Speed of Sound
Speed of Sound
The speed of sound depends on the medium the sound is travelling in. Sound travels faster in solids than in liquids, and faster in liquids than in gases. This is because the density of solids is higher than that of liquids which means that the particles are closer together. Sound can be transmitted more easily.
The speed of sound also depends on the temperature of the medium. The hotter the medium is, the faster its particles move and therefore the quicker the sound will travel through the medium. When we heat a substance, the particles in that substance have more kinetic energy and vibrate or move faster. Sound can therefore be transmitted more easily and quickly in hotter substances.
Sound waves are pressure waves. The speed of sound will therefore be influenced by the pressure of the medium through which it is travelling. At sea level the air pressure is higher than high up on a mountain. Sound will travel faster at sea level where the air pressure is higher than it would at places high above sea level.
Table: The speed of sound in different materials.
|
Substance |
v (\(\text{m·s$^{-1}$}\)) |
|
aluminium |
\(\text{6 420}\) |
|
brick |
\(\text{3 650}\) |
|
copper |
\(\text{4 760}\) |
|
glass |
\(\text{5 100}\) |
|
gold |
\(\text{3 240}\) |
|
lead |
\(\text{2 160}\) |
|
water, sea |
\(\text{1 531}\) |
|
air, 0℃ |
\(\text{331}\) |
|
air, 20℃ |
\(\text{343}\) |
Did You Know?
The speed of sound in air, at sea level, at a temperature of 21℃ and under normal atmospheric conditions, is \(\text{341}\) \(\text{m·s$^{-1}$}\).
Optional Experiment: Measuring the speed of sound in air
Aim
To measure the speed of sound.
Apparatus
-
Starter's gun or anything that can produce a loud sound in response to visible action
-
Stopwatch
Method
The speed of sound can be measured because light travels much faster than sound. Light travels at about \(\text{300 000}\) \(\text{m·s$^{-1}$}\) (you will learn more about the speed of light in the next section) while sound only travels at about \(\text{300}\) \(\text{m·s$^{-1}$}\). This difference means that over a distance of 300 m, the light from an event will reach your eyes almost instantly but there will be an approximate half a second lag before you hear the sound produced. Thus if a starter's pistol is fired from a great distance, you will see the smoke immediately but there will be a lag before you hear the sound. If you know the distance and the time then you can calculate the speed (distance divided by time). You don't need a gun but anything that you can see producing a loud sound.
Try this:
-
Find a place where you know the precise, straight-line distance between two points (maybe an athletics track)
-
Someone needs to stand at the one point to produce the sound
-
Another person needs to stand at the other point with the stop watches
-
The person with the stopwatch should start the stopwatch when they see the other person make the sound and stop the stopwatch when they hear the sound (do this a few times and write the times down)
Results
You can now calculate the speed to sound by dividing the distance by the time. Remember to work in S.I. units (metres and seconds). If you took multiple readings then you can sum them and divide by the number of readings to get an average time reading. Use the average time to calculate the speed:
\[v = \frac{D}{t}\]|
Time (s) |
Distance (m) |
\(\text{m·s$^{-1}$}\) |
|
Averages |
||
Conclusions
Some questions to ask:
-
What is your reaction time on the stopwatch? You can test this by starting it and then trying to stop it immediately.
-
What was the forecast temperature on the day of the measurement?
-
Was it humid or very dry?
Discuss what might change the speed of sound that you measured.
You can vary this experiment by trying it on days when the weather is different as this can change air pressure and temperature.
This lesson is part of:
Mechanical Waves and Sound