What is speed of sound in different media

 

SPEED OF SOUND IN DIFFERENT MEDIA

 

It is a matter of common experience that when we strike an object with a hammer, the sound of hammering is heard a short while after the actual impact. Similarly, the sound of cracker is heard only after it has exploded. These simple observations reveal that sound travels with a finite speed. Further, the flash and thunder is heard later on. The lapse of time between thunder is sometimes a few tens of a second. From this, we can easily conclude that sound travels with a speed which is much less than the speed of light.

 

Thus, the speed of sound is finite and is much less than the speed of The speed of sound depend upon :

 

1. the properties (elasticity and density) of the medium through which is propagates, and

 

2. temperature of the medium.

 

Sound (in general) has greatest speed in case of solids and least in case of gases. For liquids, the speed of sound is intermediate between these two extremes.

 

In general, speed of sound in solids > speed of sound in liquids > speed of sound in gases.

 

However, in certain solids, the speed (v) of sound is much less than that rubber) = 54 m/s and v (for hydrogen) = 1284 m/s. The speed of sound in water (a liquid) = 1531 m/s. The speed of sound in methyl alcohol (a liquid gas), its value = 1284 m/s.

 

 

The speed of sound increases with increase in temperature of the medium. The speed of sound in air at 0°C is 331 m/s. In air, it increases roughly by 0.6 m/s with rise of 1°C in temperature. Thus, a good approximation of the speed of sound in air of a particular temperature is given by

 

 

At 20°C, the speed of sound is 343 m/s, at 22°C its value is 344.2 m/s and at 25°C, it is 346 m/s.

 

 

SHOCK WAVES: SONIC BOOM

 

Sources that move faster than the speed of sound are said to have supersonic speeds. In such cases, the source advances more than the sound waves. The energy emitted in such cases is unable to move in front of the source and is concentrated on the sides. This concentration of energy as it travels outward in the form of a cone-shaped wave, is called a shock wave. Note that it is not necessary for an object to have a vibrating source of sound in order to create a shock wave. The only condition necessary is that the object moves with a speed greater than the speed of sound. The ratio of the speed of the source to the speed of sound is called the Mach number. Thus, shock waves, also called Mach waves, are generated by objects moving with a Mach number greater than one.

 

 

Sonic boom is an example of a shock wave produced by a distant supersonic aircraft. It is clearly heard by an observer at rest on the ground. The energy carried by a sonic boom which lasts only for a fraction of a second is sometimes sufficient to crack glass windows and shatter buildings. It is sometimes wrongly thought that a sonic beam is produced only at the moment an aircraft breaks the sound barrier (i.e., exceeds the speed of sound). In fact, a shock wave follows the aircraft all the time it is travelling al supersonic speeds. Thus, a series of observers on the ground hear the boom as the shock wave passes by them.

 

On Your Tips

 

1. A portion of the medium where the density (or pressure) has a larger value than its average value is called a compression (or a crest).

 

2. A portion of the medium where the density (or pressure) has a value smaller than the average value is called a trough (or a rarefaction).

 

3. The magnitude of the maximum disturbance in the medium on either side of the mean position is called the amplitude.

 

4. The change in density (or pressure) from the maximum value to the minimum value and again to the maximum value is called an oscillation.

 

5. The number of complete oscillations per second is called the frequency (v) of the sound wave. The unit of frequency is called hertz (Hz).

 

6. The time taken for one complete oscillation in density (or pressure) of the medium is called the time period (T) of the wave.

 

7. The distance between two consecutive compressions (or crests) or two consecutive rarefactions (or troughs) is called the wavelength.

 

8. The distance travelled by a sound wave in its periodic time is also called wavelength (a) of the wave

 

9. The speed of sound depends mainly on its nature and the temperature of the medium through which its propagates.

 

10. The relation between speed of the sound wave (v), its frequency (v) and wavelength (a) is v =νλ.

 

11. The sound wave is described by : (i) its speed, (ii) its frequency (or wavelength) and (iii) its amplitude. These are called the characteristics of a sound wave.

 

12. In general, speed of sound in solids > speed of sound in liquids > speed of sound in gases. However, this relation is not always valid.

 

13. Sources that move faster than the speed of sound are said to have supersonic speeds. Bullets, jet aircrafts etc. travel at supersonic speeds.

 

14. A shock wave is produced when sound producing source moves with a speed higher than the speed of sound.   15. It is not necessary for an object to be a vibrating source of sound to produce a shock wave. 16. A shock wave carries a large amount of energy.   17. The ratio of the speed of the source to the speed of sound is called the Mach Number. Shock waves are produced when Mach number is greater than one.   18. Shock waves are also called Mach waves.   19. Sonic boom is a very sharp and loud sound produced by pressure variation associated with a shock wave.

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