Science · Class 9 · Chapter 10

Sound Waves: Characteristics and Applications

Aim: Build a clear, working understanding of this chapter's key ideas, connected to real NCERT examples, worked problems, and everyday situations.
  • Vibration — Periodic to-and-fro motion that produces sound.
  • Compression / Rarefaction — Region of higher / lower than average density.
  • Longitudinal wave — Particles vibrate parallel to wave propagation.
  • Wavelength / Frequency / Time period — λ, distance between crests; ν, oscillations per second; T, time per oscillation.
  • Amplitude / Intensity — Size of density change; energy per area per time.
  • Pitch / Loudness — Perceived frequency / perceived amplitude.
  • Echo / Reverberation — A separate reflected sound / overlapping, lingering reflections.
  • Infrasonic / Ultrasonic — Below 20 Hz / above 20 kHz — outside human hearing.
40 min lesson

Chapter 10 · Concept 1 of 16

Sound Is Produced by Vibration

  • Pluck a stretched rubber band: as long as it vibrates, you hear sound. Once it stops, so does the sound.
  • The tuning fork — A struck tuning fork's vibrating prongs disturb the water they touch and the air around them — direct proof that sound comes from vibration, whether from strings, air columns, or vocal cords.

Chapter 10 · Concept 2 of 16

Sound Needs a Medium to Propagate

  • Through solids, liquids, and gases — An ear pressed to a desk hears a knock; spoons tapped underwater are still heard. Sound travels through all three states of matter.
  • But never through a vacuum — As air is pumped out of a bell jar with a ringing bell inside, the sound fades to silence — even though the bell keeps visibly ringing. This is why astronauts can't hear each other directly in space.

Chapter 10 · Concept 3 of 16

Compressions and Rarefactions

  • An oscillating piston pushes air particles forward, then pulls back — alternately bunching them together and spreading them apart.
  • Compression — A region of higher-than-average air density, where particles are pushed close together.
  • Rarefaction — A region of lower-than-average air density, where particles are spread further apart.

Chapter 10 · Concept 4 of 16

Sound Is a Longitudinal, Mechanical Wave

  • Longitudinal — Particles vibrate back and forth parallel to the direction the wave travels — unlike a transverse wave, where particles vibrate perpendicular to it.
  • Mechanical — It requires a material medium to travel — no particles, no sound. (Light, by contrast, is a transverse wave that needs no medium at all.)

Chapter 10 · Concept 5 of 16

Sound Carries Energy, Not Matter

  • Grains sprinkled on a stretched sheet jump when a loud sound is made nearby — without anything ever touching the sheet.
  • The particles don't travel with the wave — Each particle of the medium only oscillates about its own resting position. It's the energy of the disturbance — not the particles themselves — that propagates outward from the source.

Chapter 10 · Concept 6 of 16

Wavelength, Frequency, and Time Period

  • Quantity | Meaning | Symbol / Unit
  • Wavelength — Distance between two consecutive crests (or troughs) — λ, metre (m)
  • Frequency — Number of density oscillations per unit time — ν, hertz (Hz)
  • Time period — Time for one complete oscillation — T, second (s)

Chapter 10 · Concept 7 of 16

Amplitude and Intensity

  • Amplitude — The maximum change in density (above or below average) in a compression or rarefaction. Bigger amplitude means more energy carried by the wave.
  • Intensity — Sound energy passing through a unit area per unit time. As a wave spreads out from its source, the same energy covers a larger area — so intensity falls with distance.

Chapter 10 · Concept 8 of 16

Speed of Sound

  • v = λ × ν — Speed equals wavelength times frequency. The speed depends on the medium — not on the source or the frequency itself.
  • Fastest in solids, slowest in gases — Sound travels ~4–5× faster in water than in air, and ~15–20× faster in solids than in air. Warmer or more humid air also carries sound faster.

Chapter 10 · Concept 9 of 16

Putting v = λν to Work

  • Range of human hearing (344 m/s) — 20 Hz → λ = 344 ÷ 20 = 17.2 m 20,000 Hz → λ = 344 ÷ 20000 = 1.72 cm
  • Distance to a lightning strike — Thunder heard 5 s after the flash, sound at 340 m/s: Distance = 340 × 5 = 1700 m ≈ 1.7 km away.

Chapter 10 · Concept 10 of 16

Reading Frequency Off a Graph

  • A sound wave in steel (speed 5000 m/s) has a wavelength of 50 m, read from its graph.
  • Frequency and time period — ν = v ÷ λ = 5000 ÷ 50 = 100 Hz T = 1 ÷ ν = 1 ÷ 100 = 0.01 s

Chapter 10 · Concept 11 of 16

Pitch and the Range of Hearing

  • Pitch — Higher frequency sounds are perceived as higher (shriller) pitch — a whistle vs. thunder's low rumble.
  • 20 Hz – 20,000 Hz — The human audible range. Below it: infrasonic waves (elephants can detect these). Above it: ultrasonic waves (bats, dogs, dolphins can detect these).

Chapter 10 · Concept 12 of 16

Loudness, Decibels, and Noise

  • Loudness is how we perceive amplitude — larger amplitude sounds louder, and it fades with distance from the source.
  • Measured in decibels (dB) — Rustling leaves: a few dB. Conversation: ~60 dB. Firecrackers: over 100 dB. Prolonged loud, unwanted sound (noise) can damage hearing.

Chapter 10 · Concept 13 of 16

Echo

  • A reflected sound is heard as a separate echo only if it arrives at least 0.1 s after the original.
  • Minimum echo distance ≈ 17 m — At 340 m/s, sound covers 34 m (there and back) in 0.1 s — so a reflecting surface must be at least 17 m away for a distinguishable echo. Example: an echo heard after 0.5 s means the wall is (340 × 0.5) ÷ 2 = 85 m away.

Chapter 10 · Concept 14 of 16

Reverberation

  • In a large hall, sound reflects off many surfaces. If reflections arrive less than 0.05 s apart, they blend into a lingering persistence of sound.
  • Designed, not accidental — Concert halls are architecturally tuned for pleasant reverberation; soft materials like curtains and padded seats absorb excess sound to prevent garbled echoes.

Chapter 10 · Concept 15 of 16

Uses of Ultrasonic and Infrasonic Waves

  • Range | Applications
  • Infrasonic (< 20 Hz) — Detecting earthquakes, volcanic eruptions, and severe storms
  • Ultrasonic (> 20 kHz) — Ultrasonography, breaking kidney stones, industrial cleaning and welding, detecting metal flaws

Chapter 10 · Concept 16 of 16

Echolocation and SONAR

  • Bats, dolphins, whales — They emit ultrasonic bursts and sense the echoes bouncing off obstacles and prey, to navigate and hunt in the dark.
  • SONAR — Example 10.6 — A sonar signal returns after 0.90 s at 1530 m/s in seawater. One-way time = 0.45 s. Distance = 1530 × 0.45 = 688.5 m.

Quick Recap

Check Your Understanding

  1. 1Which observation best supports the idea that sound is a mechanical wave? (i) Sound shows reflection (ii) Sound needs a medium to propagate (iii) Sound has frequency (iv) Sound carries energy
  2. 2For a sound wave propagating in a medium, increasing its frequency will increase its: (i) wavelength (ii) speed (iii) number of compressions per second (iv) time period
  3. 3If 20 compressions pass a point in 4 seconds, the frequency is: (i) 80 Hz (ii) 5 Hz (iii) 10 Hz (iv) 0.2 Hz