Light vs Sound If sound is a wave

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

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

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 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.

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

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)

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

Sound Wave • Loud Sound -> Big variation in air pressure • “Crest” of wave: region of high pressure • “Trough” of wave: Region of lower-thanusual pressure

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

Sound Waves Travel As Variations in Pressure

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?

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

“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?

The 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

Perception of Sound Waves Ph. ET Simulation: “Sound”

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.