Two astronauts are repairing a space station arm together during a spacewalk. Can they hear the metal clanking as they work?
Key idea
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.
Key idea
Sound Needs a Medium to Propagate
Through solids, liquids, and gasesAn 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 vacuumAs 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.
Key idea
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.
Key idea
Sound Is a Longitudinal, Mechanical Wave
1
Longitudinal
Particles vibrate back and forth parallel to the direction the wave travels — unlike a transverse wave, where particles vibrate perpendicular to it.
→
2
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.)
Key idea
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.
Key idea
Wavelength, Frequency, and Time Period
Quantity | Meaning | Symbol / Unit
WavelengthDistance between two consecutive crests (or troughs) — λ, metre (m)
FrequencyNumber of density oscillations per unit time — ν, hertz (Hz)
Time periodTime for one complete oscillation — T, second (s)
Key idea
Amplitude and Intensity
1
Amplitude
The maximum change in density (above or below average) in a compression or rarefaction. Bigger amplitude means more energy carried by the wave.
→
2
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.
Key idea
Speed of Sound
v = λ × νSpeed equals wavelength times frequency. The speed depends on the medium — not on the source or the frequency itself.
Fastest in solids, slowest in gasesSound 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.
Thunder heard 5 s after the flash, sound at 340 m/s: Distance = 340 × 5 = 1700 m ≈ 1.7 km away.
Key idea
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 periodν = v ÷ λ = 5000 ÷ 50 = 100 Hz T = 1 ÷ ν = 1 ÷ 100 = 0.01 s
Key idea
Pitch and the Range of Hearing
1
Pitch
Higher frequency sounds are perceived as higher (shriller) pitch — a whistle vs. thunder's low rumble.
→
2
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).
Key idea
Loudness, Decibels, and Noise
Loudness is how we perceive amplitudelarger 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.
Key idea
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.
Key idea
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 accidentalConcert halls are architecturally tuned for pleasant reverberation; soft materials like curtains and padded seats absorb excess sound to prevent garbled echoes.
Key idea
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
Key idea
Echolocation and SONAR
1
Bats, dolphins, whales
They emit ultrasonic bursts and sense the echoes bouncing off obstacles and prey, to navigate and hunt in the dark.
→
2
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.
Chapter · Key terms to remember
Key Terms
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.
Chapter · Key terms to remember
More Key Terms
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.
Chapter · Key terms to remember
More Key Terms
Echo / Reverberation
A separate reflected sound / overlapping, lingering reflections.
Which 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
For 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
If 20 compressions pass a point in 4 seconds, the frequency is: (i) 80 Hz (ii) 5 Hz (iii) 10 Hz (iv) 0.2 Hz
Project as-is — students copy the questions, then the class discusses answers together.