3.4 Sound – Speed of Sound in Air
What is Speed of Sound?
The speed of sound, denoted by \$v\$, is how fast a sound wave travels through a medium. In air at room temperature it’s about 343 m s⁻¹, but it changes with temperature, humidity and pressure.
Why Measure Distance and Time?
If you know how far the sound travels (\$d\$) and how long it takes (\$t\$), you can calculate the speed using the simple relation
\$v = \frac{d}{t}\$
Think of it like a runner: the distance is the track length and the time is how long the runner takes. The faster the runner, the higher the speed. 📏⏱️
Step‑by‑Step Method
- Choose a straight, open area (e.g., a long hallway or a field). 📐
- Mark a starting point and a far point exactly \$d\$ metres apart. Use a tape measure or a laser distance meter. 🏁
- At the starting point, shout or clap loudly while a friend at the far point starts a stopwatch. 🎤⏱️
- When the sound reaches the far point, the friend stops the stopwatch. Record the time \$t\$ in seconds.
- Repeat the measurement 3–5 times to minimise error and calculate the average time.
- Insert the average \$t\$ and the known \$d\$ into \$v = d/t\$ to find the speed of sound.
Example Calculation
| Trial | Time (s) |
|---|
| 1 | 0.912 |
| 2 | 0.908 |
| 3 | 0.915 |
| Average | 0.912 |
Assuming the distance \$d\$ is 100 m, the speed of sound is
\$v = \frac{100\ \text{m}}{0.912\ \text{s}} \approx 109.6\ \text{m s}^{-1}\$
(Notice the result is lower than the typical 343 m s⁻¹ because this example uses a very short distance; in practice you’d use a longer distance to reduce timing errors.) 🚀
Key Takeaways
- Speed of sound = distance ÷ time.
- Use a long, straight path and repeat measurements.
- Average the times to improve accuracy.
- Remember that temperature, humidity and pressure affect the result.
- Have fun experimenting – it’s like being a science detective! 🕵️♂️