Contents
Physics Topics can be both theoretical and experimental, with scientists using a range of tools and techniques to understand the phenomena they investigate.
Explanation and Factors Affecting the Speed of Sound
Sound takes some time to travel from the sound producing body to our ears. The speed of sound tells us the rate at which sound travels from the sound producing body to our ears. The speed of sound depends on a number of factors. These are given below :
1. The speed of sound depends on the nature of material (or medium) through which it travels. The speed of sound is different in different materials (or different media). For example, the speed of sound in different materials like air, water and iron is different. At room temperature, the speed of sound in air is 344 m/s; the speed of sound in water is about 1500 m/s; and the speed of sound in iron is 5130 m/s.
In general, sound travels slowest in gases, faster in liquids and fastest in solids. If we convert the just given speeds of sound in air, water and iron from metres per second (m/s) into kilometres per hour (km/h), we will find that the speed of sound in air is 1238 km/h ; the speed of sound in water is 5400 km/h; and the speed of sound in iron is 18468 km/h.
2. The speed of sound depends on the temperature. For example, the speed of sound in air at a temperature of 0°C is 332 m/s but the speed of sound in air at a temperature of 20°C is 344 m/s. In fact, as the temperature of air rises, the speed of sound in it increases. Thus, the speed of sound in air will be more on a hot day than on a cold day.
3. The speed of sound depends on the humidity of air. For example, the speed of sound is less in dry air but more in humid air. In other words, sound travels slower in dry air but faster in humid air. In fact, as the humidity of air increases, the speed of sound through it also increases.
The speeds of sound in some common materials at different temperatures are given below.
Speed of Sound in Various Materials (or Media)
From the above table we can see that the speed of sound in air at room temperature is 344 metres per second (which is written as 344 m/s). This means that sound travels a distance of 344 metres in 1 second through air at the room temperature. Sound travels faster through water than through air. For example, the speed of sound in water is about 1500 metres per second (1500 m/s).
Thus, sound travels about 5 times faster in water than in air. This means that sound can be heard very fast inside water. The fact that sound can be heard very fast inside water is used by creatures living in the sea-water to communicate with one another (even when they are far away). For example, two whales which are even hundreds of kilometres away from each other under the sea can talk to each other very easily through sea-water. The sound of their talk is carried by sea-water very rapidly (due to high speed of sound in water).
Sound travels faster in solids than in liquids. For example, sound travels at a speed of 5130 metres per second through iron (or steel). This is more than 3 times the speed of sound in water and about 15 times the speed of sound in air.
Thus, sound travels about 15 times faster in iron (or steel) than in air. Here is an interesting consequence of the very high speed of sound in iron or steel. If a train is very far away from us, we cannot hear the sound of approaching train through the air.
But if we put our ear to the railway line made of steel, then we can hear the sound of the coming train easily even if it is quite far away. This is due to the fact that sound travels much more fast through the railway line made of steel than through air.
Sonic Boom
Many objects such as some aircrafts, bullets and rockets, etc., travel at speeds which are greater than the speed of sound in air. They are said to have ‘supersonic speed’. Thus, the term supersonic refers to the speed of an object which is greater than the speed of sound.
For example, when an aircraft flies with a speed greater than the speed of sound, it is said to have supersonic speed. Due to its very high speed, a supersonic aircraft produces extremely loud sound waves called ‘shock waves’ in air. The shock waves produced by a supersonic aircraft carry a great amount of energy.
The tremendous air pressure variations caused by the shock waves produce a loud burst of sound called ‘sonic boom’. We can now define sonic boom as follows : Sonic boom is an explosive noise caused by the shock waves from an Figure.
This jet aircraft flies at supersonic energy emitted by sonic boom can even shatter the glass panes of a building if the supersonic aircraft flies low over it. As long as an aircraft flies at the supersonic speed, it continues to emit unpleasant sonic boom in the surrounding area. The first supersonic jet aircraft was made in 1948.
The Race Between Sound and Light
The speed of sound in air is about 344 m/s and the speed of light in air is 300,000,000 m/s. It is clear that the speed of light is very great as compared to the speed of sound. So, though sound may take a few seconds to travel a distance of a few hundred metres, light will take practically no time to reach a distance of even a few kilometres. Some of the everyday observations based on the low speed of sound in air but very high speed of light are given below :
(i) It is a common observation that in the rainy season, the flash of lightning is seen first and the sound of thunder is heard a little later (though both are produced at the same time in clouds). It is due to the very high speed of light that we see the flash of lightning first and it is due to comparatively low speed of sound that the thunder is heard a little later.
(ii) In the game of cricket, the ball is seen to hit the bat first and the sound of hitting is heard a little later. It is due to the very high speed of light that we see the ball hitting the bat first. And it is due to comparatively low speed of sound that the sound of hitting is heard a little later.
(iii) If a gun is fired from a distance, we see the flash of gun first and the sound of gun shot is heard a little later. It is due to the very high speed of light that we see the flash of gun first, and it is because of the comparatively low speed of sound that the sound of gun shot is heard a little later.
In all the above observations we notice that the light reaches us from a distant object instantly (because of its great speed) but sound takes a little more time to reach us from the same object (due to its low speed). Thus, it is light which wins the race between light and sound. We will now solve one problem based on the speed of sound (or velocity of sound).
Example Problem.
If a thunder is heard by a man 4 seconds after the lightning is seen, how far is the lightning from the man ? (Speed of sound in air = 330 m/s).
Solution:
We know that light travels at a great speed as compared to that of sound, therefore, the flash of light of lightning’ will reach the man in no time but sound takes 4 seconds to reach the man. Now :
Speed of sound = 330 m/s
Distance = ? (To be calculated)
And Time = 4 s
We know that : Speed = \(\frac{\text { Distance }}{\text { Time }}\)
So, 330 = \(\frac{\text { Distance }}{4}\)
And, Distance = 330 × 4 m
= 1320 m
Thus, the lightning is at a distance of 1320 metres from the man.