Sound


Where : Van Allen Hall, Lecture Rm 1
When : Feb 4, 2006, Saturday
Professor P. Kleiber


Sound Waves

Sound waves are longitudinal waves. Unlike transverse wave, which move up and down, longitudinal waves are formed by alternating areas of high and low pressure in whatever media it travels through.
Sound waves are created by a vibrating source. Compressions are where the molecules in the material are more densely packed together, and rarefactions are the least dense areas.
If you plot the pressure verses time of a material when sound travels through it, the graph looks like a transverse wave.
Reflection
When a sound wave hits a barrier it bounces back, a process called reflection. The reflected wave is up side down to show that the reflected wave is "out of phase" with the source wave; the compressions become rarefactions and vice versa.
This is why you hear an echo when you shout in a large room or a canyon: the sound of your voice is reflected by the walls. The greater the distance between you and the barrier, the longer it takes for the reflection to return to you.

Sound in Various Media

The more dense a material is, the more quickly sound will travel through it. This table shows the speed of sound in various media.

Notice that it also gives the temperature of the material. In particular, the speed of sound in air at 0oC is less than that at 20oC. This is because air has a greater density at higher temperatures.

Refraction

Since sound travels at different speed in different material, sound has to change speed when it passes between two materials. If the wave front hits the boundary at an angle, then one side of the wave will speed up or slow down before the other side, causing the wave to bend. This is called refraction.
Warm air rises. As the graph to the right shows, this means that the temperature of air increases with height. Since the speed of sound depends on the temperature of the media, this causes refraction. Sound waves will eventually bend around and come back to the ground.

Diffraction

As a wave, sound bends around small barriers and radiates out in every direction from gaps in large barriers. This is called diffraction.

Ultrasound

Doctors use all of these properties of sound to reconstructed images of the insides of people's bodies from the reflections of sonic pulses. This is called ultrasound.

end

index