Chapter
6: Waves, Sounds, and Vibration
Wavelength: distance between matching points on the wave
(unit: meter; lambda λ)
Amplitude: maximum distance a point moves from its rest
position when a wave passes
Frequency: number of complete waves per second
(unit: Hertz, Hz)
Speed: distance travelled in unit time
(unit: m/s)
Oscillations: to-and-fro movements of coils
Period: time for one oscillation
(period = 1/frequency)
Overtones: weaker frequencies mixed in; differ from each
instrument
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Transverse Waves: when oscillations are upwards-and-downwards; perpendicular to the
direction of travel (Ex: light waves, radio waves)
Longitudinal Waves: when oscillations are backwards-and-forwards; parallel to the direction
of travel; bunched-up sections of coils stretched-out in between (aka compressions and
rarefactions)
*Ex: sound waves
*Compression = Air Pressure Rises
*Rarefaction - Air Pressure Falls
*Wavelength - distance between two compression
Wave Equation: Speed (m/s) = Frequency (Hz) X Wavelength (m)
*Increase frequency = Decrease wavelength
Speed Equation: Distance Travelled (m) = Speed (m/s) X Time taken (s)
Refractive Index = (speed of light in air) / (speed of light in substance)
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Oscilloscope: used to display sound waves
Waveform: a graph showing how air pressure at the microphone varies
with time
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Reflection: waves bounces off a surface in the same direction it stroke it
Law of Reflection
1.
the angle of incidence is equal to the angle of reflection
2. the incident ray, the reflected ray, and the normal all lie in the same plane
Refraction: waves bend when they pass through a boundary; when waves cross a boundary,
they change speed and change direction
(frequency unchanged = speed decreased = wavelength decrease)
Diffraction: the waves spread out when they pass through a small gap
*Increase diffraction = Waves spread out more
*Wider gap = Less diffraction
Echo: reflected sound a short time after the original sound
Echo Sounder: measures the depth of water under a boat; sends pulses of sound waves
toward sea=bed and measures the echo time (longer the time = deeper the water)
Electronic Tape-Measure: measure the distance between two walls; works like an echosounder
Radar: uses echo-sounding principle, but microwaves instead of sound waves; detects the
position of aircraft by measuring the “echo time” of microwave pulses reflected from them
Echo Time: time used for sound to reflection
Echo Equation: Speed of Sound = Distance / Time Taken = (2 X Distance to Wall) X Echo
Time
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*sound waves are caused by vibrations - any vibrating object can be a source of sound
waves; when hard objects are struck, they vibrate to produce sound waves
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*sound waves need a material to travel through - the material is called “medium”, without it
nothing to pass on any oscillations; sound waves cannot travel through completely empty space
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*the speed of sound depends on the temperature of the air - sound waves travel faster
through hot air than through cold air
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*the speed of sound does not depend on the pressure of the air - atmospheric pressure
changes = speed of sound waves stays same
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*the speed of sound is different through different materials - sound waves travel faster
through liquids than through gases, and the fastest of all through solids
Solid: particles are close together and vibrations passes from one to another more easily; sound
waves travel quickly
Gas: particles are further apart and vibration passes from particle to another less easily; sound
waves travels slowly
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Wave Theory:
A. a new wave radiates from each point on the original
wavefront. Together, the new aves combine to create a
new wavefront
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B.Refraction (bending) as waves pass from one
medium to another. A new wave travels from A to A’
in the same time as another travels from B to B’.
But AA’ is shorter than BB’ because waves travel
more slowly in the different medium. So the angel of
the wavefront changes.
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C. Diffraction (spreading) as waves pass through a gap. Only part of
the original wavefront can pass through. New waves coming from
it combine to create a new, curved wavefront.
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High Frequency = Short Wavelength = Move faster
Low Frequency = Long Wavelength = Move slower
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Scientific Pitch: Higher Note = 2X frequency of Low Note; Peaks occur 2X as often; only
half as far apart
*two sounds have different quality or timbre
*each sound has a strong fundamental frequency
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Increase Frequency = Decrease Wavelength = Same Speed
Increase Speed = Increase Wavelength = Same Frequency
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High Pitch = High Frequency = High Notes
Low Pitch = Low Frequency = Low Notes
*Increase pitch by one octave = 2X frequency
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Louder Volume = Bigger Amplitude/Oscillation
* 2X Amplitude = 4X energy delivered per second
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*sound waves travel more slowly through cold air
*sound speeds up when enters denser material
* speed of sound in air: 330m/s
*human ear frequency: 20Hz−20000Hz