| MadSci Network: Physics |
A sound wave is a pressure disturbance that travels through a medium, such as air, by particle interations. This means that they travel by pressure and density changes in the air that are transfered from place to place by the interaction of indvidual particles, such as oxygen or nitrogen molecules in the air. How does this relate to your question?
The speed that this pressue disturbance can be transfered from one particle to the next is what makes the sound wave travel faster or slower through some medium. In mediums with very high densities indiviual particles have greater inertia and they are less responsive to the interactions between neighboring particles. So at greater densities sound waves will move slower, if all other factors are equal. Thus, a sound wave will travel nearly three tmies faster in Helium gas as it will in air; this is due to the lower mass of Helium particles as compared to air particles. But, this is not the whole story.
The speed of sound in some material also depends on that materials elastic properties. These properties effect the strength of the particle interactions within a medium. The phase of matter, such as solid or gas, has a tremendous impact upon the elastic properties of the medium. In general, solids have stronger interactions than liquids which have stronger interactions than gases. For this reason, sound waves travel faster in solids than they do in liquids than they do in gases. Even though the mass/density factor may favor gases, the elastic factor has a greater influence on the speed of a wave.
The speed of a sound wave in air depends upon the properties of the air, namely the temperature and the pressure. The pressure of air will effect the density of the air and the temperature will effect the strength of the particle interactions (an elastic property). At normal atmospheric pressure, the temperature dependence of the speed of a sound wave through air is approximated by the following equation.
v = 331 m/s + (0.6 m/s/C)*T
where T is the temperature of the air in degrees Celsius. Using this
equation you can find that the speed of sound in air at 20 C is
343 m/s, which is approximately equal to 750 miles/hour.
For more information try the page at:
http://www.glenbrook.k12.il.us/gbssci/phys/Class/sound/soundtoc.html
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