1. Sound Waves

2. 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

3. What IS Sound? wavelength compressed gas rarefied gas Sound 05/02/2006
Lecture 10

4. Sound Waves Remember, sound waves are longitudinal waves
Particles of air vibrate in the same direction the wave travels

5. Why is Sound Longitudinal?
05/02/2006
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
Lecture 10

6. 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

7. Sound
05/02/2006
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
Lecture 10

8. Example Sound Speeds Medium sound speed (m/s) air (20C) 343 water
05/02/2006
Example Sound Speeds
Medium
sound speed (m/s)
air (20C)
343
water
1497
gold
3240
brick
3650
wood
3800–4600
glass
5100
steel
5790
aluminum
6420
Lecture 10

9. Temperature of Air (ºC)
Speed of Sound in Air
Temperature of Air (ºC)
Speed of Sound (m/s)
331 25
246
100
386

10. Human Hearing sound wave vibrates ear drum amplified by bones
converted to nerve impulses in cochlea

11. Sound hitting your eardrum
05/02/2006
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 1.510-4 mm = 150 nm = 1/4 wavelength of visible light!
threshold of hearing detects 510-8 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
Lecture 10

12. Human Hearing Pitch highness or lowness of a sound
depends on frequency of sound wave
human range: ,000 Hz
ultrasonic waves
subsonic waves

13. Human Hearing Intensity volume of sound
depends on energy (amplitude) of sound wave
measured in decibels (dB)

14. Sound frequency Frequency is equivalent to pitch
Humans can hear Hz
Demo: How high can you hear?
Elephants & Whales- Infrasonic
Bats & Dolphin- Ultrasonic

16. Human Hearing Humans hear sounds in a limited frequency range
20 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

17. Human Hearing
DECIBEL SCALE
120
110
100 80 70 40 18 10

18. Interaction of Sound Waves

19. Reflection of sound
When sound encounters an obstacle the sound waves bounce off and reflect This causes an ECHO

20. Refraction of Sound
Sound waves can bend if there are different temperatures in the medium
The wave always bends towards warmer temperatures

21. Refraction of sound

22. 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

23. Doppler Effect moving toward you - pitch sounds higher
change in wave frequency caused by a moving wave source
moving toward you - pitch sounds higher
moving away from you - pitch sounds lower

24. Doppler Effect- Pitch increases as an object approaches and decreases as it moves away.
Click here for simulation
Click here for simulation

25. Doppler Effect
Stationary
(non-moving) source
Moving source
Supersonic source
waves combine to produce a shock wave called a sonic boom
same frequency in all directions
higher frequency
lower frequency

26. More about the sonic boom
Shock wave generated by planes and bullets

27. Sound Barrier Mach 1- the speed of sound- 331 m/s or 741 MPH)
Breaking the Sound Barrier
SR-71 Blackbird
Mach 3.31
Chuck Yeager X-1

29. Click here to see the video!
F-18 Breaks the Sound Barrier at 741 MPH (approx.)
Click here to see the video!

30. Seeing with Sound Ultrasonic waves - above 20,000 Hz Medical Imaging
SONAR
“Sound Navigation Ranging”

31. Sound
Music
Music vs. Noise
Resonance
Harmonics
Interference
Acoustics

32. Music vs. Noise Music Noise specific pitches and sound quality
regular pattern
Noise
no definite pitch; no set pattern

33. 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.

34. 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

35. The Tacoma Narrows Bridge Disaster
Resonance
“Galloping Gertie”
The Tacoma Narrows Bridge Disaster
Wind through a narrow waterway caused the bridge to vibrate until it reached its natural frequency.

36. Tacoma Narrow Bridge 1943
VideoTACOMA.mpeg

37. Harmonics Fundamental Overtones
the lowest natural frequency of an object
Overtones
multiples of the fundamental frequency

39. Interference Interference
the ability of 2 or more waves to combine to form a new wave
Constructive - louder
Destructive - softer

40. Interference
Beats
variations in sound intensity produced by 2 slightly different frequencies
both constructive and destructive interference occur

41. Sound Wave Interference and Beats
05/02/2006
signal A
in phase: add
signal B
A + B beat
(interference)
out of phase: cancel
Lecture 10

42. Beats and Average Frequency
fa fb = fbeat
Frequency 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.

43. Another example of Beats

44. Beats

45. AM vs FM Radio

46. E. Acoustics Acoustics Reverberation the study of sound
echo effect produced by the reflection of sound
Anechoic chamber - designed to eliminate reverberation.

47. All Shapes of Waveforms
Sound
05/02/2006
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
Lecture 10

48. Sound
05/02/2006
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
Lecture 10

49. Speakers: Inverse Eardrums
Sound
05/02/2006
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
Lecture 10

50. Sound
05/02/2006
Speaker Geometry
Lecture 10