Sound Waves

Section: Physics | Syllabus: Cambridge IGCSE Combined Science (0653)

Production and Transmission of Sound

Sound is produced by a vibrating source, such as a tuning fork, a loudspeaker cone, a guitar string, or vocal cords
The vibrating source makes the particles of the surrounding medium vibrate, and this vibration passes from particle to particle away from the source
Sound is a longitudinal wave: the particles of the medium vibrate parallel to (along) the direction in which the wave, and its energy, travels
Sound needs a medium (a solid, liquid or gas) to travel through - it cannot travel through a vacuum, because there are no particles to vibrate

A vibrating tuning fork producing a sound wave in air

An electric bell is suspended inside a bell jar connected to a vacuum pump, and switched on so that it can be heard ringing
As air is pumped out of the jar, the sound of the bell becomes quieter, even though the hammer can still be seen striking the bell
As air is let back into the jar, the sound gets louder again
This shows that sound needs particles of a medium to travel, since removing the air removes the medium that carries the sound to the listener

Extended Only

As a vibrating source moves outward, it pushes particles of the medium closer together, forming a compression - a region of higher pressure
As the source moves back, particles spread further apart, forming a rarefaction - a region of lower pressure
Compressions and rarefactions travel outward from the source, one wavelength apart, carrying the sound energy through the medium

Compressions and rarefactions in a sound wave

The speed of sound in air is approximately 330-350 m/s (a commonly used value is 340 m/s)

Extended Only

In general, sound travels faster in solids than in liquids, and faster in liquids than in gases
This is because particles are closer together in solids than in liquids, and closer together in liquids than in gases, so vibrations pass from particle to particle more quickly

Stand a measured distance d from a large flat wall, such as the side of a building
Make a sharp sound, such as a clap, and start a stopwatch at the same instant
Stop the stopwatch the instant the echo is heard, to find the time t
The sound travels to the wall and back, a total distance of 2d, so the speed of sound is found using:
- Formula:
  - v = (2d/t)
- Where:
  - v = speed of sound (m/s)
  - d = distance to the wall (m)
  - t = time between making the sound and hearing the echo (s)
Repeating the measurement several times and calculating an average value for t improves the accuracy of the result

Worked Example: Measuring the Speed of Sound

Question: Muchindu stands 170 m from a cliff face, claps once, and hears the echo 1.0 s later. Calculate the speed of sound.

Given: d = 170 m, t = 1.0 s v = (2d/t) Answer

v = (2 × 170/1.0) = 340 m/s

An echo is a reflection of sound waves, heard as a repetition of the original sound
Sound reflects well off hard, flat surfaces, such as walls and cliffs, and is absorbed by soft surfaces, such as curtains and carpets

Loudness depends on the amplitude of a sound wave: a larger amplitude produces a louder sound, and a smaller amplitude produces a quieter sound
Pitch depends on the frequency of a sound wave: a higher frequency produces a higher pitch, and a lower frequency produces a lower pitch

How amplitude affects loudness and frequency affects pitch

The human ear can detect sound frequencies from approximately 20 Hz to 20 000 Hz (20 kHz) - this is the audible range
Ultrasound is sound with a frequency higher than 20 kHz, above the upper limit of human hearing

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