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1
19-21: Main Topics • vibrations and
waves
• types and effects of waves
• sound
• resonance
• musical sounds
2
Homework:
• Ch19: RQs: 2, 11, 12, 13, 16, 18. Problem 1.
• Ch20: RQs: 3, 5, 7, 8, 20, 21, 24.
• Ch21: Read pp398-401.
3
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
4
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.
5
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)
7
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.
9
natural frequency
• lowest frequency an object vibrates with when struck
• also called “resonant frequency”
• /
10
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.
12
Sound Waves
• pressure/density waves
• compressions
• rarefactions
• reflects
• refracts (similar to light)
• diffracts (bends around corners)
13
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
14
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
15
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
17
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
18
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
21
doppler effect
• the frequency heard by an observer depends on the motion of source and the observer.
22
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
23
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