Teacher sheet · Ages 15–16 · about 45–50 minutes
Sound waves, decibels and the loudest sound ever recorded
Lesson page: echoesof.science/school/sound/ages-15-16/ · Pupil worksheet: echoesof.science/school/sound/ages-15-16/print/
Learning aims
- Use v = f·λ for sound.
- Use v = 331·√(T/273 K) for the temperature dependence of the speed of sound in air.
- Explain and calculate with the logarithmic decibel scale.
- Measure the speed of sound and evaluate the method.
- Interpret historical data (Krakatoa 1883) critically.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: why do we see lightning before we hear thunder? |
| 10 min | Read the text and work through one wave-equation and one decibel example. |
| 25 min | Echo experiment outside (or as a demonstration), three runs. |
| 5 min | Calculate v and compare with the formula for today's temperature. |
| 5 min | Quiz, or the Decibel ladder game as homework. |
Materials for the experiment
- A large flat wall (a school building) with open ground in front
- A measuring tape or trundle wheel
- Two wooden blocks to clap together
- A stopwatch (or a phone)
Safety: Choose a place away from traffic. Keep clapping blocks away from fingers. Do not use starting pistols or anything explosive.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Doubling the decibels doubles the loudness. | The scale is logarithmic: +10 dB is 10 times the intensity. |
| Sound is faster in air than in water because water is 'thicker'. | It is about four times faster in water (1480 vs 343 m/s). |
| Space is silent because it is cold. | It is silent because it is (nearly) a vacuum with no medium. |
Worksheet answers
- v = f·λ. 20 Hz: λ = 343/20 ≈ 17 m. 20,000 Hz: λ = 343/20,000 ≈ 1.7 cm.
- T = 308 K: v = 331·√(308/273) ≈ 351.6 m/s, about 352 m/s.
- +20 dB = 10² = 100 times the intensity.
- Expected 320–360 m/s. Errors: reaction time and rhythm (time 20 claps, repeat), measuring d, wind, echoes from other buildings.
Quiz answers
- A sound has frequency 686 Hz in air at 20 °C (343 m/s). Its wavelength is… 0.5 m. λ = v / f = 343 / 686 = 0.5 m.[2]
- How many times more intense is a 90 dB sound than a 60 dB sound? 1000 times. +30 dB = 10³ times the intensity.[3]
- Using v = 331·√(T/273 K), the speed of sound at 0 °C is about… 331 m/s. At T = 273 K the square root is 1.[2]
- Why can't sound cross the space between Earth and the Moon? It is essentially a vacuum: no medium for a pressure wave. A pressure wave needs matter to compress.[5]
- At what level does the OpenStax table list the threshold of pain? 120 dB. Loud rock concert: 120 dB, threshold of pain.[3]
- Krakatoa's pressure wave was recorded on barometers worldwide. It went round the Earth… three to four times in each direction. Cities recorded up to seven pressure spikes over several days.[4]
Going further
Lightning flash, then thunder 6 s later: how far away was the strike? Then estimate how long Krakatoa's sound took to reach Rodrigues, 4,800 km away, at 343 m/s.
Sources used in this lesson
- [1] College Physics 2e, 17.1 Sound (OpenStax, Rice University): reliability High
- [2] College Physics 2e, 17.2 Speed of Sound, Frequency, and Wavelength (OpenStax, Rice University): reliability High
- [3] College Physics 2e AP, 17.3 Sound Intensity and Sound Level (OpenStax, Rice University): reliability High
- [4] The Sound So Loud That It Circled the Earth Four Times (Nautilus (science magazine)): reliability Medium-high
- [5] Data Sonifications (NASA Science): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/sound/sources/
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