Speed of sound

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Lesson notes

Sound Needs a Medium

Sound waves need **particles** (a medium) to travel.
Sound **cannot** travel in a **vacuum** because there are no particles.
In air, sound travels at about **340 m/s** (depends on pressure and temperature).

Speed in Different Materials

Sound travels fastest in **solids**, then **liquids**, and slowest in **gases**.
In water, speed ≈ **1500 m/s**; in steel, speed > **3000 m/s**.
Particles in solids are **closer together**, so vibrations transfer more quickly.

Measuring Speed of Sound Using Echoes

An **echo** is a reflected sound wave from a surface.
Method: stand ≥50 m from a wall, make a loud sound, and time the echo.
Total distance = **2 × distance to wall** (sound travels there and back).
Calculate speed using: **Speed = Total distance ÷ Time**.
Repeat three times and take an **average** for accuracy.

Improving Accuracy

Use a **microphone** and **oscilloscope** to remove human reaction time error.
Oscilloscope shows pulses for initial sound and echo, giving precise timing.
Human reaction time makes measured time longer, so calculated speed is **lower** than actual.

Sound Absorption

When sound reflects, some energy is **absorbed** by the medium.
Not all sound is reflected back; absorption reduces echo loudness.

Key Equations

Speed = Distance ÷ Time (or Total distance ÷ Time for echoes).
Distance = Speed × Time.
Time = Distance ÷ Speed.

Particle arrangement in solids, liquids, and gases explains why sound speed differs.

Diagram showing a person clapping near a wall and hearing the echo.

Slides

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Practice questions

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1.Which of the following statements about sound waves is correct?
Easy
- ASound waves can travel through a vacuum.
- BSound waves need particles to travel.
- CSound waves travel fastest in gases.
- DSound waves are a type of light wave.
2.What is the approximate speed of sound in air?
Easy
- A1500 m/s
- B340 m/s
- C3000 m/s
- D3400 m/s
3.Sound travels fastest in which state of matter?
Easy
- AGas
- BLiquid
- CSolid
- DVacuum
4.A student stands 50 m from a wall and claps. She hears the echo 0.40 s later. What is the speed of sound calculated from this experiment?
Medium
- A125 m/s
- B250 m/s
- C340 m/s
- D400 m/s
5.Why does human reaction time cause the measured speed of sound to be lower than the actual value?
Medium
- AReaction time adds extra time to the measurement, making speed appear lower.
- BReaction time subtracts time, making speed appear higher.
- CReaction time has no effect on the measurement.
- DReaction time causes the distance to be measured incorrectly.
6.Human reaction time is about 0.25 s. How far does sound travel in that time if the speed of sound is 340 m/s?
Medium
- A85 m
- B1360 m
- C85 cm
- D13.6 m
7.A bat uses echolocation. It sends a sound wave and detects the echo after 0.80 s. How far away is the object? (Speed of sound = 340 m/s)
Medium
- A272 m
- B136 m
- C425 m
- D68 m
8.To measure the speed of sound more accurately, a microphone and oscilloscope can be used. This reduces error from:
Hard
- AMeasuring distance
- BHuman reaction time
- CSound absorption
- DWind speed

More questions

9.In an echo experiment, the sound wave travels to the wall and back. If the distance to the wall is d, what total distance does the sound travel?
Hard
- Ad
- B2d
- Cd/2
- D4d
10.The speed of sound in water is about 1500 m/s. Compared to air, sound in water travels:
Medium
- ASlower
- BAt the same speed
- CFaster
- DCannot travel at all
11.Which of the following is the correct equation for speed?
Easy
- ASpeed = distance × time
- BSpeed = distance / time
- CSpeed = time / distance
- DSpeed = distance + time
12.An echo is the result of sound waves being:
Medium
- AAbsorbed
- BReflected
- CRefracted
- DDiffracted
13.A student uses a microphone and oscilloscope to measure the speed of sound. The oscilloscope shows two pulses: one when the sound is produced and one when the echo returns. Why does this method give a more accurate value than using a stopwatch?
Medium
- AIt removes error due to human reaction time.
- BIt measures the distance more accurately.
- CIt amplifies the sound wave.
- DIt reduces the effect of temperature.
14.A student stands 100 m from a cliff and claps. She hears the echo after 0.60 s. Using the speed of sound in air as 340 m/s, what is the percentage error in her measured speed?
Hard
- A1.96%
- B2.94%
- C3.92%
- D5.88%
15.A sound wave travels from air into water. What happens to the speed of the sound wave?
Medium
- AIt increases because particles are closer together in water.
- BIt decreases because water is denser than air.
- CIt stays the same because the frequency does not change.
- DIt increases because the wavelength decreases.
16.The table shows the speed of sound in three materials. Which material is most likely a gas?
Hard
- AMaterial A: 330 m/s
- BMaterial B: 1500 m/s
- CMaterial C: 5100 m/s
- DAll could be gases.
17.A student repeats an echo experiment five times and calculates the speed each time. The results are: 330 m/s, 335 m/s, 332 m/s, 338 m/s, and 331 m/s. What is the best estimate for the speed of sound?
Medium
- A333 m/s
- B332 m/s
- C335 m/s
- D338 m/s
18.In an echo experiment, the total distance travelled by the sound is 200 m. The time taken is 0.59 s. What is the speed of sound?
Easy
- A339 m/s
- B118 m/s
- C3390 m/s
- D200 m/s
19.A bat emits a sound wave of frequency 40 kHz. The speed of sound in air is 340 m/s. What is the wavelength of the sound wave?
Hard
- A0.0085 m
- B0.85 m
- C8.5 m
- D85 m
20.A student measures the speed of sound by clapping 50 m from a wall. She hears the echo 0.30 s later. Which calculation gives the correct speed?
Medium
- A(2 × 50) / 0.30
- B50 / 0.30
- C0.30 / 50
- D(50 × 0.30) / 2
21.Sound waves cannot travel through a vacuum because:
Easy
- Athere are no particles to transfer the vibrations.
- Blight waves travel faster than sound.
- Cthe vacuum absorbs all sound.
- Dsound waves need a magnetic field.
22.A student uses a trundle wheel to measure the distance to a wall. He then claps and times the echo with a stopwatch. Which source of error is most likely to affect his result?
Medium
- AHuman reaction time when starting and stopping the stopwatch.
- BThe trundle wheel may not measure accurately.
- CThe speed of sound changes with temperature.
- DAll of the above.
23.A sound wave travels from steel into air. What happens to its speed and wavelength?
Hard
- ASpeed decreases and wavelength decreases.
- BSpeed increases and wavelength increases.
- CSpeed decreases and wavelength increases.
- DSpeed increases and wavelength decreases.
24.Which of the following is the correct equation for the speed of sound?
Easy
- Aspeed = total distance / time
- Bspeed = total distance × time
- Cspeed = time / total distance
- Dspeed = 2 × distance / time
25.In an experiment to measure the speed of sound using an echo, a student stands 50 m from a wall and claps. She hears the echo after 0.30 s. What is the calculated speed of sound?
Medium
- A333 m/s
- B167 m/s
- C340 m/s
- D500 m/s
26.A student uses a microphone and oscilloscope to measure the speed of sound. The oscilloscope shows two pulses: one when the sound is produced and one when the echo returns. The time between pulses is 0.29 s. The distance to the wall is 50 m. What is the speed of sound?
Hard
- A345 m/s
- B172 m/s
- C340 m/s
- D290 m/s
27.Which of the following is a correct method to improve the accuracy of measuring the speed of sound using an echo?
Medium
- AUse a trundle wheel to measure distance
- BStand closer to the wall
- CUse a stopwatch with a 0.1 s resolution
- DClap louder
28.A student measures the speed of sound by clapping 100 m from a wall. She hears the echo after 0.59 s. The actual speed of sound is 340 m/s. What is the percentage error in her measurement?
Hard
- A0.3%
- B0.6%
- C1.2%
- D2.5%
29.In an echo experiment, why is it better to stand at least 50 m from the wall?
Medium
- ATo reduce the effect of human reaction time on the measurement
- BTo ensure the echo is loud enough to be heard
- CTo make the sound travel faster
- DTo avoid sound absorption by the wall
30.A student repeats an echo experiment five times and obtains times: 0.58 s, 0.60 s, 0.59 s, 0.61 s, 0.59 s. The distance to the wall is 100 m. What is the best estimate of the speed of sound?
Hard
- A340 m/s
- B338 m/s
- C341 m/s
- D339 m/s
31.Sound travels faster in steel than in air because:
Medium
- AParticles in steel are closer together
- BSteel is denser
- CSteel is a solid
- DAll of the above
32.A student uses a trundle wheel to measure the distance to a wall. He then claps and times the echo with a stopwatch. Which source of error is most likely to affect the accuracy of the speed calculation?
Medium
- AHuman reaction time when starting and stopping the stopwatch
- BThe trundle wheel may not measure exactly 50 m
- CThe clap may not be loud enough
- DThe wall may absorb some sound
33.In an experiment to measure the speed of sound, a student uses a microphone and oscilloscope. The oscilloscope trace shows two pulses. The time base is set to 0.1 s per division. The two pulses are 2.9 divisions apart. The distance to the wall is 50 m. What is the speed of sound?
Hard
- A345 m/s
- B340 m/s
- C350 m/s
- D335 m/s
34.Sound waves need a medium to travel. Which of the following is the name for the substance through which sound travels?
Easy
- AMedium
- BVacuum
- CParticle
- DWave
35.Which of the following best describes why sound cannot travel through a vacuum?
Easy
- AThere are no particles to vibrate
- BSound waves are too weak
- CVacuum absorbs sound
- DSound travels slower in a vacuum
36.The speed of sound in water is approximately 1500 m/s. Compared to its speed in air, sound in water travels:
Medium
- Afaster because particles are closer together
- Bslower because particles are closer together
- Cfaster because particles are further apart
- Dslower because particles are further apart
37.A student stands 50 m from a wall and claps. She hears the echo after 0.40 s. What is the speed of sound calculated from this experiment?
Medium
- A250 m/s
- B340 m/s
- C400 m/s
- D125 m/s
38.The answer to the echo experiment is less than the actual speed of sound (340 m/s). This is because of human reaction time. Why does human reaction time lead to a measured speed that is lower than expected?
Medium
- AThe measured time is longer than the actual time, so speed = distance/time is smaller.
- BThe measured distance is shorter than the actual distance, so speed is smaller.
- CReaction time causes the sound to travel slower.
- DThe echo is heard before the sound is produced.
39.A student uses a microphone and oscilloscope to measure the speed of sound. The oscilloscope shows two pulses: one when the sound is produced and one when the echo returns. The time between the pulses is 0.30 s. The distance to the wall is 50 m. What is the speed of sound?
Medium
- A333 m/s
- B167 m/s
- C340 m/s
- D500 m/s
40.In an experiment to measure the speed of sound, a student uses a microphone and oscilloscope. The oscilloscope trace shows two pulses. The time base is set to 0.1 s per division. The two pulses are 3 divisions apart. The distance to the wall is 50 m. What is the speed of sound?
Hard
- A333 m/s
- B1000 m/s
- C167 m/s
- D500 m/s

Revision notes

## Speed of Sound - Sound needs a medium (particles) to travel; cannot travel in a vacuum. - Speed in air ≈ 340 m/s (depends on pressure and temperature). - Sound travels faster in liquids and solids than in gases because particles are closer together, transferring vibrations more quickly. - Water: ~1500 m/s - Steel: >3000 m/s - **Speed = Total distance ÷ Time** (from echo method). - To measure speed using echo: 1. Stand ≥50 m from a wall. 2. Make a loud sound. 3. Measure time for echo to return. 4. Total distance = 2 × distance to wall. 5. Repeat three times and average. - Human reaction time causes error; using a microphone and oscilloscope improves accuracy. - Some sound is absorbed when reflected. **Key Formula:** - Speed = Total distance / Time - Distance = Speed × Time (rearranged)

Speed of sound: The speed at which sound waves travel through a medium; in air approximately 340 m/s.
Echo: A reflection of a sound wave from a surface.
Medium: The material (solid, liquid, or gas) through which a sound wave travels.
Vacuum: A space with no particles; sound cannot travel through a vacuum.
Oscilloscope: A device that displays sound waves as pulses, used to measure time accurately.

Flashcards

What do sound waves need to travel?

Sound waves need particles (a medium) to travel.

Can sound travel in a vacuum? Why?

No, because there are no particles in a vacuum.

What is the approximate speed of sound in air?

Approximately 340 m/s, but it depends on pressure and temperature.

Does sound travel faster in air or in water?

Sound travels faster in water (about 1500 m/s) than in air.

What is the approximate speed of sound in steel?

Over 3000 m/s.

Why does sound travel faster in liquids and solids than in gases?

Because particles in liquids and solids are closer together, so vibrations are transferred more quickly.

In which state of matter does sound travel slowest?

Sound travels slowest in gases.

In which state of matter does sound travel fastest?

Sound travels fastest in solids.

What is an echo?

An echo is a reflection of a sound wave from a surface.

Describe a method to measure the speed of sound using an echo.

Stand at least 50 m from a wall, make a loud sound, measure the time for the echo to return, then calculate speed = total distance (2 × distance to wall) ÷ time. Repeat three times and take an average.

Why is the total distance in the echo method twice the distance to the wall?

Because the sound travels to the wall and back.

How can we get a more accurate reading for the speed of sound than using a stopwatch?

Use a microphone attached to an oscilloscope to show pulses for the initial sound and the echo, removing human reaction time error.

What happens to some sound when it is reflected from a medium?

Some sound is absorbed, so not all sound is reflected back.

True or false: Sound waves can travel through a vacuum.

False.

True or false: Sound usually travels faster through a solid than through air.

True.

True or false: The equation to calculate speed of sound is speed = total distance × time.

False. The correct equation is speed = total distance ÷ time.

A person stands 50 m from a wall, claps, and hears an echo after 0.40 s. Calculate the speed of sound.

Total distance = 2 × 50 m = 100 m. Speed = 100 m ÷ 0.40 s = 250 m/s.

Why might the calculated speed of sound using the echo method be lower than the actual speed?

Human reaction time causes the measured time to be longer than the actual time, leading to a lower calculated speed.

Rearrange speed = distance ÷ time to give an equation for distance.

Distance = speed × time.

Human reaction time is about 0.25 s. How far does sound travel in that time at 340 m/s?

Distance = 340 m/s × 0.25 s = 85 m.

A bat hears an echo 0.80 s after sending a sound. How far away is the object? (Speed of sound = 340 m/s)

Total distance = 340 m/s × 0.80 s = 272 m. Distance to object = 272 m ÷ 2 = 136 m.

What is echolocation?

Echolocation is sending out sound waves and using echoes to estimate the distance to objects.

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