Sound
Sound is produced by vibrating bodies, travels only through a material medium (solid, liquid or gas) as oscillations, and—when excessive or unwanted—becomes noise pollution, a recognised environmental and public-health hazard.
Prelims frequently tests the physics basics here—the audible range (20 Hz–20 kHz), the amplitude-loudness and frequency-pitch links, units (Hz, dB) and ultrasound applications—often as match-the-pair or assertion-reason items. In GS-III the chapter feeds the environment-and-pollution theme: noise pollution as an environmental and public-health hazard and its regulation (Noise Pollution Rules, 2000; CPCB; Article 21). It also supplies easy science-tech facts for the General Science segment.
Understand the chapter
Sound is Produced by a Vibrating Body
Sound is produced by the to-and-fro (back-and-forth) motion of a body, called vibration; the instant the vibration stops, the sound stops. In musical instruments the actual vibrating part differs—a string, a stretched membrane, an air column or the whole body—yet the underlying principle is identical. In many cases the amplitude of vibration is too small to see, but it can still be felt.
- Vibration = to-and-fro / back-and-forth motion of an object.
- Vibrating parts: veena/sitar = stretched string; tabla/mridangam = stretched membrane; flute = air column; ghatam/manjira/kartal = whole body struck.
- Grip a struck plate to stop its vibration and the sound dies instantly.
Sound Produced by Humans
In humans, sound is produced by the voice box or larynx, located at the upper end of the windpipe, where two vocal cords are stretched leaving a narrow slit for air. When the lungs force air through this slit, the vocal cords vibrate and produce sound; attached muscles tighten or loosen them to change the quality of voice. Differences in vocal-cord length explain why the voices of men, women and children differ.
- Larynx = voice box; felt as the hard bump at the top of the windpipe that moves on swallowing.
- Vocal cords: men about 20 mm, women about 15 mm, children very short.
- Tight, thin cords versus loose, thick cords give different voice quality and pitch.
Sound Needs a Medium for Propagation
Sound requires a material medium—solid, liquid or gas—to travel, and it cannot pass through a vacuum. As air is sucked out of a vessel the sound grows fainter and finally becomes inaudible, proving the medium's role. Sound is carried in all directions: whales and dolphins communicate through water, and a toy telephone shows that sound travels through a stretched string and through solids.
- No medium = no sound; sound cannot travel through vacuum (a key contrast with light, which can).
- Travels through gases, liquids and solids alike.
- Spreads in all directions from the source.
We Hear Sound Through Our Ears
The funnel-shaped outer ear gathers sound and channels it down a canal to the eardrum, a thin stretched membrane. Sound vibrations make the eardrum vibrate; it passes these vibrations to the inner ear, which then sends the signal to the brain, producing hearing. A sharp or pointed object can rupture the eardrum and impair hearing, so nothing pointed should ever be put into the ear.
- Pathway: outer ear (funnel) -> canal -> eardrum -> inner ear -> brain.
- Eardrum = thin stretched membrane that vibrates with incoming sound.
- A damaged eardrum leads to impaired hearing.
Amplitude, Time Period and Frequency: Loudness and Pitch
Vibration is an oscillatory motion; frequency is the number of oscillations per second, measured in hertz (Hz), and time period is the time for one oscillation. Loudness depends on amplitude and is measured in decibel (dB)—crucially, loudness is proportional to the square of the amplitude, so doubling the amplitude makes the sound four times as loud. Pitch (shrillness) depends on frequency: higher frequency gives a higher pitch, lower frequency a lower pitch.
- Frequency (Hz): 1 Hz = 1 oscillation per second.
- Amplitude -> loudness (dB): large amplitude is loud, small amplitude is feeble.
- Frequency -> pitch: drum low-pitched, whistle high-pitched; a woman's voice usually has higher frequency than a man's.
- Loudness is proportional to amplitude squared; above 80 dB sound becomes physically painful.
Audible and Inaudible Sounds
The human ear hears frequencies roughly from 20 Hz to 20,000 Hz (20 kHz); frequencies below and above this range are inaudible. Sounds above 20 kHz are ultrasonic—dogs can hear them (hence high-frequency police whistles), and medical ultrasound equipment works at these frequencies to investigate and track health problems. Sounds below 20 Hz are infrasonic and also lie outside human hearing.
- Audible range: about 20 Hz to 20,000 Hz (20 kHz).
- Ultrasound (above 20 kHz): medical imaging/tracking; audible to dogs.
- Infrasound (below 20 Hz): inaudible to humans.
Noise, Music and Noise Pollution
Pleasant sound is music, whereas unpleasant, unwanted sound is noise, and the presence of excessive or unwanted sound in the environment is noise pollution. Its major sources are vehicles and horns, explosions and crackers, machines, loudspeakers and loud home appliances. It damages health—sleeplessness, hypertension, anxiety and temporary or permanent hearing loss—and is regulated under the Noise Pollution (Regulation and Control) Rules, 2000.
- Noise pollution = presence of excessive or unwanted sound in the environment.
- Harms: lack of sleep, hypertension (high BP), anxiety, hearing impairment.
- Control: silencing devices on engines/machines, fewer automobile horns, low TV/music volume, plant trees along roads, shift noisy industry away from residential and silence zones.
- Static anchors: Noise Rules 2000 framed under the Environment (Protection) Act, 1986; CPCB is the nodal body; the Supreme Court reads freedom from noise into Article 21.
Key terms
- Vibration
- The to-and-fro or back-and-forth motion of a body that produces sound.
- Amplitude
- Maximum displacement of a vibrating particle from its rest position; it decides loudness.
- Frequency
- Number of oscillations per second, measured in hertz (Hz); it decides pitch.
- Time period
- Time taken to complete one oscillation (the reciprocal of frequency).
- Loudness
- Magnitude of sound sensation, measured in decibel (dB); proportional to the square of amplitude.
- Pitch
- The shrillness of a sound, determined by frequency—higher frequency gives higher pitch.
- Larynx (voice box)
- Organ at the upper end of the windpipe housing the vocal cords that generate human sound.
- Eardrum
- Thin stretched membrane in the ear that vibrates and relays sound to the inner ear.
- Ultrasound
- Sound above 20,000 Hz, inaudible to humans, used in medical diagnostics.
- Noise pollution
- Presence of excessive or unwanted sound in the environment.
Must-know facts exam-ready
- A vibrating body produces sound; the sound ceases the moment the vibration stops.
- Sound needs a material medium and cannot travel through vacuum (unlike light).
- Human audible range is about 20 Hz to 20,000 Hz (20 kHz).
- Frequency is measured in hertz (Hz); 1 Hz = 1 oscillation per second.
- Loudness is measured in decibel (dB) and is proportional to the square of amplitude (double the amplitude gives four times the loudness).
- Loudness depends on amplitude; pitch (shrillness) depends on frequency.
- Above 80 dB sound becomes physically painful (factory ~80 dB, busy traffic ~70 dB, conversation ~60 dB, whisper ~30 dB, breathing ~10 dB).
- Vocal cords are about 20 mm in men and about 15 mm in women, and very short in children—hence different voices.
- Sound above 20 kHz is ultrasound (medical use; audible to dogs); below 20 Hz is infrasound—both inaudible to humans.
- Hearing pathway: outer ear (funnel) -> canal -> eardrum -> inner ear -> brain.
- Noise pollution is regulated by the Noise Pollution (Regulation and Control) Rules, 2000 under the Environment (Protection) Act, 1986; CPCB is the nodal body and the SC links a noise-free environment to Article 21.
Memory tricks remember it for good
Traps to avoid
- Amplitude decides loudness and frequency decides pitch—UPSC loves swapping these two.
- Loud is not the same as high-pitched: a lion's roar is loud but low-pitched, while a bird's call is feeble but high-pitched.
- Loudness is proportional to the square of amplitude, not directly proportional—doubling amplitude quadruples loudness.
- Sound cannot travel through vacuum, but light and other electromagnetic waves can—do not generalise that all waves need a medium.
- Hertz (Hz) measures frequency while decibel (dB) measures loudness—do not interchange the units.
- The voice box is the larynx at the top of the windpipe; vocal cords vibrate (they are not plucked), and it is not the pharynx.
Exam focus
🧠 Prelims angles
- Audible range (20 Hz to 20 kHz), ultrasound (above 20 kHz) and its uses (medical imaging, dog/police whistles).
- Match quantity-unit-cause: loudness-decibel-amplitude; pitch-hertz-frequency.
- Assertion-reason items on 'sound needs a medium / no sound in vacuum', contrasting sound with light.
- Decibel benchmarks: breathing 10, whisper 30, conversation 60, traffic 70, factory 80; above 80 dB painful.
- Noise pollution governance: Noise Rules 2000, Environment (Protection) Act 1986, CPCB, and silence zones (about 100 m around hospitals, schools and courts).
- Human voice production: larynx and vocal-cord lengths (men ~20 mm, women ~15 mm).
✍️ Mains angles GS-III
- Noise pollution is an under-recognised environmental and public-health hazard—examine its sources, harms and India's regulatory response.Use the Noise Rules 2000, CPCB ambient standards, silence zones and Article 21; flag weak enforcement and suggest source-control, green barriers and awareness.
- Rapid urbanisation, traffic and festivals are intensifying noise pollution—how can cities balance livelihoods and celebration with the right to a peaceful environment?Cite vehicle horns, loudspeakers and crackers and SC night-time loudspeaker curbs (10 pm to 6 am); propose zoning, low-noise technology and citizen sensitisation.
Last-minute revision tick as you recall
- Vibration makes sound; no vibration means no sound.
- Sound needs a medium—no sound in vacuum (light differs).
- Amplitude -> loudness (dB); frequency -> pitch (Hz).
- Audible 20 Hz to 20 kHz; above 20 kHz ultrasound, below 20 Hz infrasound.
- Loudness is proportional to amplitude squared; above 80 dB is painful.
- Vocal cords: men 20 mm, women 15 mm, children shortest.
- Ear path: funnel -> canal -> eardrum -> inner ear -> brain.
- Noise pollution harms: sleeplessness, hypertension, anxiety, hearing loss.
- Regulation: Noise Rules 2000 (EPA 1986), CPCB nodal, silence zone ~100 m, Article 21.
Distilled from NCERT Class 8 · Science (Class 8) for UPSC. Always cross-check facts with the original NCERT.