Measuring the speed of sound in air and solids | Exit Quiz | Oak National Academy

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

Answer explanation

The accepted value for the speed of sound in air in normal conditions is 340 m/s.

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

• 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

• If it is repeatable and reproducible, and has been checked for systematic errors

Answer explanation

Accurate results are close to the true value. Results that are repeatable and reproducible can be considered accurate if scientists are confident there is no systematic error.

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

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

• Two people should stand at that distance from the wall.

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

• The timer should be stopped when the echo is heard.

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

Answer explanation

That is the correct sequence for the steps.

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.

Answer explanation

The correct calculation is (2 x 100m) / 0.64 s = 312.5. This rounds to 310 (to 2 significant figures). Their result could be lower than 340 m/s (the expected speed of sound in air) due to reaction times making the time too long

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.

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

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

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

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

Answer explanation

That is the correct sequence for the steps.

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.

Answer explanation

The wavelength of the sound wave will be double the length of the rod, so 2.40 m. Wave speed = frequency x wavelength = 2675 x 2.40 = 6420 m/s.