Speed Of Sound Through Solids, Liquids And Gases

What is Sound and Speed of Sound ?

Sound is a form of energy, just like electricity, heat or light. When you strike a bell, it makes a loud ringing noise. Now instead of just listening to the bell, put your finger on the bell after you have struck it. Can you feel it shaking? This movement or shaking, i.e. the to and fro motion of the body is termed as Vibration. The sound moves through a medium by alternately contracting and expanding parts of the medium it is travelling through.

The speed of sound is the distance which is travelled per unit time by any sound wave which can be measured.

Speed of Sound in Air

The speed of sound is an essential parameter used in a variety of field in Physics. The speed of sound refers to the distance travelled per unit time by a sound wave propagating through a medium. The speed of sound when travelling through air at 20oC is 343.2 m/s which translates to 1,236 km/h.

Speed Of Sound

The speed of sound in gases is proportional to the square root of the absolute temperature (measured in Kelvin) but it is independent of the frequency of the sound wave or the pressure and the density of the medium. But none of the gases we find in real life are ideal gases and this causes the properties to slightly change.

Speed of Sound in Solid, Liquid and Gases

Sound can travel through wood too! In fact, sound likes travelling through solid more. In the olden days, doctors used stethoscopes consisting of thin wooden rods with broadened ends and they worked exactly like our modern ones and performed just as well. How can sound travel through solids?

Sound is nothing more than a disturbance which is propagated by the collisions between the particles; one molecule hitting the next and so forth. Solids are significantly denser than liquids or gases. This means that the molecules are closer to each other in solids than in liquids and in liquids than in gases. This closeness due to density means that they can collide very quickly. Effectively it takes less time for a molecule of a solid to bump into its neighbouring molecule. Due to this advantage, the speed of sound in a solid is larger than in gas.

Similarly, the density of a liquid is greater than the density of a gas. Therefore the distances between molecules is more in liquids than in solids but is less than in gases. Hence speed of sound in liquids lies in between the speed of sound in solids and gases.

We should remember that the speed of sound is independent of the density of the medium when it enters a liquid or solid. Since gases expand to fill the given space, density is quite uniform irrespective of the type of gas. This clearly isn’t the case with solids and liquids.

The table below lists the speed of sound in various media. The proportional variation of speed with density is clearly visible here.

Speed Of Sound

Another very curious fact is that in solids, sound waves can be created either by compression or by tearing of the solid also known as Shearing. Such waves exhibit different properties from each other and also travel at different speeds. This effect is seen clearly in Earthquakes. Earthquakes are created due to the movement of the earth plates which then sends these disturbances in the form of waves similar to sound waves through the earth and to the surface causing an Earthquake. Typically compression waves travel faster than tearing waves which is why Earthquakes always start with an up and down motion followed after some time by side to side motion. In seismic terms, the compression waves are called P-waves and the tearing waves are called S-waves. S-waves are the more destructive of the two, causing most of the damage in an earthquake.

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Practise This Question

A stone dropped from the top of a tower of height 300 m splashes into a pond of water at its base. When will the sound of the splash be heard at the top?
( Take velocity of sound = 340 ms1 and g = 9.8 ms2)