11/3/08

Post date: Nov 03, 2008 12:6:17 AM

Bell Activity: What is sound? What gets higher and lower during a sound wave?

• • Sound wave demonstration. See the video below!

• • Interference of waves

    • When objects collide, they stop or rebound off each other. Waves do not do this.

    • Two or more waves can be in the same place at the same time. Instead of bouncing off each other, they add together to make a bigger or smaller wave. This is called interference.

    • Example - two pulses sent down a spring (see page 527 for illustration). The waves move right through each other, and interfere when they are at the same spot along the spring.

  • Interference can be constructive or destructive

    • In constructive interference, both waves are at positive values or both are at negative values, and their combination makes a bigger wave

    • In destructive interference, one wave has a positive value and the other has a negative, and their combination makes a smaller wave

    • Example video on youtube: http://www.youtube.com/watch?v=P_rK66GFeI4

    • Explanation and demo at http://id.mind.net/~zona/mstm/physics/waves/interference/intrfrnc.html

    • With sound waves, we can see this by setting up 2 speakers playing the same tone and moving forward and backward between them. When there is destructive interference, it sounds quieter. When there is constructive interference, it sounds louder.

    • Bubbles show an example of wave interference with light. A light wave may bounce of the outer or inner surface of a bubble, causing the two reflections to interfere. This creates swirls of different colors on the surface of the bubble.

    • Noise-canceling headphones make sounds that interfere destructively with the outside sounds to make them quieter, then play the music along with the canceling sounds.

Chapter 15: Waves Quiz on Friday