The shape of the lesson
By the end of the lesson, learners can
- Time the round trip: Set the distance to the wall and watch the pulse travel out and return, with the stopwatch climbing to the echo time.
- Twice the distance, one timing: The factor of two is the whole trick: the sound makes a round trip, so the path is twice the distance to the wall.
- Audible range: about 20 Hz to 20 000 Hz for humans.
- Speed order: fastest in solids, then liquids, then gases. In air, about 330 to 340 m/s.
- Echo equation: 2d = v t. The 2 is the round trip. Divide by 2 to get the one-way distance.
One picture every idea returns to
An echo is sound reflected from a hard surface. Because the sound travels to the surface and back, it covers twice the distance before it returns, so the speed is found from speed = total distance divided by time. In air sound travels at about 330 to 340 m/s.
Forty-five minutes, phase by phase
| Time | Phase | What happens in the room | Grouping |
|---|---|---|---|
| 0 to 5 min | Hook: predict | Open with the question in the title, There, and back again.. Shout at a cliff and the sound comes back a moment later. | Whole class, then pairs |
| 5 to 17 min | Teach and check: Time the round trip | Set the distance to the wall and watch the pulse travel out and return, with the stopwatch climbing to the echo time. Project tab 2 of the lab (Speed and echoes) and let a student drive it while the class predicts. | Whole class, then pairs |
| 17 to 30 min | Teach and check: Twice the distance, one timing | The factor of two is the whole trick: the sound makes a round trip, so the path is twice the distance to the wall. Model the worked example Worked example on the board, one line at a time. | Connect-Extend-Challenge |
| 30 to 36 min | Check | The four Stage 1 questions from the student lesson, answered on mini whiteboards or by show of fingers. Re-teach the one most get wrong. | Individual, then whole class |
| 36 to 43 min | Plenary | The exam-style questions from Stage 2, one at a time: thirty seconds alone, then a random call. The feedback lines in the key give the examiner's wording. | Individual, random call |
| 43 to 45 min | Exit and homework | Set the worksheet below (7 questions, 18 marks). Learners self-assess against the objectives before they leave. | Individual |
Protect the parts that carry the learning
The sequence carries about fifty minutes of material, so the lab is driven by one student while the class predicts rather than explored freely. Times are a guide.
Protected: the check at 30 minutes. It is the only individual read on whether the key idea landed, and it is not cut.
Cut first if the build overruns: the last check in Twice the distance, one timing, never the check or the plenary.
In a 60 minute block: run worksheet questions 1 to 3 in class as a Rally Coach set and model both the hardest worked example and its check live.
Connect-Extend-Challenge
After the new idea, each student writes three lines: how it connects to what they knew, how it extends their thinking, and what still challenges them. Pairs compare, then the challenges become the plenary.
The full structure, with its variations and the other twelve, is on the cooperative-learning page.
What to head off, and how
| Trap learners fall into | Teaching move that pre-empts it |
|---|---|
| In an echo the sound makes a round trip, so use twice the distance to the surface. | Using only the one way distance halves the speed. And sound travels faster in liquids and solids than in air, which is the opposite of what many expect. |
Speed of sound and echoes: worksheet
7 questions, 18 marks. Written in the style of the papers; no past paper question is reproduced.
State the approximate range of frequencies that a healthy human ear can detect.
Sound travels at different speeds in solids, liquids, and gases.
(a) Place these three states in order of the speed of sound, from fastest to slowest. [1] (b) Explain, in terms of the particles, why sound travels fastest in a solid. [2]A person stands in a valley and shouts. They hear the echo from a rock face 1.5 s later. The speed of sound in air is 340 m/s. Calculate the distance to the rock face.
To measure the speed of sound, a student stands 660 m from a large cliff, claps once, and times the echo at 4.0 s. Calculate the speed of sound from this experiment.
In an echo question, a student writes d = v t and calculates the distance to a wall without dividing by two. Explain the error and its effect on the answer.
A student says that sound, like light, must slow down when it passes from air into a solid such as steel, because the solid is denser. Explain why this is wrong.
A survey ship sends a pulse of sound straight down to the seabed and detects the reflected pulse 0.40 s later. The speed of sound in seawater is 1500 m/s. Calculate the depth of the water.
Answer key and marking notes
From about 20 Hz to about 20 000 Hz (20 kHz). ✓✓
(a) Solid (fastest), then liquid, then gas (slowest). ✓
(b) In a solid the particles are very close together and tightly linked. ✓ So a vibration is passed from one particle to the next very quickly. ✓
Total path: 2d = v t
2d = 340 x 1.5 = 510 mOne way: d = 510 / 2
d = 255 m
The sound travels 510 m in total, so the rock face is half that away.
Round trip: distance = 2 x 660 = 1320 m
v = 2d / t = 1320 / 4.0v = 330 m/s
A sensible value for the speed of sound in air.
- The timed sound travels to the wall and back, so v t is the total there-and-back distance, which is 2d. ✓
- Leaving out the division by two gives a distance twice as large as the real one. ✓
- Sound actually travels faster in a solid than in air, not slower. ✓
- The particles in a solid are close together and tightly bonded, so they pass the vibration on to each other much more quickly than the widely spaced particles in air. ✓
Round trip: 2d = v t = 1500 x 0.40 = 600 m
d = 600 / 2d = 300 m
Depth is half the total path, because the pulse goes down and comes back.
2 multiple-choice questions on this lesson
From the topic question bank. Assign them in the app for a marked score in your gradebook, or run the topic as a class game.
What is the approximate range of frequencies audible to a healthy young human ear?
- A20 Hz to 20 000 Hz
- B0 Hz to 100 Hz
- C20 000 Hz to 2 000 000 Hz
- D1 Hz to 200 Hz
A person shouts toward a cliff and hears the echo 2.0 s later. The speed of sound in air is 340 m/s. How far away is the cliff?
- A85 m
- B170 m
- C340 m
- D680 m
Quiz key
Human hearing ranges from about 20 Hz to 20 000 Hz (20 kHz). Frequencies above this range are called ultrasound.
Total distance = v × t = 340 × 2.0 = 680 m for the round trip, so the cliff is half of that, 340 m. Option D forgets to halve; B halves twice.
What to project
The student lesson embeds these tabs. Open them full screen on the projector; every slider has a drag handle on the stage.
Slides for this lesson
A projectable deck for every pack is on the way. Until then, present the student lesson full screen: its sections, figure and lab tabs follow the same order as the plan above.