Why it Sounds Noisier on a Hot Day?

What happens to sound on a hot day? Does air temperature affect speed of sound? Why it sounds noisier on hot weather conditions?

Sound is a traveling disturbance that travels through matter - solid, liquid or gas. It is a mechanical wave that is propagated by the vibrations of particles in a medium. A medium is the material at which wave travels. 


Sound is a longitudinal mechanical wave. A longitudinal wave is a type of mechanical wave that propagates by compression and rarefaction.  Compression is a region of high pressure where particles of the medium are forced to compress or move closer together. Rarefaction, on the other hand, is a region of low pressure where the particles of the medium are rarefied or move farther away from each other. Particles of a medium in longitudinal wave vibrates back and forth parallel to the direction of the wave. The wavelength of a longitudinal wave is the distance measured from one compression to the next compression or from one rarefaction to the next rarefaction. 

As said, sound travels through different media (plural for medium) at different speeds. The speed of sound depends on different factors like elasticity, density and temperature. 

So, in which medium does sound travel faster?  How does elasticity affect sound speed?

Elasticity is a property of a material that gives it the ability to resist deformation. This property gives a material the ability to spring back to its original shape after the force applied is released. Metals for example have high elastic limit compared to plastic. The elasticity of metals makes them capable of transmitting sound faster than any other solid objects. 

Does sound travel faster in cold air? How density and temperature affect sound speed?

Density is an inertial property of a material. It is the ratio of mass and volume of the material. Take note that sound is propagated by the vibration of the particles of the medium. The higher is the inertial property of the material, the more difficult it is to cause neighboring particles to vibrate. The denser the material is, the slower is the speed of sound. This can be observed more in air at different temperatures. Temperature affects density of air. Cold, dense air transmit sound slower that hot, less dense air.

Density is a function of temperature. Temperature is the average kinetic energy of the particles of a substance. As temperature increases, air expands and the particles move farther away from one another. This causes a decrease in air density. The movement of particles away from one another might lead someone to think that sound travel slower at hot, less dense air because of the distance between and among particles. It may be peculiar however that sound travel faster in hot air than in cold air. This is because of the kinetic energy acquired by particles of air at higher temperature. The kinetic energy of particles of air increases with temperature, which makes sound to be transmitted faster. The speed of sound in air at 0°C is 331 m/s. In every one degree celsius rise in temperature of air, the speed of sound increases by 0.6 m/s. The formula for the speed of sound in air at different temperatures in the celsius scale can be computed as 

vair = 331 m/s + (0.6 ms-1/°C)T 

where T is the temperature in the degree celsius.


Sample Problem: What is the speed of sound in air at 20°C?

Formula: vair = 331 m/s + (0.6 ms-1/°C)T 

Solution: vair = 331 m/s + (0.6 ms-1/°C) 20°C
vair = 331 m/s + 12 m/s
vair = 343 m/s


Sample Problem: What is the temperature of air when the speed of sound is 355m/s?

Formula: vair = 331 m/s + (0.6 ms-1/°C)T 

Solution: 355 m/s = 331 m/s + (0.6 ms-1/°C) T
355 m/s - 331 m/s =  (0.6 ms-1/°C) T

24 m/s = (0.6 ms-1/°C) T
T = (24 m/s) / (0.6 ms-1/°C) 
T = 40°C

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