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# Sound

## 04 Factors Affecting Speed of Sound

#### SPEED OF SOUND

The speed of sound refers to the distance travelled per unit time by a sound wave propagating through a medium.

#### SPEED OF SOUND IN SOLID, LIQUID AND GASES

• Sound is a disturbance which is propagated by the collisions between the particles. One molecule hits the next and so forth.
• Speed of sound is independent of the density of the medium only until it enters a liquid or solid. Since gases expand to fill the given space, density is quite uniform irrespective of the type of gas, unlike solids and liquids.
• Speed of sound in a solid is larger than in gas: Solids are significantly denser than liquids or gases. So, the molecules are closer to each other in solids than in liquids and gases. Due to this, they can collide very quickly. It takes less effective time for a molecule of a solid to bump into its neighbouring molecule and thus, the speed of sound in a solid is larger than in gas.
• Speed of sound in liquid lie in between the speed of sound in solids and gases: Liquid is more dense than gas but less dense than solid. Therefore the distances between molecules is more in liquids than in solids but is less than in gases. So, speed of sound in liquids lies in between the speed of sound in solids and gases.
• The table below lists the speed of sound in various media. The proportional variation of speed with density is clearly visible here:
 MATERIAL DENSITY(g/cm) SPEED(m/s) Copper 8.90 6420 Steel 7.86 5940 Berylium 1.93 12890 Aluminium 2.58 6420 Water 1.00 1496 Ethanol 0.79 1207 Air 0.00139 331.45 Helium 0.000178 965 Fat 0.95 1450 Muscle 1.07 1580 Skull Bone 1.91 4080
• Shearing: 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.
• 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.

#### WHAT CAUSES EARTHQUAKE?

• Earthquakes are created due to the movement of the earth plates which sends disturbances in the form of waves similar to sound waves, through the earth to the surface. This causes an Earthquake.
• Typically compression waves travel faster than tearing waves.
• So, earthquakes always start with an up and down motion followed after some time by side to side motion.

#### FACTORS AFFECTING SPEED OF SOUND

Speed of sound depends on THREE FACTORS

• Nature of Material/ Medium
• Temperature
• Humidity of Air

#### NATURE OF MATERIAL/ MEDIUM:

• The speed of sound depends on the nature of material (or medium) through which it travels.
• Sound travels faster in liquids than in gases and faster in solids than in liquids.
• Sound travels about 5 times faster in water than in air.
• Speed of Sound in Air: The speed of sound when travelling through air at 20oC is 343.2 m/s which is equal to 1,236 km/h.
• Speed of Sound in Gases:
• The speed of sound in gases is proportional to the square root of the absolute temperature (measured in Kelvin)
• The speed of sound is independent of the frequency of the sound wave or the pressure and the density of the medium.
• However, the properties slightly change because none of the gases we find in real life are ideal gases.

#### TEMPERATURE:

• The speed of sound increases with the rise in temperature.
• The speed of sound in air 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.

#### HUMIDITY OF THE AIR:

Speed of sound increases with the rise in humidity.