# Speed of sound

Practical Activity for 14-16

In this activity students work with a partner at home (or in school) and use phyphox app to estimate the speed of sound.

#### Equipment

Each pair of students will need:

- Two phones with phyphox installed

#### Procedure

Ask students to:

- Place the phones 3 m apart.
- Stand next to their phone and open the phyphox app. Click on the Acoustic stopwatch and press play. Their partner should do the same.
- Clap once. When their partner hears the clap they should also clap.
- Subtract the times displayed on the phones to find the difference. This gives the time delay for the sounds to travel 6 m (twice the distance between the phones).
- Divide 6 m by the time delay to work out the speed of a sound wave.
- Repeat for two more distances and find an average value for the speed of sound.

#### Teaching notes

The acoustic watch starts counting when it hears a noise above a certain threshold and stops when a second noise is detected. The phone closest to the source of the noise will detect it before the other phone. If the phones are a distance d apart and the speed of sound is *v*, the further away phone will detect a sound wave a time *d**v* after the nearby phone. In this experiment there are two claps, and so the total distance travelled is 2*d*. Finding the difference between the times *t*_{1} and *t*_{2} displayed on the phones eliminates your partner’s reaction time and gives the following relationship for the speed of sound:

2*d**t*_{1} − *t*_{2}

Students should be able to obtain an estimate for the speed of sound of between 300 and 400 ms^{ -1}. Echoes can cause erroneous readings. Students could try to identify sources of possible echoes and eliminate them, change location or increase the threshold on the acoustic stopwatch if this proves to be an issue.

#### Learning outcome

Students describe an experiment to estimate the speed of sound.