Wave Machine Contributed By: National Science Center (Fort Discovery)

### Experiment Category:

Objective:

To experiment with wave behaviors such as wave interference, standing waves, free-end reflection and wave velocity.

What You Need:
• Transparent tape
• 11 pencils
• Two tables or chairs the same height.
• Alternate materials such as straws, shorter pencils, markers or wood splints.
To Do and Observe:

1. Cut a piece of transparent tape 32cm long. 2. Place two tables about 24 cm apart and stretch the tape between them. 3. Stick the pencils to the bottom of the tape, 2cm apart, so they are horizontal in reference to the floor. 4. Tap the tip of one pencil at one end of the tape... that should produce a wave that travels to the end and reflects back. When a wave hits a boundary between media, all or some of the wave bounces back into the first medium. This is reflection. 5. Experiment with the wave machine to create other waves

What's Going On:

Wave interference: Start waves at both ends at the same time to see interference. Interference affects the loudness of sounds; therefore it is an important property of sound waves, especially when designing auditoriums and in placing speakers. Constructive interference of sound waves will make the sound loud in some spots in the room; while destructive interference can cause \"dead\" spots where the sound can not be heard. Destructive interference is a useful property in anti-noise technology. Standing Waves: Pushing a single pencil up and down can create standing waves. Standing waves are the result of interference. When two waves of equal amplitude (height) and wavelength (length) pass through each other in opposite directions, the waves are always out of phase (out of synch) at the nodes. The nodes are the parts that remain stationary or have no amplitude. Free-end Reflection: Hang the entire wave machine vertically from the table edge. How does this wave behavior differ from the waves made when the machine was attached to the table at both ends? Wave Velocity: Changing the masses of the pencils can change wave velocity. Experiment with different objects such as: straws, shorter pencils, markers, or wood splints. Sound travels through the air at a speed of 330 to 350 meters per second. Sound travels faster through warm air; humidity also slightly increases the speed of sound. Sound travels much faster in liquids and solids than it does in gasses. For example, the speed of sound in water is about four times faster than air; while in some metals sound can travel about fifteen times faster than in air.

Parent/Teacher Tips:

Experiment with your stereo speakers (if they aren’t attached to the unit). What placement provides the best sound? What placement provides the worst sound? Make waves in a pan or tub of water. Can you see the same properties?

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