Sound 
Sound is a form of energy. All sounds happen because something is vibrating, which makes molecules in the medium (solid, liquid or gas) begin to vibrate too. The molecules themselves are not the sound, but without them there is silence. Sound can only travel if there are molecules around. In the outer-space there are no molecules so sound cannot travel. Therefore, astronauts have to talk to each other by radio. (Radio waves, like light waves, can travel where there are no molecules.)
Try this with your ruler. Put a long ruler under a thick
book on the edge of a table. Bend the ruler up and let go.
When the ruler is up, air molecules are squashed together above
the ruler (compression) and thinned out underneath
(rarefaction).
When the ruler is down, molecules
are crowded together underneath and thinned out above.
Meanwhile, the first
group of close-together molecules is now expanding and pushing the next group of
molecules above it together. The vibrating ruler is pushing the air into a
pattern of molecules that are at first close together (compression), then far apart (rarefaction).
This is the characteristic of sound as
Longitudinal Wave.
Longitudinal Waves
A longitudinal wave is a wave in which
particles of the medium move in a direction
parallel to the direction which the wave moves. The animation below shows a
one-dimensional longitudinal plane wave propagating down a tube. The particles
do not move down the tube with the wave; they simply oscillate back and forth
about their individual equilibrium positions. Pick a single particle and watch
its motion. The wave is seen as the motion of the compressed region (ie, it is a
pressure wave), which moves from left to right.
To make longitudinal waves on the slinky, bunch a few coils up near your hand, then release them. You will see a little pulse of ``bunchiness'' travel along the slinky, reflect from the other end, and return to your hand (like the animation shown below).
Click here to see more animation of Longitudinal wave. Checkout this Applet too.
Go to this site to see the movement of waves as compare to the movement of the particles of the medium.
Wave length, λ, Amplitude, a and frequency, f
Wave length, λ of Longitudinal wave is the distance between 2 adjacent compressions or rarefactions. eg. AC, CE, BD, DF.
.
AC = CE = BD = DF = wave length, λ
Visit this site to play with wave length of Longitudinal wave.
The loudness of the sound depends on the amplitude, a of the sound wave.
The pitch of a note is determined by the frequency of the sound wave. High pitch sound has a high frequency sound wave while low pitch sound has a low frequency sound wave.
Speed of sound waves, v
Sound being a mechanical wave needs a medium to propagate. The speed of sound in solids is greater than the speed of sound in liquids which is greater than the speed of sound in gases. vsolid > vliquid > vgas
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Velocity of sound waves in different medium.
The speed of sound is not affected by pressure. If the atmospheric pressure changes, the speed of sound in air remains constant.
The speed of sound is affected by temperature. Sound travels faster in hot air than in cold air. At the peak of high altitude, the speed of sound is less than that at sea level. This is not due to the lower pressures but instead it is because of lower temperature at the peak of the mountain. vhot air > vcold air
The relationship between v, f and λ
v=fλ
[Home] [Transverse wave] [Reflection of wave] [Refraction of wave] [Diffraction of wave] [Interference of wave]
Last updated on: 11/02/2008
