Please read the following...
1. Reflection- bouncing off of waves.
It is dependent on the following factors:
* Type of boundary
a. Reverberation - often occurs in a small room with height, width, and length dimensions of approximately 17 meters or less. The reflected sound will only travel 0.1s in a 340 m/s, at a room temperature.Thus the sound seems to prolonged.It is observed when talking in an empty room, when honking the horn while driving through a highway tunnel or underpass, or when singing in the shower. In auditoriums and concert halls, reverberations occasionally occur and lead to the displeasing garbling of a sound.
b. Echo - occur when a reflected sound wave reaches the ear more than 0.1 seconds after the original sound wave was heard. The reflected sound is now perceived as second sound raher than the prolonging of the 1st sound.
* Shape of the surface
- Curved (Parabolic) surface - to collect sound and to focus sound into one point.
2. Diffraction -
It involves a change in direction of waves as they pass through an opening or around a barrier in their path. Diffraction of sound waves is commonly observed; we notice sound diffracting around corners or through door openings, allowing us to hear others who are speaking to us from adjacent rooms.
3. Refraction - bending due to change in medium. The figure below explains the refraction of sound. Sound waves bend due to change in temperature.
For example, during the day the air is warmest right next to the ground and grows cooler above the ground. This is called a temperature lapse. Since the temperature decreases with height, the speed of sound also decreases with height. This means that for a sound wave traveling close to the ground, the part of the wave closest to the ground is traveling the fastest, and the part of the wave farthest above the ground is traveling the slowest. As a result, the wave changes direction and bends upwards. This can create a "shadow zone" region into which the sound wave cannot penetrate. A person standing in the shadow zone will not hear the sound even though he/she might be able to see the source. The sound waves are being refracted upwards and will never reach the observer.
A temperature inversion is when the temperature is coolest right next to the ground and warmer as you increase in height above the ground. Since the temperature increases with height, the speed of sound also increases with height. This means that for a sound wave traveling close to the ground, the part of the wave closest to the ground is traveling the slowest, and the part of the wave farthest above the ground is traveling the fastest. As a result, the wave changes direction and bends downwards. Temperature inversions most often happen at night after the sun goes down when the ground (or water in a lake) cools off quickly, while the air above the ground remains warm. This downward refraction of sound is why you can hear the conversations of campers across the lake, when otherwise you should not be able to hear them. (remember that they can probably hear you too!)
(Text, images, and animations are ©2001 Dan Russell)
4. Interference : Two traveling waves which exist in the same medium will interfere with each other. If their amplitudes add, the interference is said to be constructive interference, and destructive interference if they are "out of phase" and subtract. Patterns of destructive and constructive interference may lead to "dead spots" and "live spots" in auditorium acoustics.
Interference of incident and reflected waves is essential to the production of resonant
standing waves.
Interference has far reaching consequences in sound because of the production of "
beats" between two frequencies which interfere with each other.
Please try the different links for better understanding and read the above concepts.
take home exam will be posted on Sunday.
Due of the take home exam will be on Tuesday (8/12/11).
Good luck everyone!!!
Ma'am G.
