Sound

Why This Chapter Matters

Sound questions show up in almost every SSC and RRB science paper, usually one or two marks, and they are some of the easiest marks on the entire paper if you know four numbers: the speed of sound in air, the audible range of the human ear, the frequency ranges for ultrasonic and infrasonic waves, and the name SONAR. Miss these and you lose a free question. This chapter covers exactly that ground: what sound is, how fast it travels, how pitch and loudness differ, echoes and SONAR, musical instruments, and noise pollution limits.

The single biggest mistake aspirants make here is mixing up frequency and amplitude. Frequency decides pitch, how shrill or deep a sound feels. Amplitude decides loudness, how soft or loud it feels. A mosquito's whine is high-pitched but soft; a bass drum is loud but deep. Exam-setters love building a question around exactly this confusion, so lock it in now: frequency = pitch, amplitude = loudness.

Sound Is a Wave, and It Needs a Medium

Sound is a form of energy that travels as a mechanical wave, produced whenever an object vibrates. Pluck a guitar string, strike a bell, or clap your hands, and you set the surrounding particles vibrating. Those vibrations push into neighbouring particles, which push into the next ones, and the disturbance travels outward. This is the one fact you must never forget: sound needs a material medium to travel. It can move through solids, liquids, and gases, but it cannot travel through vacuum, because there are no particles there to carry the vibration.

This is why the classic "bell in a vacuum jar" experiment is a textbook favourite. Ring an electric bell inside a sealed glass jar and slowly pump the air out. As the air thins, the sound grows fainter, and once a near-vacuum is reached, you see the hammer still striking the bell but hear nothing. It is also why astronauts on a spacewalk cannot talk to each other directly through open space; they need radio (an electromagnetic wave, which needs no medium) to communicate.

Exam trap: Light and radio waves are electromagnetic and travel through vacuum. Sound is mechanical and cannot. A question that asks "which travels through vacuum, light or sound" is really just testing whether you remember this one distinction.

Sound travels as a longitudinal wave. Picture a slinky spring stretched across a floor. Push one end sharply, and you see coils bunching together (compression) and spreading apart (rarefaction) moving down the spring, all in the same direction the push travelled. That is exactly how sound moves through air: molecules of air bunch into high-pressure compressions and spread into low-pressure rarefactions, and this pattern of compressions and rarefactions is the sound wave itself.

Speed of Sound: Solids Beat Liquids Beat Gases

Here is a fact that trips up a lot of students because it feels backwards: sound travels fastest in solids, slower in liquids, and slowest in gases. The reason is particle spacing. In a solid, particles are packed tightly and locked in position relative to each other, so a vibration passes from one to the next almost instantly. In a gas, particles are far apart and have to physically collide to pass on the disturbance, which takes longer.

Memory hook: Think of a crowded local train at rush hour versus an empty compartment. If someone at one end of a packed train nudges the person next to them, the nudge reaches the far end almost instantly because everyone is squeezed together. In an empty compartment, a person has to walk over before they can nudge the next one. Tightly packed solids pass on sound the fastest, the same way a packed train passes on a nudge fastest.

Approximate speeds you should carry into the exam hall:

Medium	Approx. speed of sound
Air (0°C, dry)	332 metres/second
Air (25°C, typical room)	about 346 metres/second
Water	about 1,480 metres/second
Steel/iron	about 5,000–5,900 metres/second
Vacuum	0 (does not travel)

The exact figure quoted most often in SSC-level material is speed of sound in air ≈ 332 m/s at 0°C, rising as temperature increases. This is because warmer air molecules move faster and collide more often, passing the vibration along more quickly. That is also why sound travels a little faster on a hot afternoon than on a cold winter morning, all else being equal.

Exam trap: Students often confuse this with the speed of light, 3 × 10⁸ m/s. Remember the enormous gap between them; it is exactly why you see lightning before you hear thunder, even though both are produced at the same instant.

Frequency, Pitch, and Loudness

Frequency is the number of complete vibrations (or wave cycles) produced per second. It is measured in hertz (Hz), named after the German physicist Heinrich Hertz. A frequency of 20 Hz means 20 vibrations every second. Frequency is what your ear perceives as pitch, how high or low a sound feels. A tabla's high-pitched strokes have a high frequency; a dhol's deep boom has a low frequency.

Amplitude is the maximum displacement of vibrating particles from their resting position, essentially how "big" each vibration is. Amplitude decides loudness, measured practically in decibels (dB). A gentle tap on a table has small amplitude and low loudness; a hard slam has large amplitude and high loudness, even if the pitch (frequency) stays the same.

Think of a cricket match commentary. When the commentator speaks softly, that is low amplitude, low loudness. When he suddenly shouts "SIX!" at a high pitch of excitement, that is a change in loudness (amplitude), and separately his voice may also rise in pitch (frequency). The two properties are independent of each other, and an exam question likes to test whether you know that a sound can be loud but low-pitched (thunder) or soft but high-pitched (a whisper at high pitch).

Timbre or quality is the property that lets you tell a violin apart from a flute even when both play the same note at the same loudness. It depends on the mix of overtones each instrument produces.

The Human Audible Range and Beyond

The human ear can normally detect sound frequencies between 20 Hz and 20,000 Hz (20 kHz). This range is called the audible range. Anything outside it is inaudible to humans, though many animals hear it perfectly well.

• Infrasonic sound: frequency below 20 Hz. Elephants and whales use infrasonic calls to communicate over long distances; earthquakes and volcanic activity also produce infrasonic waves, which is why some animals appear to sense earthquakes before humans do.
• Ultrasonic sound: frequency above 20,000 Hz. Bats and dolphins use ultrasonic waves for navigation (echolocation) and hunting. Ultrasonic waves are also the working principle behind medical ultrasound scanning, ultrasonic cleaning of jewellery and lenses, and SONAR.

Memory hook: "Infra is low, Ultra goes above the flow." Infra- (as in infrared, infra-structure below ground) sits below the audible range; ultra- (as in ultraviolet, going beyond) sits above it. If you remember that infrared light has a lower frequency than visible light and ultraviolet has a higher one, the same infra/ultra logic carries straight over to sound.

Exam trap: A common wrong-option trick swaps the numbers: "audible range is 20,000 Hz to 20 Hz" written the wrong way round, or claims humans can hear up to 200 kHz. Fix the numbers 20 Hz–20,000 Hz firmly in memory, in that order.

Echo and SONAR

An echo is a reflected sound wave that reaches your ear a noticeable time after the original sound. For the human ear to distinguish an echo from the original sound as two separate events, there must be a gap of at least 0.1 second between them. Since sound travels at roughly 344 m/s in air (at typical room temperature), this means the reflecting surface must be at least about 17 metres away for you to hear a distinct echo (the sound travels to the surface and back, so the total distance covered is double the distance to the wall).

This is exactly why you hear a clear echo shouting into a canyon or a large empty hall, but never in a small room; the walls are simply too close for the reflected sound to arrive late enough to register as separate.

SONAR stands for SOund Navigation And Ranging. It is a technique that uses ultrasonic waves to detect the presence, distance, and direction of underwater objects. A SONAR device sends out an ultrasonic pulse, and by measuring the time taken for the reflected pulse (the echo) to return, it calculates the distance to the object. SONAR is used on ships and submarines to measure ocean depth, detect other submarines, locate shipwrecks, and find shoals of fish for the fishing industry.

Exam trap: Do not confuse SONAR (uses sound, works underwater) with RADAR (RAdio Detection And Ranging, uses radio waves, works in air and space to detect aircraft). SSC papers frequently swap these two in wrong options; SONAR is sound-based and underwater, RADAR is radio-based and aerial.

Reverberation is a related but different phenomenon: it is the persistence of sound in a large hall due to multiple reflections off walls and ceiling, causing sound to "linger" rather than form a distinct separate echo. This is why concert halls and auditoriums are designed with sound-absorbing materials on walls, to control reverberation without killing the sound entirely.

How Musical Instruments Produce Sound

Every musical instrument produces sound through vibration, but the vibrating part differs by category, and this is a favourite fact-recall zone for exams.

Category	What vibrates	Examples
String instruments (Chordophones)	A stretched string	Sitar, veena, violin, guitar
Wind instruments (Aerophones)	A column of air	Flute, shehnai, trumpet, harmonium
Percussion instruments (Membranophones)	A stretched membrane	Tabla, dhol, drum, mridangam
Solid-body percussion (Idiophones)	The body of the instrument itself	Cymbals, manjira, xylophone, ghungroo

In string instruments, plucking or bowing sets the string vibrating; a shorter, tighter, or thinner string vibrates faster and produces a higher pitch, which is exactly why musicians press down on a guitar string to shorten it and raise the note. In wind instruments, blowing sets a column of trapped air vibrating; a longer air column (as in a bass flute) vibrates slower, giving a deeper note than a shorter one. In membrane instruments like the tabla, striking the taut skin sets it vibrating; tightening the skin raises the pitch, which is why a tabla player adjusts the leather straps before a performance.

Analogy: Think of a matka (earthen pot) used as a folk percussion instrument. Strike the open mouth, and the trapped air inside vibrates as a column, similar in principle to a wind instrument, even though you struck it like a percussion piece. The same physics of a vibrating air column explains why a nearly-empty water bottle gives a lower, hollower note than a nearly-full one when you blow across its mouth; a full bottle has a shorter air column, and shorter columns vibrate faster, giving a higher note.

Noise Pollution

Noise is unwanted or unpleasant sound that causes discomfort, and prolonged exposure to it is officially recognised as noise pollution. It is measured in decibels (dB). As a rough working benchmark used in Indian exam material, sound above about 85 dB on sustained exposure is considered harmful to human hearing, and levels above roughly 120 dB can cause immediate pain and permanent hearing damage.

Common sources of noise pollution include traffic, industrial machinery, loudspeakers, construction work, and firecrackers. Its effects go beyond hearing loss: it disturbs sleep, raises stress and blood pressure, reduces concentration and productivity, and can harm wildlife that depends on sound for navigation and communication, such as bats and marine animals disturbed by ship engine noise.

India regulates permissible noise levels through the Noise Pollution (Regulation and Control) Rules, 2000, framed under the Environment (Protection) Act, 1986, which set different day-time and night-time decibel limits for industrial, commercial, residential, and silence zones (areas near hospitals, schools, and courts). Silence zones carry the strictest limits, which is exactly why you see "Silence Zone" boards near hospitals and schools in Indian cities.

Exam trap: Do not confuse the unit of loudness (decibel, dB) with the unit of frequency (hertz, Hz). A question about "unit used to measure noise pollution" wants decibel, not hertz.

Reflection, Refraction, and Other Behaviours of Sound

Sound behaves like other waves in several ways that exams like to probe. It undergoes reflection, bouncing off a hard surface, which is the basis of both echo and reverberation discussed above. Hard, smooth surfaces such as a bare concrete wall or a metal sheet reflect sound well; soft, porous materials such as curtains, carpets, and acoustic foam absorb sound rather than reflecting it. This is exactly why recording studios and cinema halls line their walls with thick, soft panels: to cut down unwanted reflections and keep the sound crisp rather than muddy.

Sound also undergoes refraction, bending when it passes from one medium into another, or even within the same medium if temperature varies. This explains a genuinely useful everyday observation: sound from a source such as a loudspeaker often carries further and sounds clearer at night than during the day. During the day, air near the ground is warmer than air higher up, so sound bends upward, away from listeners on the ground. At night, this pattern reverses, air near the ground is cooler and denser than air above, so sound waves bend back down towards the ground, letting them travel further before fading out. A distant train's whistle or temple bell often feels louder at night for exactly this reason.

Sound can also show interference, where two sound waves overlap and combine, sometimes reinforcing each other into a louder sound and sometimes cancelling into a quieter patch, a principle used deliberately in modern noise-cancelling headphones, which generate a wave that is the exact mirror image of unwanted ambient noise to cancel it out.

Resonance: Why a Glass Can Shatter to a Voice

Resonance occurs when a vibrating object forces another nearby object to vibrate at its own natural frequency, dramatically increasing the amplitude of that second object's vibration. Every object that can vibrate has a natural frequency it prefers to vibrate at. If an external sound wave matches that natural frequency exactly, the object absorbs energy efficiently and its vibrations build up rapidly.

The oft-quoted example, a trained singer shattering a wine glass by singing a sustained note, works on exactly this principle: the singer's note matches the glass's natural frequency, the glass vibrates with growing amplitude, and eventually the vibration becomes violent enough to break the glass. On a gentler note, this is also roughly how a radio tuner works, picking out one specific broadcast frequency out of the many radio waves in the air by resonating with it while ignoring the rest, and it is why soldiers are traditionally ordered to break step while marching across a bridge, since rhythmic marching close to the bridge's natural frequency could in principle build up dangerous resonant vibrations.

Exam trap: Do not confuse resonance (one vibrating object driving another at a shared natural frequency) with an echo (a single wave simply reflecting back). They are different phenomena that both happen to involve sound.

The Doppler Effect in Sound

The Doppler effect is the apparent change in the frequency (and hence pitch) of a sound when the source and listener are moving relative to each other. You have almost certainly experienced this without naming it: stand at a railway platform as a train approaches sounding its horn, and the pitch sounds higher and higher as it nears you, then suddenly drops to a noticeably lower pitch the instant it passes and starts moving away.

The physics behind it is straightforward. As the source approaches, successive sound waves get compressed closer together before they reach you, effectively raising the frequency you perceive, hence the higher pitch. As the source moves away, successive waves get stretched further apart, lowering the frequency you perceive, hence the drop in pitch. The actual frequency produced by the horn never changes; only what you perceive changes, purely because of the relative motion.

Analogy: Picture cars merging onto a single-lane bridge during rush hour. Cars approaching the bridge from behind you bunch closer together (like compressed wave peaks, higher frequency); cars that have already crossed and are pulling away spread further apart (like stretched wave peaks, lower frequency). The rate at which cars pass a fixed point standing at the bridge changes even though every car is moving at a similar underlying speed, exactly how perceived pitch changes even though the actual sound frequency at the source stays constant.

The Doppler effect for sound was first explained by the Austrian physicist Christian Doppler in 1842, and the same underlying principle, applied to light and radio waves instead of sound, is what lets astronomers tell whether a distant star is moving towards or away from Earth, and lets traffic police radar guns measure a vehicle's speed.

A Closer Look at Ultrasonic Applications

Because ultrasonic waves carry usefully high frequency and can be focused into narrow, directional beams, they have earned a wide range of practical uses well beyond SONAR that competitive exams enjoy testing individually.

In medicine, ultrasound scanning (sonography) sends ultrasonic pulses into the body and reads the pattern of echoes reflected by internal organs and tissues to build an image, used routinely for prenatal check-ups during pregnancy, examining the abdomen, and guiding certain needle biopsies, all without the radiation risk that comes with X-rays. In industry, ultrasonic waves are used for non-destructive testing, checking for hidden cracks inside metal welds, pipelines, and machine parts without having to cut them open. Jewellers and opticians use ultrasonic cleaning baths, where high-frequency vibrations shake loose dirt trapped in the tiny grooves of rings, chains, and spectacle lenses far more thoroughly than manual scrubbing could. Ultrasonic waves are further used to break down kidney stones into fine fragments that the body can pass naturally, a treatment called lithotripsy, sparing the patient invasive surgery.

Memory hook: "SCUB" for ultrasonic uses: Sonography (medical scanning), Cleaning (jewellery, lenses), Underwater SONAR, Breaking kidney stones (lithotripsy). Four very different fields, one shared trick: precisely focused high-frequency sound.

Quick Revision — One-Line Facts

• Sound is a mechanical, longitudinal wave produced by vibrating objects.
• Sound cannot travel through vacuum; it needs a medium (solid, liquid, or gas).
• Speed of sound is highest in solids, lower in liquids, lowest in gases.
• Speed of sound in air at 0°C is approximately 332 m/s.
• Speed of sound in water is roughly 1,480 m/s.
• Speed of sound increases with a rise in temperature of the medium.
• Frequency is measured in hertz (Hz), named after Heinrich Hertz.
• Frequency determines pitch; amplitude determines loudness.
• The human audible range is 20 Hz to 20,000 Hz.
• Sound below 20 Hz is called infrasonic; sound above 20,000 Hz is ultrasonic.
• Bats and dolphins use ultrasonic waves for echolocation.
• Elephants and whales can produce and sense infrasonic sound over long distances.
• SONAR stands for SOund Navigation And Ranging and uses ultrasonic waves.
• SONAR is used to measure ocean depth and detect submarines and shipwrecks.
• RADAR uses radio waves, not sound, and works in air, not underwater.
• An echo needs a reflecting surface at least about 17 metres away and a minimum gap of 0.1 second to be heard distinctly.
• Reverberation is the persistence/lingering of sound due to repeated reflections in an enclosed space.
• Loudness is measured in decibels (dB).
• String instruments (sitar, violin) produce sound from a vibrating string.
• Wind instruments (flute, shehnai) produce sound from a vibrating air column.
• Percussion instruments (tabla, dhol) produce sound from a vibrating membrane.
• Idiophones (cymbals, manjira) produce sound from the vibration of the instrument's own body.
• A shorter or tighter string vibrates faster, giving a higher pitch.
• A longer air column vibrates slower, giving a lower pitch.
• Ultrasonic waves are used in medical ultrasound scanning and cleaning of jewellery/lenses.
• Sustained exposure above roughly 85 dB is considered harmful to hearing.
• Noise pollution in India is regulated under the Noise Pollution (Regulation and Control) Rules, 2000.
• Astronauts cannot hear each other directly in space because sound cannot travel through vacuum; they use radio instead.
• Timbre (sound quality) is what lets you distinguish two instruments playing the identical note.
• Lightning is seen before thunder is heard because light travels far faster than sound.
• A ringing bell in a jar with air pumped out becomes progressively fainter, then silent.
• Sound bends towards the ground at night because cooler, denser air near the surface refracts it back down.
• Resonance is a sharp rise in an object's vibration amplitude when a driving sound matches its natural frequency.
• The Doppler effect explains why a train's horn sounds higher-pitched approaching and lower-pitched moving away.
• The Doppler effect for sound was first explained by Austrian physicist Christian Doppler in 1842.
• Ultrasonic waves are used in sonography, ultrasonic cleaning, SONAR, and breaking kidney stones (lithotripsy).
• Soft, porous materials like curtains and foam absorb sound; hard smooth surfaces reflect it.

Memory Tables

Table 1: Speed of Sound in Different Media

Medium	Approximate speed	Why
Vacuum	0 (cannot travel)	No particles to carry vibration
Air (0°C)	332 m/s	Gas particles are widely spaced
Air (25°C)	~346 m/s	Warmer particles collide more often, so sound speeds up
Water	~1,480 m/s	Liquid particles are closer than gas
Steel/Iron	~5,000–5,900 m/s	Solid particles are tightly packed

Table 2: Sound Frequency Ranges and Uses

Range	Frequency	Who uses/hears it	Key application
Infrasonic	Below 20 Hz	Elephants, whales, earthquakes	Long-distance animal communication, earthquake precursors
Audible	20 Hz–20,000 Hz	Humans	Everyday hearing, speech, music
Ultrasonic	Above 20,000 Hz	Bats, dolphins	Echolocation, SONAR, medical ultrasound, ultrasonic cleaning

Table 3: Instrument Families and Their Vibrating Part

Family	Vibrating part	Examples
Chordophones	String	Sitar, veena, guitar, violin
Aerophones	Air column	Flute, shehnai, trumpet
Membranophones	Stretched membrane	Tabla, dhol, mridangam
Idiophones	Body of the instrument	Cymbals, manjira, xylophone

Practice MCQs

Q1. Sound cannot travel through which of the following? (a) Water (b) Steel (c) Vacuum (d) Air

Q2. What is the approximate speed of sound in air at 0°C? (a) 3 m/s (b) 332 m/s (c) 3,32,000 m/s (d) 1,480 m/s

Q3. In which medium does sound travel the fastest? (a) Gases (b) Liquids (c) Solids (d) Vacuum

Q4. The pitch of a sound is determined by its: (a) Amplitude (b) Frequency (c) Wavelength speed (d) Loudness

Q5. SONAR stands for: (a) SOund Navigation And Ranging (b) Sound Of Nature And Radiation (c) Signal Object Notation And Ranging (d) SOund Node And Radio

Q6. The unit of loudness of sound is: (a) Hertz (b) Decibel (c) Newton (d) Pascal

Q7. The human ear can normally hear sounds within which frequency range? (a) 2 Hz to 2,000 Hz (b) 20 Hz to 20,000 Hz (c) 200 Hz to 2,00,000 Hz (d) 2,000 Hz to 20,000 Hz

Q8. Sound waves with frequency above 20,000 Hz are called: (a) Infrasonic (b) Ultrasonic (c) Supersonic (d) Subsonic

Q9. Which animal is well known for using ultrasonic waves to navigate in the dark? (a) Elephant (b) Bat (c) Owl (d) Whale

Q10. For a person to hear a distinct echo, the minimum distance to the reflecting surface should be about: (a) 1.7 metres (b) 17 metres (c) 170 metres (d) 1,700 metres

Q11. In a tabla, sound is produced mainly due to the vibration of the: (a) Wooden body (b) Stretched membrane (c) Air column inside (d) Metal rim

Q12. Lightning is seen before thunder is heard mainly because: (a) Lightning happens before thunder is produced (b) Light travels much faster than sound (c) Sound is absorbed by clouds (d) Thunder is produced a few seconds after lightning

Q13. Which Indian legislation primarily regulates permissible noise levels in residential, commercial, and silence zones? (a) Air (Prevention and Control of Pollution) Act, 1981 (b) Noise Pollution (Regulation and Control) Rules, 2000 (c) Environment Impact Assessment Notification, 2006 (d) Water (Prevention and Control of Pollution) Act, 1974

Q14. SONAR and RADAR differ mainly because: (a) SONAR uses sound waves and works underwater; RADAR uses radio waves and works in air (b) Both use identical radio waves (c) RADAR works only underwater (d) SONAR uses light waves

Q15. Two instruments playing the same note at the same loudness still sound different from each other because of a property called: (a) Frequency (b) Amplitude (c) Timbre (d) Wavelength

Answer Key

Q	Answer	Reason
1	(c) Vacuum	Sound is a mechanical wave and needs particles of a medium to travel; vacuum has none.
2	(b) 332 m/s	This is the standard exam figure for speed of sound in dry air at 0°C.
3	(c) Solids	Tightly packed particles in solids transmit vibrations fastest, unlike widely spaced gas particles.
4	(b) Frequency	Frequency (vibrations per second, in Hz) decides pitch; amplitude decides loudness, not pitch.
5	(a) SOund Navigation And Ranging	SONAR uses ultrasonic pulses and their echoes to detect underwater objects and measure depth.
6	(b) Decibel	Loudness is measured in decibels (dB); hertz measures frequency, a separate property.
7	(b) 20 Hz to 20,000 Hz	This is the standard human audible range; anything outside it is infrasonic or ultrasonic.
8	(b) Ultrasonic	Frequencies above 20,000 Hz are ultrasonic, used in SONAR and medical scanning.
9	(b) Bat	Bats emit ultrasonic waves and use the returning echoes to navigate and hunt in darkness.
10	(b) 17 metres	A 0.1 second gap is needed to perceive a distinct echo, which at sound's speed in air works out to a reflecting surface roughly 17 m away.
11	(b) Stretched membrane	The tabla is a membranophone; striking its taut skin produces the vibration that creates sound.
12	(b) Light travels much faster than sound	Light's speed (3 × 10⁸ m/s) vastly exceeds sound's speed (~332 m/s), so light reaches you first even though both are produced together.
13	(b) Noise Pollution (Regulation and Control) Rules, 2000	These rules, under the Environment (Protection) Act, set zone-wise and day/night decibel limits in India.
14	(a) SONAR uses sound waves and works underwater; RADAR uses radio waves and works in air	This is the key distinguishing fact exam papers test between the two similarly-named technologies.
15	(c) Timbre	Timbre (sound quality) depends on the mix of overtones an instrument produces, letting the ear tell instruments apart even at identical pitch and loudness.