Sound
Sound as a longitudinal wave, the range of human hearing, the speed of sound and echoes, and ultrasound.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 3.4.1Describe the production of sound by vibrating sources and its transmission as a longitudinal wave.
- 3.4.2State the approximate range of human hearing (20 Hz to 20 000 Hz).
- 3.4.3Describe how to measure the speed of sound in air and use echo methods.
6 minute read
Sound
Sound is produced by a vibrating source — a speaker cone, a guitar string, vocal cords — and travels as a longitudinal wave, made of compressions (particles pushed closer together) and rarefactions (particles spread further apart).
Because it needs particles to pass the vibration along, sound cannot travel through a vacuum. It can travel through solids, liquids and gases, generally fastest through solids and slowest through gases, because particles are closer together and transmit vibrations more efficiently.
Human hearing
The normal range of human hearing is approximately 20 Hz to 20 000 Hz. Sound above this range is called ultrasound, and below it, infrasound.
Measuring the speed of sound
One method: stand a known distance from a large flat wall, make a sharp sound (such as a clap), and measure the time for the echo to return. The sound travels to the wall and back, so:
speed = (2 × distance to wall) / time for the echo
This "there and back" detail is the most common place marks are lost — the distance in the calculation is double the distance to the wall.
Think of it like this
A longitudinal sound wave is like a Slinky spring pushed sharply at one end: a pulse of squeezed coils (compression) travels down the spring, followed by a stretched-out section (rarefaction), while no single coil actually travels along the spring's length.
Worked examples
Method, step by step
A student stands 85 m from a cliff and claps. The echo is heard 0.50 s later. Calculate the speed of sound.
- 1Total distance travelled = 2 × 85 = 170 m
- 2speed = distance / time
- 3speed = 170 / 0.50
speed = 340 m/s
Common misconceptions
- Thinking sound is a transverse wave like light. Sound is longitudinal — the vibrations are parallel to the direction of travel, not perpendicular.
- Forgetting the echo pulse travels there AND back, so the distance to the reflecting surface is half the total distance travelled, not the same as it.
- Believing sound travels fastest through air. It generally travels faster through solids and liquids than through gases.
In the exam
- For echo calculations, always halve the total distance (or double the single distance) — state this step explicitly to secure the method mark even if the arithmetic slips.
- When asked why sound cannot travel through space, the answer must mention that sound needs a medium (particles) to transmit compressions and rarefactions — "space is empty" alone is not enough.