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Sound

Sound. Principles of Physics. Sound. Result of vibration of air particles around a source Longitudinal wave – air particles get compressed and spread apart as vibration propagates. Characteristics of Sound Waves. Pitch = frequency

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Sound

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  1. Sound Principles of Physics

  2. Sound • Result of vibration of air particles around a source • Longitudinal wave – air particles get compressed and spread apart as vibration propagates

  3. Characteristics of Sound Waves • Pitch = frequency • Human ear most sensitive to sounds between 100 Hz and 2000 Hz Thunder 20 Hz λ = 17 m Fire Truck Siren 2000 Hz λ = 17 cm Highest pitch heard by humans 20,000 Hz λ = 1.7 cm

  4. Characteristics of Sound Waves • Loudness = amplitude • The more the particles move back and forth the more energy the wave has Decibel Scale (dB) – used to measure loudness of sound times greater than dB level the threshold of hearing Whisper 20 100 Conversation 60 1,000,000 Pain Threshold 120 1,000,000,000,000

  5. Characteristics of Sound Waves • Mechanical Wave – Sound requires a medium • There is no sound in space • Speed depends on the medium • Sound travels faster through solids and liquids than gases • Why did outlaws put their ears to the train tracks or ground to listen for the sheriff? Sound travels faster through solids than gases • In air at Standard Temperature and Pressure (STP) the speed of sound is 331 m/s

  6. Resonance Resonance- increase in amplitude when energy is added to a wave at its natural frequency • usually occurs when a wave interferes with its reflections • standing wave pattern results • can also occur when two identical waves pass through each other in opposite directions • Boundary Conditions

  7. Resonance Fundamental Frequency – natural frequency of the wave which depends on the conditions under which the wave is produced (distance between boundaries, material, etc) • Longest wavelength and shortest frequency standing wave possible for the conditions Harmonics – series of standing waves that forms between boundaries • Wavelength decreases, and frequency increases with each successive harmonic

  8. End Conditions Fixed end – end that cannot move (node) Open end – end that can move (antinode)

  9. Fixed Ends 1st harmonic = fundamental frequency (½λ fits between the boundaries) 2nd harmonic = 1 λ fits between the boundaries • Wavelength is less than the fundamental • Frequency is greater than the fundamental 3rd harmonic = 1.5 λ fit between boundaries • Wavelength is less than the 2nd harmonic • Frequency is greater than the 2nd harmonic Standing Waves, Medium Fixed At Both Ends

  10. Open Ends 1st harmonic = fundamental frequency (½ λ fits between the boundaries) 2nd harmonic = 1 λ fits between the boundaries • Wavelength is less than the fundamental • Frequency is greater than the fundamental 3rd harmonic = 1.5 λ fit between boundaries • Wavelength is less than the 2nd harmonic • Frequency is greater than the 2nd harmonic Standing Waves, Medium Open At Both Ends Fundamental 1 node, 2 antinodes 2nd harmonic 2 nodes, 3 antinodes 3rd harmonic 3 nodes, 4 antinodes

  11. One Fixed End, One Open End 1st harmonic = fundamental frequency (1/4 λ between boundaries) 3rd harmonic = ¾ λ between boundaries • Wavelength is less than the fundamental • Frequency is greater than the fundamental 5th harmonic = 1.25 λ between boundaries • Wavelength is less than the 2nd harmonic • Frequency is greater than the 2nd harmonic Standing wave: One open end, one fixed end Fundamental 1 node, 1 antinodes 2nd harmonic 2 nodes, 2 antinodes 3rd harmonic 3 nodes, 3 antinodes

  12. Tacoma Narrows Bridge Collapse "Gallopin' Gertie"

  13. Doppler Effect • Apparent change in frequency as a result of the source or observer moving • The frequency of the source does not change *Frequency is related to - pitch for sound (how high or low it is) - color for light

  14. Doppler Effect Occurs when the source, observer or both are moving Source and observer move closer together • Source notices higher frequency Source and observer move away from each other • Source notices lower frequency

  15. t= 1 s t= 2 s t= 3 s Experiences higher frequency than source Experiences lower frequency than source

  16. Point

  17. Doppler Effect Observer and source move closer together • Wave fronts bunch up • Increased frequency • Sound – higher pitch • Light – color shifts toward blue • Decreased wavelength Observer and source away from each other • Wave fronts spread apart • Decreased frequency • Sound – lower pitch • Light – color shifts toward red (Red Shift) • Increased wavelength

  18. Beats • When 2 waves with slightly different frequencies interfere • When wave pattern is the same • constructive interference • loud • When wave pattern is different • destructive interference • soft

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