BharatNotes 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
| Category | Frequency Range | Sources | Uses / Threats |
|---|---|---|---|
| Infrasound | Below 20 Hz | Earthquakes, volcanic eruptions, nuclear explosions, ocean waves, elephants, whales | Disaster early warning; nuclear test detection (CTBTO); animal communication |
| Audible sound | 20 Hz – 20,000 Hz | Human voice, musical instruments, machinery | Communication, music, noise pollution |
| Ultrasound | Above 20,000 Hz | Bats, dolphins, specialized equipment | Medical imaging; industrial testing; SONAR; cleaning |
Speed of Sound in Different Media
| Medium | Approximate Speed | Why Faster? |
|---|---|---|
| Air (20°C) | ~343 m/s | Least dense; molecules far apart |
| Water (25°C) | ~1,480–1,500 m/s | Denser; molecules closer; more rigid |
| Steel | ~5,000–6,000 m/s | Highly rigid; vibrations propagate very efficiently |
| Vacuum | 0 (cannot travel) | No medium; no molecules to vibrate |
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
| Source | Approximate dB | Effect |
|---|---|---|
| Rustling leaves | ~10–20 dB | Barely audible |
| Normal conversation | ~60 dB | Comfortable |
| Heavy traffic | ~80–85 dB | Prolonged exposure → hearing damage |
| Industrial machinery | ~90–100 dB | Occupational hazard; hearing protection needed |
| Rock concert | ~110–120 dB | Immediate pain risk; permanent damage possible |
| Jet engine (at 30 m) | ~140 dB | Threshold of pain; permanent hearing damage |
| Firecracker (close range) | ~140–150 dB | Explosive; can rupture eardrums |
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.
Longitudinal wave: Particles of the medium vibrate parallel to the direction of wave propagation. Sound waves in air are longitudinal. (Contrast with transverse waves — e.g., light waves, waves on a string — where particles vibrate perpendicular to wave direction.) Sound in solids can also travel as transverse waves, but in fluids (liquids, gases) only longitudinal.
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).
Acoustic design matters: The sabine formula calculates reverberation time (RT60 — time for sound to decay by 60 dB). Concert halls are designed for RT60 ~1.8–2.2 seconds (music sounds rich); lecture halls ~0.6–0.8 seconds (speech remains clear). Poor acoustic design in India's Parliament annexe, courts, or public halls leads to communication failures — an underappreciated governance infrastructure issue.
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)
UPSC GS3 — Naval Technology, Submarine Detection, and SONAR:
Active SONAR:
- Ship or submarine emits an ultrasound pulse (typically 1–100 kHz) → pulse reflects off submarines, underwater mines, sea floor, or fish shoals → detected by hydrophones → time of return → distance and direction of target
- Used by: Indian Navy surface ships, patrol aircraft (P-8I Poseidon — India acquired 12 from the US); helicopter-deployed sonobuoys; towed array sonar systems
- Limitation: Active sonar reveals the position of the emitting platform — submarines use it sparingly to avoid detection
Passive SONAR:
- Simply listens to underwater sound — does not emit anything
- Detects noise from enemy submarines (propeller cavitation noise, engine/reactor sounds, crew activity)
- Submarines are designed to be "acoustically quiet" — anechoic tiles (rubber coatings that absorb sonar pulses), vibration-isolated machinery, pump-jet propulsors
- India's Scorpène class (Kalvari class) submarines use diesel-electric propulsion; when running on battery (silent mode) — extremely quiet; very hard to detect by passive SONAR
SONAR applications beyond submarine warfare:
- Ocean floor mapping (bathymetry): Multi-beam sonar maps sea floor topography; used in: (a) Samudrayaan mission (India's first crewed deep sea mission — MATSYA 6000 submersible by NIOT/MoES/VSSC; titanium sphere, 3 aquanauts, 6,000 m depth capability, 12-hour endurance; [Additional] completed wet harbour tests Jan-Feb 2025 at L&T Shipbuilding, Kattupalli; shallow-water test ~500 m planned early 2026; deep-water trials mid-2027; scientific missions 2027-28); (b) Polymetallic nodule surveys in India's Exclusive Economic Zone (EEZ) and the Indian Ocean (India holds exploration rights to 75,000 sq km of seabed in Central Indian Ocean Basin — CIOB)
- Fish detection: Commercial fishing fleets use fish-finder sonars (echo sounders)
- Pipeline and cable inspection: Sonar surveys for undersea infrastructure
India's maritime domain awareness (MDA):
- NATGRID and NaMPOL for maritime information fusion
- Information Fusion Centre for Indian Ocean Region (IFC-IOR) at Gurugram — real-time tracking of vessels; White shipping agreements with 50+ countries
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)
UPSC GS3 — Nuclear Test Detection and CTBTO:
Comprehensive Nuclear-Test-Ban Treaty (CTBT): Adopted by UN General Assembly in 1996. Bans all nuclear explosions (military and civilian). Has not entered into force — requires ratification by all 44 states listed in Annex 2; 8 have not ratified: USA, China, India, Pakistan, Israel, Egypt, Iran, North Korea.
India's position: India has NOT signed the CTBT, arguing it is discriminatory (does not include a time-bound disarmament commitment by nuclear weapons states) and that it constrains India's right to develop its nuclear deterrent. India conducted its last nuclear tests in Pokhran in May 1998 (Pokhran-II / Operation Shakti — 5 tests including thermonuclear and fission devices).
CTBTO International Monitoring System (IMS): Despite CTBT not being in force, the CTBTO Preparatory Commission operates the IMS — a global network of 337 monitoring stations using four technologies:
- Seismic: 170 stations detect underground nuclear explosions via seismic waves
- Infrasound: 60 stations detect atmospheric nuclear explosions via infrasound waves
- Hydroacoustic: 11 stations detect underwater nuclear explosions via T-waves (sound waves in ocean)
- Radionuclide: 80 stations detect radioactive particles/gases in the atmosphere
North Korea's 2017 nuclear test (estimated ~250 kt yield) was detected by all four monitoring technologies. The infrasound stations detected the atmospheric pressure wave from the underground explosion venting.
Disaster early warning:
- CTBTO's IMS seismic and infrasound data shared with tsunami warning centres (Pacific Tsunami Warning Centre, INCOIS in Hyderabad — India's tsunami early warning system)
- Post-2004 Indian Ocean tsunami (December 26, 2004 — 227,000+ deaths), India established INCOIS (Indian National Centre for Ocean Information Services) as the operational hub for the Indian Ocean Tsunami Early Warning System
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
UPSC GS2 — PC&PNDT Act and Sex-Selective Abortion:
Pre-Conception and Pre-Natal Diagnostic Techniques (Prohibition of Sex Selection) Act, 1994 (PC&PNDT Act): Regulates the use of ultrasound machines and prenatal diagnostic techniques to prevent sex-selective abortions.
Key provisions:
- Ultrasound machines must be registered; clinics must maintain records of all procedures
- Sex of foetus must not be revealed (by the technician or doctor) — criminal offence with up to 3 years imprisonment and Rs 50,000 fine (first offence)
- Clinics must display notice: "Disclosure of sex of foetus is a punishable offence"
- Central Supervisory Board and state/UT supervisory boards oversee implementation
Why important: India's sex ratio at birth (SRB) remains skewed — ~898 girls per 1,000 boys (declining since 2000s but still below natural 952 girls/1,000 boys). States like Haryana, Rajasthan, UP have historically low SRB. Beti Bachao Beti Padhao scheme (2015) targets districts with lowest child sex ratio.
Industrial ultrasound:
- Non-Destructive Testing (NDT): Checking for cracks in metal components (aircraft fuselage, bridge cables, railway tracks) without disassembly
- Cleaning: Ultrasonic cleaners use cavitation (bubble formation and collapse) to clean delicate instruments (surgical instruments, jewellery, electronic components)
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
[Additional] 12a. Earthquake Early Warning — P-wave vs S-wave Speed and Disaster Science
The chapter covers CTBTO seismic monitoring (nuclear test detection) and INCOIS/tsunami early warning, but misses a distinct third application: Earthquake Early Warning (EEW) — which exploits the different speeds of P-waves and S-waves, a direct sound/wave-speed physics concept.
Why EEW is physically possible — P-wave and S-wave: An earthquake generates two types of seismic waves simultaneously at the focus:
- P-waves (Primary/Pressure waves): Longitudinal waves (same as sound — compressions and rarefactions); travel at ~6–8 km/s through rock; arrive first; relatively weak shaking; can travel through solids AND liquids
- S-waves (Secondary/Shear waves): Transverse waves; travel at ~3–4 km/s (slower); arrive second; cause the destructive side-to-side shaking that damages buildings; can only travel through solids
The time gap between P-wave and S-wave arrival at a seismometer = the warning window. At 100 km from the epicentre, S-waves arrive ~10–20 seconds after P-waves — enough time for automated systems to send alerts and for people to take protective action (Drop-Cover-Hold On).
This is precisely why the speed of sound/waves in different media (already in this chapter) matters for disaster management.
[Additional] India's Earthquake Early Warning (EEW) System — GS3 (Disaster Management / Science & Technology):
India's seismic vulnerability: India has 5 seismic zones (I to V); Zones IV and V are highest risk — covering the entire Himalayan belt (including Uttarakhand, Himachal Pradesh, J&K, Northeast India), Andaman & Nicobar Islands, and parts of Gujarat. These areas together house hundreds of millions of people.
Current EEW status:
- National Centre for Seismology (NCS), Ministry of Earth Sciences: Piloting an EEW system for the NW Himalayas; 10 P-Alert instruments installed across Himachal Pradesh for rapid P-wave detection; NCS is developing algorithms to issue alerts automatically within 5-10 seconds of P-wave detection
- Uttarakhand: First state to launch an operational public EEW system (August 4, 2021); uses 170 accelerometers across seismic zones IV and V; sends alerts via SMS and sirens to give 10-30 seconds of advance warning
- No nationwide EEW system exists as of 2026 — the technology is in pilot/development phase; a national rollout requires thousands of sensors and a robust communication network
Limitations of EEW:
- EEW is NOT earthquake prediction (which is not currently scientifically possible for exact timing/location)
- Warning time is seconds to tens of seconds — useful for: trains to slow, surgeons to pause operations, gas mains to auto-close, elevators to open at nearest floor, school children to Duck-Cover-Hold
- Warning time decreases to near-zero for people very close to the epicentre — those people are also at highest risk
Sendai Framework (2015-2030): India is a signatory. Target C: Reduce direct disaster economic loss globally. Target G: Increase availability of and access to multi-hazard early warning systems. EEW is a direct Sendai implementation mechanism.
UPSC synthesis: EEW directly links this chapter's wave physics (P-wave = fast longitudinal/compressional; S-wave = slow transverse/shear) to disaster science (GS3), the Ministry of Earth Sciences mandate (GS2 institutions), and Sendai Framework commitments. The Uttarakhand operational system is the most recent concrete example (2021); NCS pilots are ongoing as of 2026.
[Additional] 12b. Undersea Cables and Distributed Acoustic Sensing — Sound Waves as Surveillance Infrastructure
The chapter covers passive SONAR (listening for submarines) and IFC-IOR (maritime domain awareness). A missing dimension is Distributed Acoustic Sensing (DAS) — using existing undersea fibre-optic cables as passive acoustic sensors — and India's undersea cable vulnerability as critical infrastructure.
[Additional] Undersea Cables and DAS — GS3 (Critical Infrastructure / Maritime Security / Sound Technology):
India's submarine cable dependence:
- India has 17 international submarine cables landing at 14 stations in five cities: Mumbai, Chennai, Kochi, Tuticorin, and Thiruvananthapuram
- Critical vulnerability: 15 of 17 cables converge within a 6-km stretch near Versova Beach, Mumbai — a single geographic chokepoint for India's international internet connectivity
- September 7, 2025 incident: A submarine cable-severing incident in the Red Sea (attributed to Houthi activity) caused large-scale internet outages across South Asia, Africa, and the Middle East — demonstrating the physical vulnerability of undersea cables to deliberate or accidental damage
Distributed Acoustic Sensing (DAS) — Sound Waves in Fibre: DAS converts existing submarine cables into passive acoustic sensors using backscattered laser light:
- A laser pulse is sent through the optical fibre
- Natural impurities in the glass cause a tiny fraction of light to scatter back
- When a sound wave (from a ship's propeller, submarine, or seismic event) passes near the cable, it causes microscopic strain in the fibre
- This changes the backscatter pattern — the sensor measures the time and intensity of the change to reconstruct the acoustic event
- The cable effectively becomes a passive acoustic hydrophone array hundreds of km long
Why DAS matters for India's maritime security:
- Existing undersea cables already traverse India's EEZ, the Bay of Bengal, and the Arabian Sea
- DAS can detect: submarine propeller cavitation, surface ship signatures, seismic events, and underwater explosions — without deploying any additional hardware on the seafloor
- The National Maritime Foundation (NMF) and Indian Navy are studying DAS integration with IFC-IOR and IMAC (Information Management and Analysis Centre) for undersea domain awareness
- Countries already deploying DAS: USA (SOSUS legacy + modern DAS); UK; Australia — India is assessing feasibility
UPSC angle: DAS is a dual-use technology (civilian cable + military acoustic surveillance) — relevant to GS3 (science & technology, dual-use tech), maritime security (IOR strategy, Quad), and cybersecurity/critical infrastructure (cables as both communication arteries and potential sensor networks). It also illustrates how a fundamental sound wave principle (mechanical vibration → signal detection) is applied at strategic scale.
[Additional] 12b. Noise Pollution — CPCB Standards, Urban Reality, and Health Impact
The chapter covers sound, its properties, and qualitative mention of noise pollution. It does not cover India's legally enforceable noise standards, the regulatory authority enforcing them, the documented violation levels in Indian cities, or the health and governance implications — all UPSC-relevant.
Key Terms — Noise Pollution Standards:
| Term | Meaning |
|---|---|
| Noise Pollution (Regulation and Control) Rules, 2000 | Central rules notified under the Environment (Protection) Act, 1986; prescribe area-wise noise limits for day and night; enforced by CPCB + State PCBs |
| Ambient Noise | The background sound level in an area at a given time; measured in decibels (dB) using a Sound Level Meter |
| Silence zone | As defined under the 2000 Rules: area within 100 metres of hospitals, educational institutions, courts, religious places; strictest limits (50 dB day / 40 dB night) |
| CPCB (Central Pollution Control Board) | India's apex pollution monitoring and regulation body; operates under Ministry of Environment, Forest and Climate Change (MoEFCC); monitors ambient noise and enforces standards at national level |
| State PCB (Pollution Control Board) | State-level body that enforces the Noise Rules at the local level; issues closure/penalty notices to violating industries |
| Continuous Equivalent Sound Level (Leq) | The steady sound level that would contain the same energy as the measured fluctuating sound over a period; used in noise standards |
[Additional] Noise Pollution — CPCB Standards, India's Urban Reality, and Health Governance (GS3 — Environment / GS2 — Governance):
India's noise standards — Noise Pollution (Regulation and Control) Rules, 2000:
| Area category | Day limit (6 AM – 10 PM) | Night limit (10 PM – 6 AM) |
|---|---|---|
| Industrial | 75 dB(A) | 70 dB(A) |
| Commercial | 65 dB(A) | 55 dB(A) |
| Residential | 55 dB(A) | 45 dB(A) |
| Silence zones | 50 dB(A) | 40 dB(A) |
Statutory basis:
- Noise Pollution (Regulation and Control) Rules, 2000 — notified under Section 6 of the Environment (Protection) Act, 1986
- State governments can set lower limits (more restrictive) but NOT higher limits
- Firecrackers: permitted only between 8 PM – 10 PM in non-silence zones; banned completely in silence zones
India's actual noise levels — CPCB monitoring:
| City | Measured level | Standard | Exceedance |
|---|---|---|---|
| Delhi (residential areas, 2023 CPCB survey) | 65–75 dB | 55 dB (day) | +10 to +20 dB above permissible |
| Delhi NCR (traffic corridors, peak hours) | 80–100 dB | 65 dB (commercial) | Far above standards |
| Mumbai (CPCB data) | 55–75 dB (residential areas) | 55 dB | At or above limits |
| Bengaluru | 60–70 dB (residential zones near traffic) | 55 dB | Above limits |
Note on decibel scale: The dB scale is logarithmic — 10 dB increase = 10× energy increase; 20 dB above the 55 dB standard means 100 times more energy than permitted.
WHO guidelines on noise:
| Source | WHO recommended limit |
|---|---|
| Road traffic noise | ≤53 dB (day, outdoor) |
| Railway noise | ≤54 dB (day, outdoor) |
| Aircraft noise | ≤45 dB (day, outdoor) |
| Night noise (all sources) | ≤40 dB (outdoor) for sleep protection |
| India's residential night limit | 45 dB — already 5 dB above WHO recommendation |
Health impacts of noise — scientific consensus:
| Noise level | Health effect |
|---|---|
| >85 dB (prolonged exposure) | Noise-Induced Hearing Loss (NIHL); cochlear hair cell damage; irreversible |
| 65–85 dB (chronic) | Cardiovascular disease risk increase (hypertension, heart attack); WHO estimates 1 million DALYs lost per year in Western Europe from traffic noise alone |
| 55–65 dB (nighttime) | Sleep disruption → fatigue, metabolic disorders, reduced immunity |
| >40 dB (nighttime, children) | Cognitive impairment: reduced reading ability, memory, concentration; documented in schools near airports |
Governance and judicial interventions:
| Event | Detail |
|---|---|
| Supreme Court on loudspeakers | SC has consistently held that using loudspeakers in violation of noise rules = breach of Article 19(1)(a) subject to reasonable restrictions under Article 19(2) |
| Article 21 linkage | SC (multiple rulings including 2024 cases) = right to live in pollution-free environment (noise included) = part of Article 21 (Right to Life) |
| Noise in urban planning | Smart Cities Mission and National Urban Policy Framework both incorporate noise reduction as a parameter (urban green buffers, traffic management) |
| CPCB National Ambient Noise Monitoring Network | CPCB operates real-time noise monitors in major cities; data published on CPCB website |
| Firecrackers ban | SC 2018: banned firecrackers that exceed 125 dB at 4 metres in the manufacture; allowed only "green crackers"; states can impose total ban around Diwali |
UPSC synthesis: Key exam facts: Noise Pollution Rules notified under Environment (Protection) Act 1986 = promulgated in 2000; limits = Residential 55 dB day / 45 dB night, Silence zones 50 dB day / 40 dB night, Industrial 75 dB day / 70 dB night; enforced by CPCB (central) + State PCBs; Delhi residential areas measured 65–75 dB = up to 20 dB above permissible; WHO recommends ≤53 dB road noise (India's residential limit is already looser at 55 dB); prolonged exposure >85 dB = NIHL (Noise-Induced Hearing Loss); SC 2024 affirmed noise violations = breach of Article 21; firecracker use limited to 8 PM – 10 PM under Rules. Prelims trap: The noise standard for silence zones = 50 dB day / 40 dB night (NOT 0 dB — "silence" is a regulatory category, not absolute silence); the decibel scale is logarithmic (10 dB increase = 10× sound energy, NOT 10% more); CPCB monitors noise (under MoEFCC) — do NOT confuse with NDMA (National Disaster Management Authority) or PCPNDT (women's health); the Noise Rules are under the Environment (Protection) Act 1986 (NOT the Air (Prevention and Control of Pollution) Act 1981 — that Act covers air quality, not noise).
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
Practice Questions
Prelims:
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 monitoringWith 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:
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)
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)
