The speed of sound in air is about 340 m/s.
The speed of sound in air can be measured using an echo method and using speed = distance/time.
The speed of sound in a metal rod can be found by measuring the length and the frequency produced when struck.
A result is accurate if it is close to the true value.
A result can be considered accurate if it is repeatable, reproducible and if systematic errors have been checked for.

Keywords

Synchronisation - Synchronisation is the process of ensuring two (or more) things happen at the same time.
Period - The period of a wave is the time taken for one oscillation.
Wavelength - The wavelength of a wave is the distance a wave travels during one period.
Accurate - A result is accurate if it is close to the true value.
Systematic error - A systematic error affects all results by the same amount.

Common misconception

If measurements are repeatable (or precise) then they will also be accurate.

Repeatable (or precise) measurements may involve a systematic error making them inaccurate.

A teacher should be able perform the demonstration of the metal rod experiment quite easily. There are many free apps that can be downloaded onto a mobile phone that will measure the frequency, make sure you have tested one out before the lesson.

Teacher tip

Content guidance

Risk assessment required - equipment

Supervision

Adult supervision required

Licence

This content is © Oak National Academy Limited (2024), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Starter quiz

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6 Questions

Q1.

What is an echo?

When a person repeats a word over and over again

Correct answer: When sound waves reflect back so a sound is heard more than once

When sound waves refract from a surface

When a sound wave is absorbed into the air so it can be heard for a longer time

Q2.

The material or substance that a wave is travelling through is called the wave .

Correct Answer: medium, Medium

Q3.

A wave has a frequency of 2 Hz and a wavelength of 10 m. What is the wave speed?

0.2 m/s

5 m/s

12 m/s

Correct answer: 20 m/s

100 m/s

Q4.

The time taken for one oscillation is called the of a wave.

Correct Answer: period, time period

Q5.

Which one of these does not describe the wavelength of a wave?

Correct answer: the distance from peak to trough

the distance from one peak to the next peak

the distance from one trough to the next trough

the distance from one part of a wave to the same part of the next wave

the distance travelled by a wave during one period

Q6.

Which word means a result is close to the true value?

precise

repeatable

reproducible

Correct answer: accurate

Exit quiz

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6 Questions

Q1.

Give the missing number in the following sentence. The accepted value for the speed of sound in air is m/s.

Correct Answer: 340

Q2.

Which two of the below would allow a result to be considered accurate?

Correct answer: If it is close to the true value

If repeat measurements cluster closely together in a small range

If it is repeatable and precise, and has been checked for random errors

Correct answer: If it is repeatable and reproducible, and has been checked for systematic errors

Q3.

Starting with the measurement of distance, put the steps for the echo method of measuring the speed of sound into the correct order.

1 - Use a trundle wheel to measure a distance of at least 100m from a vertical wall.

2 - Two people should stand at that distance from the wall.

3 - One person should make a sudden noise, the other starts a timer at the same time

4 - The timer should be stopped when the echo is heard.

5 - The speed of sound is calculated using (2 x distance to the wall)/time taken

Q4.

Andeep and Sofia are 100 m from a wall. Andeep makes a sound, Sofia measures the time for the echo to return as 0.64 s. Their result for the speed of sound, to 2 significant figures, is m/s.

Correct Answer: 310

Q5.

Starting with the measurement of the rod length, put the steps for the method of measuring the speed of sound in a metal rod into the correct order.

1 - Measure the length of the rod (L) with a meter rule. Wavelength will be 2L.

2 - Suspend the rod using elastic bands and strike the end so the bar rings.

3 - Use a frequency-meter to determine the frequency of the rod's oscillations.

4 - Calculate the speed of sound in the bar by multiplying frequency and wavelength.

Q6.

Alex and Jun suspend a 1.20 m long metal rod on elastic bands. Jun strikes the rod and Alex uses a phone app to find the frequency of the sound: 2675 Hz. The speed of sound in the rod is m/s.

Correct Answer: 6420

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