sound - Sound LongitudinalWaves Interference PressureGraphs...

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Sound Longitudinal Waves Pressure Graphs  Speed of Sound Wavefronts  Frequency & Pitch  (human range) The Human Ear Sonar & Echolocation Doppler Effect  (and sonic booms)  Interference Standing Waves in a String: Two fixed ends Standing Waves in a Tube: One open end Two open ends Musical Instruments  (and other complex sounds) Beats Intensity Sound Level (decibels)
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Longitudinal Waves As you learned in the unit on waves, in a longitudinal wave the particles in a medium travel back & forth parallel to the wave itself. Sound waves are longitudinal and they can travel through most any medium, so molecules of air (or water, etc.) move back & forth in the direction of the wave creating high pressure zones (compressions) and low pressure zones (rarefactions). The molecules act just like the individual coils in the spring. The faster the molecules move back & forth, the greater the frequency of the wave, and the greater distance they move, the greater the wave’s amplitude. wavelength , λ Animation rarefaction compression molecule
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Sound Waves:  Molecular View When sound travels through a medium, there are alternating regions of high and low pressure. Compressions are high pressure regions where the molecules are crowded together. Rarefactions are low pressure regions where the molecules are more spread out. An individual molecule moves side to side with each compression. The speed at which a compression propagates through the medium is the wave speed, but this is different than the speed of the molecules themselves. wavelength , λ
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Pressure    vs.    Position The pressure at a given point in a medium fluctuates slightly as sound waves pass by. The wavelength is determined by the distance between consecutive compressions or consecutive rarefactions. At each com- pression the pressure is a tad bit higher than its normal pressure. At each rarefaction the pressure is a tad bit lower than normal. Let’s call the equilibrium (normal) pressure P 0 and the difference in pressure from equilibrium P. P varies and is at a max at a compression or rarefaction. In a fluid like air or water, P max is typically very small compared to P 0 but our ears are very sensitive to slight deviations in pressure. The bigger P is, the greater the amplitude of the sound wave, and the louder the sound. wavelength , λ
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B Pressure    vs.    Position Graph P x A C A: P = 0; P = P 0 B: P > 0; P = P max C: P < 0; P = P min λ animation
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Pressure    vs.    Time The pressure at a given point does not stay constant. If we only observed one position we would find the pressure there varies sinusoidally with time, ranging from: P 0 to P 0 + P max back to P 0 then to P 0 - P max and back to P 0 The time it takes to go through this cycle is the period of the wave.
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sound - Sound LongitudinalWaves Interference PressureGraphs...

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