Sound Waves
description
Transcript of Sound Waves
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Sound Waves
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What is sound?
• Sound is really tiny fluctuations of air pressure– units of pressure: N/m2 or psi (lbs/square-inch)
• Carried through air at 345 m/s (770 m.p.h) as compressions and rarefactions in air pressure
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What IS Sound?
wavelengthcompressed gas
rarefied gas
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Sound Waves
• Remember, sound waves are longitudinal waves– Particles of air vibrate in the same direction the
wave travels
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Why is Sound Longitudinal?• Waves in air can’t really be transverse, because the
atoms/molecules are not bound to each other– can’t pull a (momentarily) neighboring molecule sideways– only if a “rubber band” connected the molecules would this work– fancy way of saying this: gases can’t support shear loads
• Air molecules can really only bump into one another• Imagine people in a crowded train station with hands in pockets
– pushing into crowd would send a wave of compression into the crowd in the direction of push (longitudinal)
– jerking people back and forth (sideways, over several meters) would not propagate into the crowd
– but if everyone held hands (bonds), this transverse motion would propagate into crowd
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Speed of Sound
• 344 m/s in air at 20°C• Depends on:
– Temperature of medium• travels faster at higher temps
– Type of medium• travels better through liquids and solids• can’t travel through a vacuum
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Speed of Sound
• Sound speed in air is related to the frantic motions of molecules as they jostle and collide– since air has a lot of empty space, the communication that a
wave is coming through has to be carried by the motion of particles
– for air, this motion is about 500 m/s, but only about 350 m/s directed in any particular direction
• Solids have faster sound speeds because atoms are hooked up by “springs” (bonds)– don’t have to rely on atoms to traverse gap– spring compression can (and does) travel faster than actual atom
motion
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Example Sound SpeedsMedium sound speed (m/s)
air (20C) 343
water 1497
gold 3240
brick 3650
wood 3800–4600
glass 5100
steel 5790
aluminum 6420
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Speed of Sound in Air
Temperature of Air (ºC) Speed of Sound (m/s)
0 331
25 246
100 386
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Human Hearing
sound wave
vibrates ear drum
amplified by bones
converted to nerve impulses in cochlea
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Sound hitting your eardrum
• Pressure variations displace membrane (eardrum, microphone) which can be used to measure sound– my speaking voice is moving your eardrum by a mere 1.510-4 mm =
150 nm = 1/4 wavelength of visible light!– threshold of hearing detects 510-8 mm motion, one-half the diameter
of a single atom!!!– pain threshold corresponds to 0.05 mm displacement
• Ear ignores changes slower than 20 Hz– so though pressure changes even as you climb stairs, it is too slow to
perceive as sound• Eardrum can’t be wiggled faster than about 20 kHz
– just like trying to wiggle resonant system too fast produces no significant motion
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Human Hearing
• Pitch– highness or
lowness of a sound
– depends on frequency of sound wave
– human range: 20 - 20,000 Hz
ultrasonic waves
subsonic waves
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Human Hearing
• Intensity– volume of sound– depends on energy (amplitude) of sound wave– measured in decibels (dB)
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Sound frequency• Frequency is equivalent to pitch Humans can hear 20- 20 000 Hz
Demo: How high can you hear?
Elephants & Whales- Infrasonic
Bats & Dolphin- Ultrasonic
http://www.cbmwc.org/education/echo.asp
http://www.seaworld.org/animal-info/info-books/bottlenose/communication.htm
http://www.whaleacoustics.com/audiobaleenwhales.html
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Human Hearing
• Humans hear sounds in a limited frequency range20 Hz-20,000 Hz
• Any sound below the human range of hearing is known as an infrasound
• Any sound above the human range of hearing is known as an ultrasound
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Human Hearing
7080
100110
120
40
1810
0
DECIBEL SCALE
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INTERACTION OF SOUND WAVES
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Reflection of sound
When sound encounters an obstacle the sound waves bounce off and reflect
This causes an ECHO
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Refraction of Sound
• Sound waves can bend if there are different temperatures in the medium
• The wave always bends towards warmer temperatures
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Refraction of sound
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Diffraction of Sound• Since sound diffracts, we can hear sound
around corners or through openings• This is why we use megaphones that are
shaped like cones
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Doppler Effect
• Doppler Effect– change in wave frequency
caused by a moving wave source
moving toward you - pitch sounds higher
moving away from you - pitch sounds lower
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Doppler Effect- Pitch increases as an object approaches and decreases as it moves away.
Click here for simulation
Click here for simulation
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Doppler Effect
Stationary (non-moving) source Moving source Supersonic source
same frequency in all directions
waves combine to produce a shock wave called a sonic boom
higher frequency
lower frequency
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More about the sonic boom
• Shock wave generated by planes and bullets
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Sound Barrier•Mach 1- the speed of sound- 331 m/s or 741 MPH)
•Breaking the Sound Barrier
Chuck Yeager 1947 X-1 SR-71 BlackbirdMach 3.31
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F-18 Breaks the Sound Barrier at 741 MPH (approx.)
Click here to see the video!
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Seeing with Sound
• Ultrasonic waves - above 20,000 Hz
Medical Imaging SONAR“Sound Navigation Ranging”
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Sound
Music Music vs. Noise Resonance Harmonics Interference Acoustics
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Music vs. Noise
• Music– specific pitches and sound quality– regular pattern
• Noise– no definite pitch; no set pattern
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Resonance
• Forced Vibration– when one vibrating object forces
another object to vibrate at the same frequency
– results in a louder sound because a greater surface area is vibrating
– used in guitars, pianos, etc.
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Resonance• Resonance
– A phenomenon that occurs when two objects naturally vibrate at the same frequency
– special case of forced vibration
– object is induced to vibrate at its natural frequency
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Resonance
“Galloping Gertie”The Tacoma Narrows Bridge Disaster
Wind through a narrow waterway caused the bridge to vibrate until it reached its natural frequency.
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Tacoma Narrow Bridge 1943
VideoTACOMA.mpeg
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Harmonics
• Fundamental– the lowest natural frequency of an object
• Overtones– multiples of the fundamental frequency
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Constructive - louder
Interference
• Interference– the ability of 2 or more waves to combine to form a
new wave
Destructive - softer
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Interference
• Beats– variations in sound
intensity produced by 2 slightly different frequencies
– both constructive and destructive interference occur
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Sound Wave Interference and Beats
signal A
signal B
A + B beat(interference)
in phase: add
out of phase: cancel
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Beats and Average Frequency
fa - fb = fbeatFrequency A – frequency B = Beat frequency
Ex1: A couple is taking a walk. If Jim takes 264 steps per minute and Sue takes 262 steps per minute, they will be in step twice during a one minute walk and these two steps will be louder than others. The listener will hear a total of 263 steps.
Ex2: If tuning fork A vibrates 264 times each second and fork B vibrates 262 times each second, they will be in step twice each second and the listener will hear a beat of 2 Hz and an overall tone of 263 hertz.
Musicians use beats to tune instruments. Dolphins use to detect motion.
http://library.thinkquest.org/19537/java/Beats.html
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Another example of Beats
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Beats
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AM vs FM Radio
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E. Acoustics
• Acoustics– the study of sound
• Reverberation – echo effect produced by
the reflection of sound
Anechoic chamber - designed to eliminate reverberation.
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UCSD: Physics 8; 2006
All Shapes of Waveforms• Different Instruments have
different waveforms– a: glockenspiel– b: soft piano– c: loud piano– d: trumpet
• Our ears are sensitive to the detailed shape of waveforms!
• More waveforms:– e: french horn– f: clarinet– g: violin
http://www.st-and.demon.co.uk/AudioMisc/asymmetry/asym.html
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How does our ear know?• Our ears pick out frequency
components of a waveform• A DC (constant) signal has no
wiggles, thus is at zero frequency
• A sinusoidal wave has a single frequency associated with it
• The faster the wiggles, the higher the frequency
• The height of the spike indicates how strong (amplitude) that frequency component is
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Speakers: Inverse Eardrums
• Speakers vibrate and push on the air– pushing out creates compression– pulling back creates rarefaction
• Speaker must execute complex motion according to desired waveform
• Speaker is driven via “solenoid” idea:– electrical signal (AC) is sent into coil that surrounds a permanent
magnet attached to speaker cone– depending on direction of current, the induced magnetic field
either lines up with magnet or is opposite– results in pushing or pulling (attracting/repelling) magnet in coil,
and thus pushing/pulling on center of cone
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Speaker Geometry