Edexcel IGCSE Physics · Spec 3.21-3.23
Sound Waves
How sound waves travel and how we hear them.
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Sound Waves
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Explained
Sound, and measuring how fast it travels
Sound is a longitudinal wave. The particles of the medium vibrate backwards and forwards along the same line the wave travels, rather than across it, and the wave passes through as a series of compressions where the particles are pushed together and rarefactions where they are spread apart.
Because it moves particles, sound needs a medium. It travels through solids, liquids and gases, and it cannot cross a vacuum, which is why a ringing bell inside a jar falls silent as the air is pumped out.
It travels fastest through solids, more slowly through liquids, and slowest through gases, which is the opposite order to light. The reason is that the particles in a solid are closest together, so a vibration is passed on to the next particle sooner.
Pitch and loudness
Pitch is determined by frequency. More vibrations per second means a higher pitch.
Loudness is determined by amplitude, the size of the vibration. A larger amplitude carries more energy and sounds louder.
On an oscilloscope trace these are read from two different directions, which is the whole skill. A higher pitched sound has waves squeezed closer together across the screen, so the time period is shorter. A louder sound has taller waves, so the trace is stretched further up and down. Comparing two traces means checking both dimensions separately.
The human hearing range runs from about 20 Hz to about 20 000 Hz. Above that is ultrasound, which is used in medical scanning and in sonar, and the upper limit of the range falls with age.
Measuring the speed of sound by echo
Stand a measured distance from a large flat wall, make a sharp sound, and time how long the echo takes to come back.
The distance in the calculation is twice the distance to the wall, because the sound goes there and back. Missing that doubling is the single most common error in this experiment, and it gives an answer half the size it should be.
So the speed is twice the distance to the wall, divided by the time for the echo. In air at room temperature you should get somewhere near 330 to 340 metres per second.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme takes this experiment apart across four parts, and its notes read almost as a list of the ways it goes wrong.
For measuring the distance it credits a suitable instrument, listing a trundle wheel, a tape measure or a laser range finder, and condoning a ruler. It then instructs the examiner to ignore any attempt at using the speed of sound, which would be circular, since the speed of sound is what the experiment is trying to find.
On the calculation itself, the worked example uses 130 m for a wall 65 m away and reaches 370 metres per second for three marks. The note then says that an answer rounding to 190 metres per second scores two marks, and gives the reason in brackets: use of 65 m. So the doubling is worth exactly one mark, and the mark scheme has anticipated the error rather than treating it as a wrong answer.
Asked to criticise the method, it credits four ideas from six: that the original method did not repeat and average, human reaction time, the accuracy of the distance measurement, the sound becoming inaudible at a large distance, the fact that a large distance reduces the effect of timing errors, and uncontrolled variables such as temperature, humidity, wind and noise.
Two of those pull in opposite directions, which is what makes this a good evaluation question. A longer distance gives a longer time, so a fixed timing error matters less. But too long and the echo is too faint to hear. The best distance is a compromise, and saying so is a stronger answer than simply asking for a longer one.
For improvements it credits datalogging with a microphone or sound meter, and electronic starting and stopping of the timer, while ignoring use of an oscilloscope. The theme is removing the human from the timing, because reaction time is the largest error in the whole experiment.
The wave equation applied to sound
Wave speed equals frequency multiplied by wavelength, and it applies to sound exactly as it does to any other wave.
What is useful is that the speed of sound in a given medium is fixed. So if the frequency rises the wavelength must fall, and the two are inversely proportional. A high pitched note has a short wavelength; a low one has a long wavelength.
When a sound passes from one medium into another the speed changes, and the frequency does not, because the frequency is set by whatever made the sound. So the wavelength is what changes, and that is what causes refraction.
Spec 3.21-3.23
What you need to know
- Describe sound as a longitudinal wave
- State the human hearing range
- Read pitch and loudness on an oscilloscope
Active recall
Quick check
Answer each question before opening the answer.
What type of wave is sound, and what does it need to travel?
A longitudinal wave; it needs a medium and cannot travel through a vacuum.
What is the approximate range of human hearing?
About 20 Hz to 20 000 Hz (20 kHz).
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