Measuring the Speed of Sound

There are several experiments that can be carried out to determine the speed of sound
Three methods are described below
- The apparatus for each experiment is given in bold

Method 1: Measuring Sound Between Two Points

Measuring the Speed of Sound Using a Loud Noise

sound-method-1, IGCSE & GCSE Physics revision notes

Measuring the speed of sound directly between two points

Two people stand a distance of around 100 m apart
The distance between them is measured using a trundle wheel
One person has two wooden blocks, which they bang together above their head
The second person has a stopwatch which they start when they see the first person banging the blocks together and stops when they hear the sound
This is then repeated several times and an average value is taken for the time
The speed of sound can then be calculated using the equation:

$speed of sound = \frac{distance travelled}{time taken}$

Method 2: Using an Oscilloscope

Measuring the Speed of Sound Using Microphones

sound-method-3, IGCSE & GCSE Physics revision notes

Measuring the speed of sound using an oscilloscope

Two microphones are connected to an oscilloscope and placed about 5 m apart using a tape measure to measure the distance
The oscilloscope is set up so that it triggers when the first microphone detects a sound, and the time base is adjusted so that the sound arriving at both microphones can be seen on the screen
Two wooden blocks are used to make a large clap next to the first microphone
The oscilloscope is then used to determine the time at which the clap reaches each microphone and the time difference between them
This is repeated several times and an average time difference calculated
The speed can then be calculated using the equation:

$speed of sound = \frac{distance between microphones}{time between peaks}$

Measuring Wave Speed in Water

Ripples on water surfaces are used to model transverse waves
- The speed of these water waves can be measured

Ripples on Water

Creating ripples in water

Choose a calm flat water surface such as a lake or a swimming pool
Two people stand a few metres apart using a tape measure to measure this distance
One person counts down from three and then disturbs the water surface (using their hand, for example) to create a ripple
The second person then starts a stopwatch to time how long it takes for the first ripple to get to them
The experiment is then repeated 10 times and an average value for the time is calculated
The average time and distance can then be used to calculate the wave speed using the equation:

$average speed = \frac{distance moved}{time taken}$

Examiner Tip

When you are answering questions about methods to measure waves, the question could ask you to comment on the accuracy of the measurements

In the case of measuring the speed of sound:
- Method 2 is the most accurate because the timing is done automatically
- Method 1 is the least accurate because the time interval is very short

Whilst this may not be too important when giving a method, you should be able to explain why each method is accurate or inaccurate and suggest ways of making them better (for example, use greater distances)

For example, if a manual stopwatch is being used there could be variation in the time measured which can be up to 0.2 seconds due to a person's reaction time
The time interval could be as little as 0.3 seconds for sound travelling in air
This means that the variation due to the stopwatch readings has a big influence on the results and they may not be reliable

Speed of Sound in Air

Sound waves travel at a speed of about 340 m/s in air at room temperature
- The higher the air temperature, the greater the speed of sound

The speed of sound in air varies from 330 – 350 m/s

Speed of Sound in Materials

Sound travels at different speeds in different mediums:
- Sound travels fastest in solids
- Sound travels slowest in gases
Some typical speeds of sound in solids, liquids and gases are:
- Solids $\approx$ 5000 m/s
- Liquids $\approx$ 1500 m/s
- Gases $\approx$ 350 m/s

Measuring the Speed of Sound (Cambridge O Level Physics)

Revision Note