Interference

= __ Interference of Sound __ =

General Explanation- When two waves meet, the resulting wave will have the combined displacement of the two waves. If two peaks meet, the resulting wave will be larger, if two troughs meet, the same thing will happen in reverse. This is the Principle of Superposition at work. Principle of Superposition; when two waves meet, they combine and form a wave that is the algebraic (accounts for both positive and negative) sum of the displacements of each single wave.

** Displacement of a Wave **

 The displacement of any particle in a sound wave at any given time is represented by the mathematical equation represented in the figure below. The maximum displacement of any given particle is the amplitude of the wave.

** Constructive Interference **



If the displacement of the resulting wave is greater than that of either of the original waves, then it is considered constructive interference. Constructive interference produces louder sounds. A real life example of such would be an acoustic center (like concert hall). Acoustic centers use constructive interference to amplify the music and create a sound for the audience.

** Destructive Interference **

When the combined wave has a displacement less than that of either of the original waves, this is destructive interference. Headphones use destructive interference to quiet ambient noise.



Two waves are considered to be "in phase" when they are identical.

** Interference Involving Different Mediums ** Sound travels at different speeds in different mediums and will travel straight out from the source until another medium gets in the way, then the wave is refracted because part of the wave enters the new medium first and that part of the wave changes speeds before the rest of the wave can catch up, bending the wave at an angle. The angle of refraction depends on the medium, and the angle at which the wave encounters the new medium at. As the angle between the wave and the new medium gets smaller, the angle of refraction gets closer to the barrier.

** Real Life Application of Interference ** media type="file" key="Video 22.wmv.wmv" align="left" width="300" height="300" media type="file" key="Video+24.mov" width="300" height="300" align="right"

Interference also produces sound beats. Sound beats are just that, the sound seems to grow loud and then quiet alternately. The waves are actually combining an alternating between constructive and destructive interference, creating the "beat" effect.  Instruments can actually be tuned using interference. When tuning a string instrument, you play two notes that should be the same on two different strings (like open E on the first string of a standard tuned bass guitar) and E on the second string (7th fret on the second, or A string) and to tune the second string to the first string, you strum both notes simultaneously and you will hear the sound beats created by the two different notes. You would then adjust the A string tuning peg until the beats stop, meaning the strings are playing the same note, or they are in tune (or in phase).

** References **

http://www.school-for-champions.com/science/sound_create.htm http://sprott.physics.wisc.edu/demobook/chapter3.htm 9.8.10 http://www.howstuffworks.com/amplifier.htm/printable http://www.ndt-ed.org/EducationResources/HighSchool/Sound/interference.htm Principles of Physics version 2.10.018 © 2001-2010 Kinetic Books Company