Sound Propagation• Clicker issues• Review sound waves• Sound propagation– Reflection– Refraction– Diffraction– Interference (Spatial and Temporal)Clickers• Register your clicker– http://capa.colorado.edu/cgi-bin/RegisterAFS– I have a list of registered clickers• Carry spare batteries• Find good clicker spots in the room• If clicker stops working, find an instructor immediately• I’ll post the grades after classWaves• A wave is a disturbance that propagates through a mediumSound Waves• A wave is a disturbance that propagates through a mediumPressure fluctuationAirCompressionRarefactionCT 7.1A sound wave is an example of a:a) dispersive waveb) conservative wavec) transverse waved) longitudinal waveSound Waves• A sound wave is longitudinal– Molecules’ motion is parallel to propagation directionPressure fluctuationsAir moleculesRuben’s Flame Standing Wave Tube DemoReflection• Waves bounce off of objectsReflections• The displacement of the reflected wave is affected by the boundaryWave Applethttp://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swfCT 7.2The back side of a window pane acts like a free boundary for light waves. The reflected wave has a(n):a) inverted displacementb) preserved displacementc) lower frequencyd) higher frequencyReflections of Transverse WavesFixed boundary inverts the displacementFree boundary preserves the displacementRefraction• Direction of propagation bends when the wave changes speedfaster vslower vRefraction• Wavelength also changes when the wave changes speedλλλ = v / fAs v, λCT 7.3Refraction occurs when a wave changes speed. What would cause a sound wave to refract as it passes over a parking lot?a) warming of the overlying airb) reflection off of the rigid surfacec) absorption by the surrounding bushesRefraction by Warmer AirDiffraction• Waves bend around obstaclesDiffraction• Large waves diffract more than small ones• Diffraction is effective when wavelength larger than objectRipple Tank• Didn’t work so well. We’ll try this again.CT 7.4If you are standing around the corner from a loudspeaker, which waves diffract more towards your ear?a) large amplitudeb) low frequencyc) small amplituded) high frequencyDiffraction• Low frequencies have long wavelength and they diffract moreShadowRegionInterferenceWaves add togetherConstructive InterferenceDestructive InterferenceInterference (Spatial)Spatial Interference Demos• Pohl’s plates, overlapping circles on transparencies• Same audio signal through two speakersInterference (Temporal)00.20.4=0 0.05 0.1100 Hz105 Hzfbeats= f1–f2= 5 Hzτ = 0.2 sf = 1 / τ = 5 Hz(remember 1/s = 1 Hz)Doppler ShiftSwing buzzer overhead in circlesConclusion• Sound waves reflect (bounce) off of boundaries• Sound waves diffract (bend) around boundaries– Large wavelengths diffract more than smaller ones• Sound waves interfere (add together)– Constructively (crests match up and troughs match up...two waves reinforce each other)– Destructively (crests match up with troughs...two waves cancel each
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