# 6.1.4 Measuring the Speed of Waves

### Measuring the Speed of Waves

#### Experiments to Determine 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 directly between two points

1. Two people stand a distance of around 100 m apart
2. The distance between them is measured using a trundle wheel
3. One person has two wooden blocks, which they bang together above their head
4. 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
5. This is then repeated several times and an average value is taken for the time
6. The speed of sound can then be calculated using the equation:

Method 2: Using Echoes

Measuring the speed of sound using echoes

1. A person stands about 50 m away from a wall (or cliff) using a trundle wheel to measure this distance
2. The person claps two wooden blocks together and listens for the echo
3. The person then starts to clap the blocks together repeatedly, in rhythm with the echoes
4. A second person has a stopwatch and starts timing when they hear one of the claps and stops timing 20 claps later
5. The process is then repeated and an average time calculated
6. The distance travelled by the sound between each clap and echo will be (2 × 50) m
7. The total distance travelled by sound during the 20 claps will be (20 × 2 × 50) m
8. The speed of sound can be calculated from this distance and the time using the equation:

Method 3: Using an Oscilloscope

Measuring the speed of sound using an oscilloscope

1. Two microphones are connected to an oscilloscope and placed about 5 m apart using a tape measure to measure the distance
2. 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
3. Two wooden blocks are used to make a large clap next to the first microphone
4. The oscilloscope is then used to determine the time at which the clap reaches each microphone and the time difference between them
5. This is repeated several times and an average time difference calculated
6. The speed can then be calculated using the equation:

#### Measuring Wave Speed in Water

Creating ripples in water

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

#### Worked Example

Small water waves are created in a ripple tank by a wooden bar. The wooden bar vibrates up and down hitting the surface of the water. The diagram below shows a cross-section of the ripple tank and water.

Which letter shows:

a) The amplitude of a water wave?
b) The wavelength of the water wave?

Part (a)

Step 1: Recall the definition of amplitude

• Amplitude = The distance from the undisturbed position to the peak or trough of a wave

Step 2: Mark the undisturbed position on the wave

• This is the centre of the wave

Step 3: Identify the arrow between the undisturbed position and a peak

• The amplitude is arrow D

Part (b)

Step 1: Recall the definition of wavelength

• Wavelength = The distance from one point on the wave to the same point on the next wave

Step 2: Draw lines on each horizontal arrow

• This helps to identify the points on the wave the arrows are referring to

Step 3: Identify the arrow between two of the same points on the wave

• The wavelength is arrow C

#### Exam 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 3 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 (use bigger 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

### Author: Katie

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.
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