### Experiments to Determine the Speed of Sound

- In your IGCSE exam, you might be asked to describe a method of measuring the speed of sound
- When giving your method, try and include the following things:
- List all of the apparatus that you will need
- Choose a suitable (realistic) distance over which you will measure the sound
- Describe how you will measure this distance
- Explain how you will produce a loud enough sound
- Explain how you will time the sound (and how the timer will be started and stopped)
- Explain how you will calculate the speed (give an equation)
- State that you will then repeat the experiment several times and take an average

- Three methods for carrying out this experiment are given below:

- Two people stand a distance of around 100m apart
- The distance between them is measured using a trundle wheel
- One of the people has two wooden blocks, which he bangs together above his head
- The second person has a stopwatch which he starts when he sees the first person banging the blocks together and stops when he hears 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:

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

- Two microphones are connected to an oscilloscope and placed about 5 m apart

(This distance can be measured using a tape measure) - 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:

#### Exam Tip

Method 3 is the most accurate method (because the timing is done automatically) whilst 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).