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19-21: Main Topics • vibrations and

waves

• types and effects of waves

• sound

• resonance

• musical sounds

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Homework:

• Ch19: RQs: 2, 11, 12, 13, 16, 18. Problem 1.

• Ch20: RQs: 3, 5, 7, 8, 20, 21, 24.

• Ch21: Read pp398-401.

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Vibrations

• back and forth motion

• regularly repeating

• cycle time = period of motion, T (seconds)

• frequency = 1/T (1/s = hertz)

• amplitude = maximum displacement from non-vibrating position

• have energy

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Simple Pendulum• mass on light string

• amplitude of motion is angle of swing

• “vibrates” when disturbed

• T, period of motion = cyclic time

• T is very nearly constant for all oscillations less than ~5 degrees.

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Waves

• moving vibrations in fluids and solids

• wave vibrations have amplitude (A) and frequency (f)

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Wave Speed

• speed (v) of waves = length of wave () divided by time (T) for wave to pass by.

• v = /T

• but 1/T = f, the wave frequency

• v = f (a general wave formula)

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Types of Waves

• Transverse Longitudinal(e.g. light) (e.g. sound)

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wave interference

• waves reaching the same point “add”

• waves can add constructively to produce higher intensity

• or, can add destructively to produce lower intensity

• Examples: noise-canceling headphones, colors on soap bubble.

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natural frequency

• lowest frequency an object vibrates with when struck

• also called “resonant frequency”

• /

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Standing Transverse Waves on Strings

Nodes (N) are places with zero amplitude

Anti-nodes (A) are places with maximum amplitude

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resonance

• objects absorb more energy when the frequency of a disturbance hits it at the objects natural frequency

• example: a guitar string tuned to produce sound of frequency 256 Hz, will resonate when exposed to a sound of 256 Hz.

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Sound Waves

• pressure/density waves

• compressions

• rarefactions

• reflects

• refracts (similar to light)

• diffracts (bends around corners)

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sound in air

• speed increases with temperature at a rate of 0.6m/s per degree C (from 0 to 20C)

• speed is about 340 m/s (760 mph) at atmospheric pressure and 20 degrees Celsius

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Sound Speed

• in gases: ~ 300 to 1200 m/s

• (e.g. air v = 331 + 0.6TC m/s)

• in liquids: ~ 1100 to 1500 m/s

• in solids: ~ 1800 to 5000 m/s

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Pitch

• subjective perception

• frequency is a measurement

• Ex. The note A above middle C played on any instrument is perceived to be of the same pitch as a pure tone of 440 Hz, but does not necessarily contain a partial having that frequency. (Wikipedia)

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factors affecting frequency

• length, l

• string tension, T

• string density

source: Wikipedia

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Sound Intensity and Loudness

• intensity = power per unit area

• “loudness” is the human perception of sound intensity

• sound level dB (decibels) is a logarithmic scale of intensity

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hearing

• “audible range” of humans is defined to be 20 Hz to 20,000 Hz (1 Hz = 1 cycle per second)

• individual audible ranges vary greatly and depend on age, genetics, and exposure (music, workplace, etc)

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summary

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sound quality

• accuracy with which a device records or emits original sound waves

• mp3 reduces amount of information in real sounds, can reduce sound quality.

• quality measured by bit rate in kbits/s

• “excellent” at 224...320 kbit/s

• “very good” at 192...224 kbit/s,

• “good” at 128...192 kbit/s

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doppler effect

• the frequency heard by an observer depends on the motion of source and the observer.

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character of doppler effect

• let f be the frequency of the source

• let fo be the observed frequency

• observer moves toward source: fo increases

• source moves toward observer: fo increases

• observer moves away from source: fo decreases

• source moves away from observer: fo decreases

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wave speed p.82

• 7a,b: two crests pass per second f = 2 cycles/s T = 1/f = ½ s/cycle

• 7c: if wavelength = 1.5m and f = 2 Hz, then v = f = (2cycle/s)(1.5m/cycle)= 3m/s

• 7d: period of wave = 1/f = ½ second