval active sonar linked to beaching of whales)

India's noise pollution regulatory framework:

• Noise Pollution (Regulation and Control) Rules, 2000 — framed under the Environment (Protection) Act, 1986
• Standards: Day (6 AM – 10 PM): Residential: 55 dB; Commercial: 65 dB; Industrial: 75 dB; Silence zones (hospitals, schools, courts): 50 dB. Night: 5 dB lower in each category
• Silence zones: 100-metre radius around hospitals, educational institutions, and courts — no honking, no loudspeakers
• Loudspeakers/PA systems: Regulated; firecrackers banned between 10 PM and 6 AM (Supreme Court order, 2018)
• Central Pollution Control Board (CPCB): Monitors noise levels; has Noise Monitoring Network in major cities
• Occupational Safety, Health and Working Conditions Code, 2020: Mandates hearing protection for workers exposed to 85+ dB

Exam Strategy

Prelims traps:

• Sound is a longitudinal wave (NOT transverse); it requires a medium; cannot travel in vacuum
• Speed of sound: air < water < solid — often reversed in trick questions
• For an echo, minimum distance = 17 metres (not 34 m — 34 m is total path length, but distance to wall is 17 m)
• Infrasound = below 20 Hz; ultrasound = above 20,000 Hz; audible = 20 Hz to 20,000 Hz — exact thresholds are tested
• PC&PNDT Act regulates ultrasound for sex determination — it does NOT ban ultrasound use itself
• CTBT — India has NOT signed it; last Indian tests were May 1998 (Pokhran-II)
• CTBTO's IMS uses 4 technologies: seismic, infrasound, hydroacoustic, radionuclide — all four are tested
• Decibel scale is logarithmic — 10 dB increase = 10× intensity (not linear)

Mains linkages:

• SONAR → submarine technology → India's Scorpène class → Kalvari class → Project-75 / Project-75I → maritime security → IOR strategy
• Infrasound → CTBT → India's nuclear doctrine (NFU, No First Use) → non-proliferation regime → India's stand
• Noise pollution → Noise Rules 2000 → urban governance → right to peaceful environment (Article 21 jurisprudence) → firecrackers ban SC ruling

Previous Year Questions

Prelims:

1. Which of the following technologies is used by the CTBTO's International Monitoring System to detect atmospheric nuclear explosions specifically?
   (a) Seismic monitoring
   (b) Infrasound monitoring
   (c) Hydroacoustic monitoring
   (d) Radionuclide monitoring

2. With reference to the PC&PNDT Act of India, which of the following statements is correct?
   (a) It bans all use of ultrasound machines in India
   (b) It prohibits disclosure of the sex of the foetus and mandates registration of ultrasound clinics
   (c) It is administered by the Ministry of Women and Child Development
   (d) It applies only to private hospitals and clinics, not government facilities

Mains:

1. Despite the PC&PNDT Act being in force for over two decades, India's sex ratio at birth remains skewed in several states. Critically analyse the reasons for the continued practice of sex-selective practices and the measures needed to address the issue. (CSE Mains 2019, GS Paper 2, 15 marks)

2. Discuss India's position on the Comprehensive Nuclear-Test-Ban Treaty (CTBT). How does India's stance reflect its broader nuclear doctrine and strategic interests? (CSE Mains 2021, GS Paper 3, 15 marks)