Textbook Solution: Sound | IGCSE Cambridge Science for Year 6 - Class 6 PDF Download

How Sound Travels

Q1. How many different ways can you think of to make air vibrate and create sound?

Ans: Air can be made to vibrate and create sound in several ways, such as by blowing across a bottle opening, striking a drum, plucking a guitar string, or via the vocal cords when speaking or singing.

Q2. Why do drums have large surfaces?

Ans: Drums have large surfaces to allow for more air to be moved, which produces a louder sound. The larger the surface, the more it can vibrate, enhancing the sound's volume and quality.

Q3. How are sounds made?

Ans: Sounds are made when objects vibrate, creating waves in the air that the human ear perceives as sound.

Q4. How does sound travel through the air?

Ans: Sound travels through the air as a series of longitudinal waves, where air molecules compress and decompress as the wave passes through.

Q5. Explain why sound cannot travel through a vacuum.

Ans: Sound cannot travel through a vacuum because there are no molecules to transmit the sound waves. Sound requires a medium (like air, water, or solids) to travel.

Echoes

Q1. Why do you not hear an echo if you shout when you are very close to the cliff?

Ans: You do not hear an echo when close to a cliff because the sound waves do not have enough distance to travel and reflect back as a distinct sound after the initial sound. The echo merges with the original sound due to the short distance.

Q2. A girl shouts at a cliff. She hears the echo 1.5 seconds later. Sound takes about 3 seconds to travel 1 km. Approximately how far away from the cliff is she? [Hint: remember the sound has to travel to the cliff and back again.]

Ans: 

First, convert the rate given into a speed in meters per second:

• Sound travels 1 km (which is 1000 meters) in 3 seconds.
• Speed of sound = 1000 meters / 3 seconds = 333.33 meters per second.

The total time for the sound to travel to the cliff and back is 1.5 seconds.

This means the sound travels to the cliff and back, covering a distance we denote as 2d (where d is the one-way distance to the cliff).

Total distance for the round trip = speed of sound * time

2d = 333.33 meters/second * 1.5 seconds

2d = 500 meters

To find the one-way distance to the cliff (d), divide the total distance by 2:

d = 500 meters / 2

d = 250 meters

Q3. Joe shouts at a cliff 300 m away. He uses a stopwatch to time how long it takes for him to hear the echo. He times it as 2.1 seconds. Amina also tries the experiment. She times the echo for her shout as 2.4 seconds. Use the results to make a conclusion.

Ans: The slight difference in timing between Joe and Amina's experiments could be due to variations in how quickly they started/stopped the stopwatch, differences in the exact position where they shouted, or environmental factors affecting sound travel.

Q4. What is the main source of error in their measurements and what effect has this had on your conclusion?

Ans: The main source of error is likely human reaction time in using the stopwatch. This introduces uncertainty in the precise timing, affecting the accuracy of the calculated distance from the cliff.

Q5. Why are the seats in theatres usually covered in soft materials?

Ans: Seats in theatres are covered in soft materials to absorb sound, reducing unwanted echoes and reverberations. This improves the clarity of the sound from the stage.

Q6. Why are echoes a problem in music recording studios?

Ans: Echoes can distort the sound quality of recordings by causing unwanted reverberations and blurring the clarity of musical notes and vocals.

Q7. What would you suggest to do to reduce this problem?

Ans: To reduce echoes in recording studios, it is advisable to use sound-absorbing materials on walls and ceilings, design the room to minimize sound reflections, and possibly use diffusers to scatter the sound waves.

End of the Chapter Review

Q1. Use the words in the box to fill in the gaps in the paragraph below:

Ans: Sound is made by vibrations. Sound can travel through all solids, liquids, and gases. Sound travels through the air as a wave when the particles of air push each other. Sound cannot travel through a vacuum.

Q2. What do we call the substance sound travels through?

Ans: Medium

Q3. Why can't sound travel through a vacuum?

Ans: Because there are no particles.

Q4. What is an echo?

Ans: When a sound wave hits a surface and reflects back.

Q5. Copy and complete the following sentences about using echoes.

Ans: Submarines can use sonar to find out how deep water is. Sound waves are sent down to the bottom of the sea. When they hit the sea bed, the waves reflect. The distance the wave has travelled can be calculated by measuring the time between sending out the wave and receiving its echo.

Q6. In the slinky spring model of a sound wave, each coil vibrates backwards and forwards but goes back to its original position. What does this show about how sound travels through air?

Ans: The sound source makes the air next to it vibrate and this air passes on the vibration.

Q7. The apparatus shown on the right is often used to prove that sound cannot travel through a vacuum.

(a) Why does this experiment prove that sound cannot travel through a vacuum?

Ans: It demonstrates that without air (in a vacuum), the bell cannot be heard, indicating sound needs a medium to travel.

(b) Explain why sound cannot travel through a vacuum.

Ans: Sound waves require a medium (such as air, water, or solids) to transmit their vibrations. In a vacuum, there are no particles to transmit these vibrations, hence sound cannot travel.

Q8. The warmer air is, the further apart the air particles are likely to be. Do you think the speed of sound through warm air would be faster or slower than through cold air?

Ans: Faster, because the particles are more energetic and can transmit vibrations more quickly.

Q9. There is no atmosphere on the Moon. Explain why, even if we were very close, we could not hear an explosion that happened on the Moon.

Ans: Without an atmosphere, there are no particles to carry the sound waves from the explosion to our ears, making it impossible to hear the sound.

Q10. Sound dissipates very little energy when it passes through some materials. This means that the sound does not get quieter.

(a) Design an experiment using a sound source and a microphone connected to a loudspeaker to compare how well wood, glass, and steel reduce sound levels.
Ans: 

(b) Why would it be better to use a sound meter rather than a microphone and loudspeaker?

Ans: A sound meter measures sound levels more accurately and directly, while a microphone and loudspeaker might introduce variables that affect the measurements.

(c) State the precautions you would take to make sure your results were accurate and reliable.

Ans: Ensure all equipment is calibrated, conduct the experiment in a controlled environment to minimize external noise, and repeat tests to verify results.

(d) A singer wants to make one room in their house soundproof. How would the singer use the results of your experiment to decide which material to use?

Ans: By comparing the sound reduction capabilities of each material (wood, glass, steel) demonstrated in the experiment, the singer can choose the material that best meets their soundproofing needs.

Q11. A girl sees the lightning in a thunderstorm 8 seconds before she hears the thunder. If sound travels at 340 m/s in air, how far away is the thunderstorm?

Ans: 2720 meters away. This is calculated by multiplying the time delay (8 seconds) by the speed of sound (340 m/s).

Q12. A boy is standing 400 m away from a cliff. He makes a loud noise. If sound travels at 340 m/s, how long will it be before he hears the echo?

Ans: 2.35 seconds. The sound must travel to the cliff and back (800 m total), and at 340 m/s, this takes about 2.35 seconds.