Sound Begins With A Wiggle

1
Sound Begins With A Wiggle

• A vibration is a wiggle. When a wiggle moves
  through space and time it is a wave\
• Light and sound are both wavesbut differ greatly
• Light can travel through a vacuum
• Light is the vibration of an E-M field
• Light is pure Energy
• Matter is not required

2
But Sound.

• Sound requires matter
• No vibrating matterNO SOUND
• Sound moves through solids, liquids, gases
• Sound is longitudinal wavesiea series of
  compressions then rarefactions of matter

3
Frequency

• Recall that Frequency is the number of vibrations
  per second
• 1 vib/sec is 1 Hz
• Heinrich Hertz demonstrated radio waves in 1886
• Radio stations are the frequencies of the waves
• Cant hear radio waves but can be converted into
  the movement of material to vibrate our eardrums

4
KHz vs MHz

• AM radio dial is in the kiloHertz ranges
• FM radio stations are in the MegaHerta ranges
• So 960 on your AM dial is 960,000
  vibrations/second
• 101.7 on the FM dial is 101,700,000 Hz
• What this means is that the electrons on the
  broadcasting antenna are forced to vibrate at
  those frequencies

5
The Period

• Recall that the period of a wave is the TIME it
  takes for one vibration.
• Suppose have F 2HZ then the TIME is 1/f or ½
  sec.
• 3 Hz has T 1/3 sec.
• F and T are inverse of each other.

6
Try these

• An electric toothbrush has F 90 cycles/sec so
  its T ?
• Answer?
• 1/90th of sec.
• The sears Tower sways back and forth in the wind
  one cycle every 10 sec. So its F ? And its T?
• F .1 Hz T 1/.1 10 sec.

7
Waves transmit energy

• Energy is carried by waves but not matter
• Matter may be disturbed but it will return to its
  normal posit as wave passes
• Much like the wave at a stadium you may bob up
  and down but then you return to where you were
• A baby crawling under a blanket..blanket stays
  baby moves

8
Wave motion contd

• Speech 340 m/s
• Speed frequency times wavelength
• The energy of your voice disturbs molecules of
  air but the air itself does not travel across the
  room at this speed otherwise speech would be a
  windy experience!!

9
Longitudinal

• Direction of wave is along the line of vibrating
  source parallel to one another
• Matter that is compressed and stretched
• Producing areas that are dense and compact and
  other areas less dense or rarefied
• Sound is longitudinal when a source pushes on
  molecules and then pulls back like a tuning fork

10
Wavelength of sound

• This is distance between successive compressions
  or rarefactions
• Shorter wavelengths tend to be higher energy and
  higher pitch

11
PITCH

• Pitch is really the frequency of a sound
• High pitched sounds have high frequency so
  20,000HZ is a higher pitch that a FD 20 HZ
• Human ear can hear between 20 20,000 Hz
• Below 20 Hz is INFRASONIC we cant hear but some
  animals may be able tolike??? Report on such
  for extra credit
• Above 20,000 HZ is ULTRASONIC we cant hearbut
  dogs can

12
Transmitters

• Any elastic material can transmit sound
• Recall elastic means material that can be
  distorted and then RESTORE ITSELF
• So steel is quite elastic but neither lead nor
  silly putty will conduct sound very effectively
• Many substances conduct sound faster than air.

13
A Radio

• A radio loudspeaker is a paper cone that vibrates
  in rhythm with an electrical signal.
• Air molecules nearest the vibrating cone sets up
  vibrations in the air which in turn sets up the
  neighboring air molecules etc.
• The rhythmic patterns of compressed and rarefied
  air emanates from loudspeaker and eventually sets
  your eardrum vibrating

14
And then.

• The vibrating eardrum sends cascades of rhythmic
  impulses along the cochlea nerve canal to the
  brain which makes sense out of the noise that
  is otherwise just noise

15
The Speed of Sound

• If you watch a ballgame from a distance you will
  see the batter hit the ball BEFORE you hear it
• You see lightning before you hear the thunder
• Sound is slower than light
• Sound takes time to travel through matter from
  one place to another

16
Speed depends on.but not on.

• The speed of sound in air depends on the wind
  conditions,temperature,humidity
• Speed of sound does NOT depend on the loudness of
  the sound nor upon the frequency of the sound
• All sounds, low soft high pitched or low, all
  travel at same speed in given conditions

17
But the Frame of Reference is.

• The established standard of V of sound in dry air
  is set at ZERO degrees C is about 330 m/s, or
  about 1200 km/h
• Convert to miles per hour using DA

18
Speed varies

• Sound travels faster in humid air than in dry air
  and travels faster in warm air than in cold
• Sound travels faster through denser more elastic
  material and faster through faster moving
  molecules since they bump into each other more
  often and thus transmit pulses in less time

19
More about speed

• For each one degree rise in temp above 0 C, the
  speed of sound in air increases by 0.6 m/s
• So in normal 20 C room temperature the speed of
  sound is about 340 m/s
• In Water, sound 4 times faster than speed of
  sound in air
• In Steel sound 15 times speed of sound in air

20
Lightning strikes!!!

• Wow that was close
• If you see lightning and then 3 seconds later you
  her the thunder then D ST so
• 340 m/s x 3 sec. 1020 m or about 1 Km away

21
All waves can reflect and/or refract

• Reflect means to strike a surface and bounce back
• The type, smoothness of surface determines how
  much of a wave will bounce back
• The reflection of sound is called an Echo
• A large fraction of sound energy reflects from a
  rigid, smooth surface
• Less sound is reflected off soft, rough surfaces

22
Reflection contd

• Sound energy that is not reflected is either
• TRANSMITTED through the material surface OR it is
  ABSORBED by the material surface
• Sound reflects same way LIGHT does
• The rule says the angle of INCIDENCE the angle
  of REFLECTION

23
You ought to be in pictures

• You sound great when you sing in the
  showerright?
• That is because some reflected sound improves the
  quality of our voices, reflected waves makes
  sound fuller and richer
• BUT
• When surfaces are TOO smooth, too reflective,
  then sound becomes garbledcalled
• REVERBERATION
• In handball courts conversation is nearly
  impossible

24
On the Other Hand

• If too much sound is absorbed by soft rough
  surfaces then sound levels will be low dull and
  flat.
• It is the job of the acoustical engineer to
  design halls and rooms with a balance of
  reflection and absorption in auditoriums and
  concert halls
• Good acoustics require highly reflective surfaces
  behind the stage to direct sound OUT toward
  audience along with reflective surfaces suspended
  above the performers.

25
All waves can refract

• Whenever waves enter a new mediumthe speed of
  the wave will change
• Whenever temperature changes the speed of the
  wave will change
• This change of speed causes the wave to change
  direction or to bend
• Refract means to bend

26
And so.

• On a warm day when air closer to the ground is
  warmer than the air higher above the ground, the
  speed of sound near the ground increases so the
  sound waves bend UP and away from the ground,
  thus sound does NOT travel as well
• Likewise when the air near the ground is cooler,
  the layer above is warmer then the sound wave
  bends DOWNWARD toward the ground

27
In water.

• Refraction occurs underwater as well
• Speed of sound varies with water density and
  temperature changes
• This causes problems for ships using SONAR waves
  that bounce off the bottom of the ocean
• This is good for subs who use the refraction to
  escape detection

28
Underwater

• Layers of water at different temperatures are
  called thermal gradients
• This results in the bending of sound toward or
  away, distorting the sound so that gaps or blind
  spots appear
• This is where subs can hide is these distortion
  blind spots

29
Ultrasound

• Physicians use multiple reflections-refractions
  of ultrasonic waves of sound to produce
  ultrasound pictures.
• Can do this because ultrasound waves are more
  strongly from outside the organs than from inside
  the organs this produces a good outline of the
  organs
• Dolphins and Bats do this

30
Forced vibrations and natural frequency

• Whenever one vibrating substance CAUSES another
  object to vibrate that is forced vibration
• Example hold a tuning fork,strike it, the sound
  will be faint
• Set tuning fork on hard table, strike it,sound is
  louder since table is forced to vibrate and thus
  a larger surface is set to move the air in waves.
  
• Nearly all objects can be set into vibration,
  forced into vibrating by some other vibrating
  object.

31
Natural frequency

• All objects vibrate at some frequency that is
  characteristic of the objects designthe
  arrangement of its atomsrecalling that electrons
  vibrate around the nucleus of atoms and the
  protons and neutrons are shivering in the
  nucleus

32
Forced Vibrations

• A factory floor needs to be built strong enough
  so that it does not collapse when the hum of
  heavy machinery sets the floor to vibrating
• Stringed instruments, violins, guitars, piano,
  are not very useful without a sounding board
  which can be set to vibrating and enhance the
  sound
• Like wood of violin , body of piano, amplifier of
  guitar.

33
You can tell the difference

• Drop a wrench, drop a baseball bat onto a
  concrete floor and you can tell the difference
  since each vibrates differently when striking the
  floor
• Every object vibrates characteristically
• Any object made of elastic material vibrates at
  its characteristic frequency forming a special
  sound

34
And so..natural frequencies

• Natural frequencies depend on
• Amount of elasticity of the material
• The shape of the object
• Even planets vibrate
• When Jupiter was struck by comet fragments,
  Jupiter shivered, it rang like a bell.

35
Resonance and Sympathetic Vibrations

• An object may be forced to start vibrating by
  another vibrating object
• When the frequency of the forced vibration
  happens to be the same as the objects natural
  frequency, there will be an increases in
  amplitude, loudness, energy flow
• This is called resonance, means, sounding again.

36
No resonance

• Putty doesnt resonate, it isnt elastic enough
• A dropped handkerchief is too limp

37
Higher!!

• Example of resonance is a swing
• Pumping a swing
• When you swing your legs in rhythm with the
  frequency of the swings rise and fall, the force
  of your pull on the swing is in rhythm with the
  swing so that the amplitude increases
• More important than the force you use to swing is
  the timing with the rhythm of the swing

38
Demo

• An example of resonance is to set two tuning
  forks one meter apartstrike one, the other
  should begin to vibrateif conditions are
  correct.
• The continuous compressions push the fork prongs
• When the pushes match the natural frequency the
  successions of pushes increases the amplitude of
  the vibrations of the second fork
• The motion of the second fork is called a
  sympathetic vibration
• When the timing is off,the frequencies are not
  the same then resonance does not occur.

39
Tune the radio

• Tuning the radio is adjusting the natural
  frequency of the electrons in the radio to match
  one of the signals in the air so that the radio
  resonates with one frequencyone station
• Resonance occurs whenever impulses are applied to
  a vibrating object in rhythm with its natural
  frequency

40
Danger!!

• A troop of soldiers crossed a bridgein
  stepcavalry actuallythey set up a vibration
  that was the same as the natural frequency of the
  bridgethe bridge collapsed since its amplitude
  became greater than the supports could hold
• Today all groups break stride when crossing
  bridges to avoid such a disaster

41
Ask to see the video

• In 1940 the Tacoma Narrows bridge in the state of
  Washington was destroyed by wind generated
  resonance
• The wind set up vibrations that were the same as
  the natural frequency of the bridge material
• The amplitude of the swaying bridge increased
  until it collapsed
• We have this on tapewant to see it?

42
Interference

• Waves can do something matter cannot dowaves can
  interfere with each other
• Interference may be CONSTRUCTIVE OR DESTRUCTIVE
• Constructive is when two waves meet in phase2
  crests meet moving toward each other so that the
  amplitude increases
• Destructive is when waves meet out of phasea
  crest meets a troughso that the amplitude
  decreases

43
Interference in sound

• In sound the compressions are like the crests and
  the rarefactions are like troughs.
• Destructive sound interference is useful in
  anti-noise technology
• Can design earphones that can generate mirror
  images of waves so that a worker with a
  jackhammer cannot hear the jackhammer! Thus
  destructive interference
• Noise canceling earphones.

44
And the Beat Goes On

• Beat is the effect of sound interference
• A TONE is a sound of distinct PITCH and duration
• When two tones of slightly different frequencies
  sound together a fluctuation in loudness is
  heardthis is the beat due to constructive/destruc
  tive interference

45
Career!

• A piano tuner is much in demand
• When tuning a piano, tuner listens for BEATS
• When they disappear, the frequencies are
  identical.

46
Standing Waves

• The effect of waves passing through each other is
  called interference
• Where parts of the wave are standing called
  NODES
• Nodes are areas of cancellation where
  displacement is zero
• Distance between nodes ½ of a wavelength

47
Antinodes

• Positions on a standing wave where displacement
  is maximum ANTINODES
• These are between two nodesand the distance is ½
  the wavelength
• See Walker page 443 for description
• Standing waves produced whenever two sets of
  waves of equal amplitude and wavelength pass
  through each other!

48
The Doppler Effect

• The effect is named after Christian Doppler
• What did he do? A brief, to the point report will
  earn you 5 points ECTO THE POINT
• The Doppler effect can be measured in
  wavescommonly in light and sound
• Doppler occurs when the frequency of the wave
  changes due to motion of either source or receiver

49
High and Low

• If the waves are coming at you, then the
  frequency of the light or sound his higher than
  if the waves are moving away
• Higher pitch in sound
• Color change in lightred is low frequency and
  violet is high frequencythus Red Shift.
• Recall that waves of light/sound travel outward
  in three dimensions like a balloon expanding

50
Wave barriers and Bow(as in bow wow) waves

• When a source of waves is moving as fast as the
  wave it emits then a wave barrier is produced
• The waves pile up directly in front of the
  source. (see note)
• The source has great deal of trouble to get
  through the barrier

51
(No Transcript)
52
Chuck Yeager

• In a plane, the barrier of compressed air piled
  up on the leading edges of the wings.
• Thrust is required to push through
• Once through, one can move faster without similar
  opposition
• Once over the barrier, the rest is smoother and
  undisturbed
• In the case of the sound barrierthe speed beyond
  is supersonic

53
Bow Wave

• When the source moves faster than the wave speed
  a V-shaped bow wave appears dragging behind the
  source from overlapping periodic circular waves
• As source moves faster the V-shape becomes
  narrowerBOW wave is like the bow of a ship

54
Shock waves and Sonic Booms

• Sock waves are 3-D
• Conical waves spread out until it reaches the
  ground
• Dont hear sonic booms from slower than sound
  speedsubsonic because the waves reach ears one
  at a time
• But at supersonic the waves reach the ear all at
  once in a single burst
• Increases air pressure..BOOM

55
BOOM!!!

• The sudden increase in pressure has the same
  EFFECT as a sudden expansion of air produced by
  an explosioneffect is the same
• Anything moving at the speed of sound will make a
  sound
• Plan
• Bullet
• Crack of a whip

56
And Now

• Read chapter 14
• Answer the problems assigned
• See appropriate videos
• Do the appropriate worksheets
• Labs??? Well see what we have available
• That is all!!!