The faster a sound wave travels, the more distance it will cover in the same period of time. Faster waves cover more distance in the same period of time. The speed of any wave depends upon the properties of the medium through which the wave is traveling.
Typically there are two essential types of properties that affect wave speed - inertial properties and elastic properties. Elastic properties are those properties related to the tendency of a material to maintain its shape and not deform whenever a force or stress is applied to it.
A material such as steel will experience a very small deformation of shape and dimension when a stress is applied to it. Steel is a rigid material with a high elasticity. On the other hand, a material such as a rubber band is highly flexible; when a force is applied to stretch the rubber band, it deforms or changes its shape readily.
A small stress on the rubber band causes a large deformation. Steel is considered to be a stiff or rigid material, whereas a rubber band is considered a flexible material. When a force is applied in an attempt to stretch or deform the material, its strong particle interactions prevent this deformation and help the material maintain its shape. Rigid materials such as steel are considered to have a high elasticity. Elastic modulus is the technical term. The phase of matter has a tremendous impact upon the elastic properties of the medium.
In general, solids have the strongest interactions between particles, followed by liquids and then gases. For this reason, longitudinal sound waves travel faster in solids than they do in liquids than they do in gases. Even though the inertial factor may favor gases, the elastic factor has a greater influence on the speed v of a wave, thus yielding this general pattern:. Inertial properties are those properties related to the material's tendency to be sluggish to changes in its state of motion.
The density of a medium is an example of an inertial property. The greater the inertia i. As stated above, sound waves travel faster in solids than they do in liquids than they do in gases. However, within a single phase of matter, the inertial property of density tends to be the property that has a greatest impact upon the speed of sound. A sound wave will travel faster in a less dense material than a more dense material.
Thus, a sound wave will travel nearly three times faster in Helium than it will in air. This is mostly due to the lower mass of Helium particles as compared to air particles. The speed of a sound wave in air depends upon the properties of the air, mostly the temperature, and to a lesser degree, the humidity.
Humidity is the result of water vapor being present in air. Like any liquid, water has a tendency to evaporate. As it does, particles of gaseous water become mixed in the air. This additional matter will affect the mass density of the air an inertial property. The temperature will affect the strength of the particle interactions an elastic property.
At normal atmospheric pressure, the temperature dependence of the speed of a sound wave through dry air is approximated by the following equation:. Using this equation to determine the speed of a sound wave in air at a temperature of 20 degrees Celsius yields the following solution. The above equation relating the speed of a sound wave in air to the temperature provides reasonably accurate speed values for temperatures between 0 and Celsius. The equation itself does not have any theoretical basis; it is simply the result of inspecting temperature-speed data for this temperature range.
Non-stiff materials such as air and water have relatively slow speeds of sound, while stiff materials such as diamond and iron have high speeds of sound. The key component is the stiffness of the chemical bonds involved and not just the type of molecules that are present. For instance, water molecules bound in ice form have a speed of sound more than twice as fast as in liquid water. However, we have to take into account more than the chemical bonds the springs.
We also have to take into account the atoms themselves the metaphorical balls in the grid. More massive balls have more inertia to overcome and therefore take longer to respond to a push from a neighbor.
In general, heavier materials those with higher mass densities have slower speeds of sound, all else being equal. In determining the speed of sound in a given material, the material's stiffness and density tend to work against each other. While solids usually have a higher speed of sound than liquids because solids are stiffer than liquids, this generalization is not always true because density also plays a role.
While water is denser than air, its stiffness is enough greater than air to compensate for the high density and make the speed of sound greater in water. But the fact that sound travels faster in water than in air just brings up the next question: Why is it harder to talk to someone underwater than in air?
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