 # Principles of Physics. Sound Result of vibration of air particles around a source Longitudinal wave – air particles get compressed and spread apart as.

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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 Human ear most sensitive to sounds between 100 Hz and 2000 Hz Thunder 20 Hzλ = 17 m Fire Truck Siren2000 Hzλ = 17 cm Highest pitch heard by humans 20,000 Hzλ = 1.7 cm

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 Whisper20 100 Conversation601,000,000 Pain Threshold120 1,000,000,000,000

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

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

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

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

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

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

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

Tacoma Narrows Bridge Collapse "Gallopin' Gertie"

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

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

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

Point

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

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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