Echoes and the speed of sound — questions

1. identify

A pupil shouts towards the tall brick wall of a sports hall across an empty playground and hears the shout again a moment later. Which statement explains the second sound?

Circle one answer.

• The sound waves reflect from the wall and travel back to the pupil.
• The sound waves pass through the wall and return from its far side.
• The pupil’s ears keep vibrating for a moment after the shout.
• The wall starts to vibrate and makes a new sound of its own.

Nudge (only if you are stuck): Where does the sound go after it leaves the pupil, and what does a hard wall do to a wave that reaches it?

2. calculate

A hiker shouts across a canyon and hears the echo from the far wall 2.5 s later. The speed of sound in air is 340 m/s. Calculate the distance from the hiker to the far wall.

Distance to the far wall: ______________ unit ________

Nudge (only if you are stuck): First find how far the sound travels in 2.5 s. Is that the distance to the wall?

3. identify

A class wants to hear an echo of a clap that is clear and separate from the clap itself. The table shows four places where the class could stand. Which row would give a clear, separate echo?

Circle one answer.

• A
• B
• C
• D

Nudge (only if you are stuck): Check two things in each row: how well the surface reflects sound, and whether the echo would come back late enough to hear on its own.

4. calculate

A pupil stands 119 m from a tall building and claps once. The speed of sound in air is 340 m/s. Calculate the time between the clap and its echo.

Time from clap to echo: ______________ unit ________

Nudge (only if you are stuck): How far does the sound travel before the pupil hears the echo?

5. calculate / explain

A pupil claps in a small tiled shower room. The nearest wall is 1.7 m away. The clap sounds longer, as if it rings, but no separate echo is heard. (a) Calculate the time for the reflection from that wall to return, using 340 m/s. (b) Explain why no separate echo is heard.

(a) Time for the reflection to return: ______________ unit ________

(b) Why no separate echo is heard

Nudge (only if you are stuck): Compare the return time with the gap your ear needs to hear two short sounds separately.

6. describe

A pupil uses echoes from a large wall to determine the speed of sound in air. Put the steps of the method in order.

Write the number of each step in order.

1. The partner stops the stop-clock on the 20th clap after that.
2. Measure 100 m out from the wall with a trundle wheel and stand at the mark.
3. Calculate the speed from 2 × 100 m × 20 ÷ the stop-clock time.
4. A partner starts the stop-clock on one clap, counting it as zero.
5. Clap and adjust the rhythm until each clap is made as the echo of the one before arrives.

• Step 1: ______
• Step 2: ______
• Step 3: ______
• Step 4: ______
• Step 5: ______

Nudge (only if you are stuck): Which measurement has to be made before anyone can clap from the right place?

7. calculate

A pupil stands 75 m from a wall and claps in time with the echoes. A partner times 30 intervals between claps, and the stop-clock reads 13.2 s. Calculate the speed of sound. Give your answer to 3 significant figures.

Speed of sound: ______________ unit ________

Nudge (only if you are stuck): How far does the sound travel in one interval? How far in 30 intervals?

8. calculate / explain

A pupil stands 50 m from a wall, claps once and times the echo by hand. The stop-clock reads 0.45 s. (a) Calculate the speed of sound from this one timing. Give your answer to 2 significant figures. (b) The value is far from about 340 m/s. Explain why, and explain how timing 20 intervals instead would improve the measurement.

(a) Speed from the one timing: ______________ unit ________

(b) Why the value is far off, and how 20 intervals help

Nudge (only if you are stuck): Estimate the true echo time from 50 m. How big is a reaction delay of 0.1–0.2 s compared with it?

9. identify / calculate

A group stands 120 m from a wall and times 20 intervals between claps five times (table). (a) Identify the anomalous time. (b) Calculate the mean of the other four times. (c) Calculate the speed of sound from your mean. Give this speed to 3 significant figures.

(a) Anomalous time: ______________ unit ________

(b) Mean of the other four: ______________ unit ________

(c) Speed of sound: ______________ unit ________

Nudge (only if you are stuck): Which time is far from the rest? Leave it out before taking the mean. Each interval is a journey to the wall and back.

10. calculate / explain

Two pupils stand 450 m apart on a playing field. One holds two boards high and bangs them together. The other starts a stop-clock on seeing the boards meet and stops it on hearing the bang. The mean of several timings is 1.30 s. (a) Calculate the speed of sound. Give your answer to 2 significant figures. (b) Explain why the time light takes to cross the field can be ignored.

(a) Speed of sound: ______________ unit ________

(b) Why the light time can be ignored

Nudge (only if you are stuck): How many times does the sound cross the gap between the pupils? How quickly does light cross it?

11. calculate

A walker stands between two parallel cliffs and claps once. The first echo is heard 0.80 s after the clap and the second 1.40 s after it. The speed of sound in air is 340 m/s. Calculate the distance between the two cliffs.

Distance between the cliffs: ______________ unit ________

Nudge (only if you are stuck): Find the distance to each cliff separately. Each echo time is a journey there and back.

12. explain

A pupil plans to measure the speed of sound with echoes from a wall 100 m away and says: “Clap as fast as you can. More claps in the same time make the measurement more accurate.” Explain why this plan is incorrect.

Why the plan is incorrect

Nudge (only if you are stuck): What must the time between two claps be equal to for speed = 2dN ÷ T to work?