Sound • Topic 1 of 3

Nature & Propagation of Sound

What is sound? Sound is a form of energy that our ears detect as hearing. It is produced by vibrating objects — a plucked guitar string, the stretched skin of a drum, the reed of a harmonium, or our own vocal cords. Touch your throat while you speak and you can feel the buzz; the vibration is the source of the sound. When the vibration stops, the sound stops too.

Sound needs a material medium to travel. Unlike light, sound cannot move through empty space. It needs particles — of a solid, a liquid, or a gas — to carry it from the source to your ear. The classic bell-jar experiment proves this: an electric bell ringing inside a glass jar grows fainter and fainter as a pump removes the air, and falls silent in a near-vacuum even though the hammer is still striking. Because of this, sound is called a mechanical wave.

How the wave moves. A vibrating object pushes the layer of air next to it forward, squeezing those particles closer together. This crowded region of high pressure and high density is a compression. The object then moves back, leaving the particles spread apart in a region of low pressure and low density called a rarefaction. As the object keeps vibrating, a train of compressions and rarefactions travels outward.

Sound is a longitudinal wave. Each air particle simply oscillates back and forth about its own position — parallel to the direction the wave travels — handing the disturbance on to its neighbour. The particles do not move along with the sound; only the energy and the pattern of compressions and rarefactions advance.

  • Compression (C): particles bunched together, pressure and density are maximum.
  • Rarefaction (R): particles spread apart, pressure and density are minimum.
  • One full wave = one compression + one adjacent rarefaction.
  • The wave carries energy, not matter, away from the source.

Why no sound in space. Outer space is almost a perfect vacuum, so astronauts cannot talk to each other directly even an arm's length apart — they use radios, which work on electromagnetic waves that need no medium. This single idea, that sound is mechanical and needs particles, explains why a ticking clock sealed in a vacuum flask goes quiet and why you hear a swimmer's splash through water but not through the empty gap of space.

Longitudinal sound wave showing compressions and rarefactionsSound as a Longitudinal Wavetuning forkCRCRCdirection of wave travel (particles vibrate parallel to this)C = compression (high pressure)R = rarefaction (low pressure)
Solved Examples
1
Worked Example
Explain why an astronaut on the Moon cannot hear the sound of a rock falling beside them, even though they would on Earth.
Solution
  1. Sound is a mechanical wave, so it needs a material medium (solid, liquid or gas) whose particles can be set vibrating.
  2. The wave travels as compressions and rarefactions passed from one particle to the next.
  3. The Moon has practically no atmosphere — its surface is a near-vacuum with no air particles.
  4. With no medium, the disturbance from the falling rock cannot be carried to the astronaut's ear.

Answer: Because the Moon has no air (a vacuum), there are no particles to carry the sound, so it cannot be heard.

2
Worked Example
In the bell-jar experiment, the ringing of the electric bell becomes fainter as air is pumped out. State what this shows and what would happen in a perfect vacuum.
Solution
  1. As the pump removes air, the number of particles available to carry the sound steadily decreases.
  2. Fewer particles mean fewer collisions to relay the compressions and rarefactions, so the sound grows fainter.
  3. The hammer still strikes the gong, so the source is unchanged — only the medium is being removed.
  4. In a perfect vacuum there would be no particles at all to carry the wave.

Answer: It shows sound needs a medium to travel; in a perfect vacuum no sound would be heard even though the bell still vibrates.

3
Worked Example
Distinguish between a compression and a rarefaction in a sound wave travelling through air.
Solution
  1. When the vibrating source moves forward, it pushes air particles together, forming a crowded region.
  2. This crowded region has the maximum density and maximum pressure — it is a compression.
  3. When the source moves back, it leaves the particles spread out, forming a sparse region.
  4. This sparse region has the minimum density and minimum pressure — it is a rarefaction.

Answer: A compression is a high-pressure, high-density region (particles close together); a rarefaction is a low-pressure, low-density region (particles spread apart).

4
Worked Example
Sound is called a longitudinal wave. Justify this name using the motion of air particles.
Solution
  1. In a longitudinal wave, the particles of the medium vibrate parallel to the direction in which the wave travels.
  2. In a sound wave, each air particle moves back and forth along the same line as the wave's motion.
  3. This back-and-forth motion creates alternate compressions and rarefactions along that line.
  4. The particles do not travel with the wave; they only oscillate about their fixed positions while the energy moves forward.

Answer: Because the air particles vibrate parallel to the direction of wave travel, sound is correctly classified as a longitudinal wave.

5
Worked Example
A child claims sound carries air from the speaker to the ear. Explain why this is wrong.
Solution
  1. In a sound wave, each air particle only oscillates back and forth about its own mean position.
  2. A particle pushes its neighbour and then returns; it does not move bodily across the room.
  3. What actually travels from source to ear is the energy and the pattern of compressions and rarefactions.
  4. If air itself were transported, there would be a permanent wind from every sound source, which we never observe.

Answer: Sound transfers energy through the medium, not the medium itself; the air particles merely vibrate in place, so no air is carried to the ear.

6
Worked Example
Name the medium and the cause of sound when (a) a drum is beaten and (b) a tuning fork is struck. State what is common to both.
Solution
  1. (a) Beating a drum sets its stretched membrane vibrating, which disturbs the surrounding air.
  2. (b) Striking a tuning fork sets its prongs vibrating, which likewise disturbs the air.
  3. In both cases the medium carrying the sound to the listener is the air around the source.
  4. The common cause in every case is a vibrating object producing the disturbance.

Answer: Both produce sound through a vibrating object (drum membrane / fork prongs), and in both the medium carrying it here is air; vibration is the common cause.

Key Points

  • Sound is a form of energy produced by a vibrating object; when vibration stops, the sound stops.
  • Sound is a mechanical wave and needs a material medium (solid, liquid or gas) — it cannot travel through a vacuum.
  • It is a longitudinal wave: particles vibrate parallel to the direction the wave travels.
  • A compression is a region of high pressure/density; a rarefaction is a region of low pressure/density.
  • The wave carries energy outward, not the particles of the medium themselves.
Quick Check — 4 MCQs
Tap an option to check your answer0 / 4
Q1.Sound is produced by
Explanation: Every sound originates from something that vibrates; stop the vibration and the sound stops.
Q2.Sound cannot travel through
Explanation: Sound is mechanical and needs particles; a vacuum has none, so no sound travels through it.
Q3.In a sound wave, a region where particles are crowded together with high pressure is called a
Explanation: A compression is the high-pressure, high-density region; a rarefaction is its low-pressure opposite.
Q4.Sound is classified as a longitudinal wave because the particles of the medium
Explanation: In a longitudinal wave the particles oscillate parallel to the direction of propagation.
Practice more MCQs → 📝 Assignment · 30 marks