1. Light vs. Sound If sound is a “wave,” what’s “waving?”

2. Light vs. Sound What makes a light wave different from a sound wave?

3. Light vs. Sound What makes a light wave different from a sound wave? Light travels a lot faster than sound: Speed of light in air = 300,000,000 meters per second Speed of sound in air (at 0 Celsius) = 331 meters per second At room temperature this increases to about 343 m/s.

4. Light vs. Sound What makes a light wave different from a sound wave? Light can travel in empty space …Sound can’t because sound is the compression of the medium For sound traveling in air, sound wave is made of variations in the pressure of the air

5. Air Pressure Sound Wave = variation in pressure Pressure = Force / Area Air exerts a force (presses against surfaces) Normally air pressure is not very noticeable because it tends to press on all side of a surface evenly Air pressure is greatest at sea level (P = 1 atmosphere, or 1 atm)

6. Air Pressure: Examples Plastic bottle on airplane Using a straw Vacuum cleaner Vacuum Chamber experiments

7. Sound Wave Loud Sound -> Big variation in air pressure “Crest” of wave: region of high pressure “Trough” of wave: Region of lower-than- usual pressure

8. Animation from Physics Classroom.com: http://www.physicsclassroom.com http://www.physicsclassroom.com Gray = atmospheric pressure Dark = high pressure Light = low pressure

9. Sound Waves Travel As Variations in Pressure

10. Transverse vs. Compression Light is a transverse wave: Transverse means that the wave travels perpendicular to the displacement Sound is a compression wave The wave travels in the same direction as the displacement Waves on a slinky? “The Wave” in a stadium? Waves on a string?

11. Earthquakes What’s the “medium” of an earthquake? P-waves are compression waves S-waves are transverse waves Travel at different speeds

12. “Seeing” Sound Waves A microphone converts pressure waves to electrical signals. An oscilloscope takes an electrical signal as “input” and displays a graph of the signal as a function of time Do you think an oscilloscope could be used to measure frequency, wavelength, or both?

13. The Octave Octave = 8 notes apart Going an “octave” higher means doubling the frequency of a note Going an octave lower means halving the frequency of a note Octave demo with oscilloscope: http://www.animations.physics.unsw.edu.au/jw/sound-pitch-loudness-timbre.htm

14. Perception of Sound Waves PhET Simulation: “Sound”

15. Review What determines how fast a wave travels? Can you make a wave go faster by putting more energy into the wave? What, physically, is sound? Give examples of longitudinal waves and transverse waves.