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Waves. the transfer of energy without the transfer of matter. A Simple Look at a Wave. How many wavelengths in the wave below?. crest. amplitude. A. wavelength. λ. trough. how many λ pass per second. frequency =. units: per second. time for 1 λ to pass by. period=. or.

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waves
Waves

the transfer of energy without the transfer of matter

a simple look at a wave
A Simple Look at a Wave
  • How many wavelengths in the wave below?

crest

amplitude

A

wavelength

λ

trough

ex the wave below takes 10 seconds to pass by
Ex.The wave below takes 10 seconds to pass by.

What is the period?

What is the frequency?

types of waves
Types of Waves

Transverse wave:

medium vibrates at right angles to the direction the energy moves

λ

Compression wave:

(longitudinal wave)

medium vibrates in the same direction as the direction the energy moves

electromagnetic waves
Electromagnetic Waves
  • Mechanical waves require a mediumin order to travel.
    • examples:
  • electromagnetic waves do not require a medium

water,

earthquakes,

and sound

electromagnetic waves1
Electromagnetic Waves

All EM waves travel at 3.0 x 108m/s

electromagnetic waves2
Electromagnetic Waves

Radio waves are the longest of the spectrum

electromagnetic waves3
Electromagnetic Waves

Visible light is a tiny section

electromagnetic waves4
Electromagnetic Waves

Visible light is betweenultraviolet (UV) light and Infrared (IR) light

v

v

interference
Interference
  • Superposition: when two or more waves overlap
  • Waves do not affect each others identity
interference1
Interference
  • waves add together
  • overall amplitude is sum of individual wave amplitudes
  • Interference Example
practice
Practice

Each wave pulse moves 1 grid at a time

practice1
Practice

Each wave pulse moves 1 grid at a time

practice2
Practice

sketch where wave would be if alone

practice3
Practice
  • Waves above rest are positive
  • below are negative
practice4
Practice
  • Add ‘em up

+2

0

practice5
Practice
  • Add ‘em up

+2

+2

0

practice6
Practice
  • Add ‘em up

0

+2

-2

practice7
Practice
  • Add ‘em up

0

+2

-2

practice8
Practice
  • Add ‘em up

-2

0

-2

practice9
Practice
  • Move another grid and repeat process
  • (remember, if both same sign then they stack/add)
  • Now try it yourself
sound
SOUND
  • Sound is a compressionalwave
  • Speed of sound depends on the medium
  • fastest in solids (6000 m/s in steel)
  • slower in liquids (1500 m/s in water)
  • slowest in gases
speed of sound in air
Speed of sound in Air

speed of sound at 0 °C

At average temperature:

20 °C

If no temperature is given in a problem, assume 20° C (343 m/s)

distance
Distance
  • If you hear lightning 3.0 seconds after seeing it, how far away did it strike?
characterizing sound
Characterizing Sound
  • Sounds waves are characterized by their:
  • speed
  • pitch
  • loudness
  • quality
pitch
Pitch
  • Pitch depends on frequency
  • Young healthy human ear has a range of 20 Hz to 20,000 Hz
  • Human voice: 120 Hz to 1,100 Hz
  • Baby cry: 2,000 – 3,000 Hz
  • Test your range
octaves
Octaves
  • The octave is important in music
  • Octave is the doubling in ƒ
  • Ear can hear a range of ≈ 10 octaves
  • 20 Hz  40 Hz  80 Hz 

160 Hz 

320 Hz 

640 Hz 

1280 Hz 

2560 Hz 

5120 Hz 

10,240 Hz 

20,480 Hz

just noticeable difference jnd
Just Noticeable Difference (JND)
  • The range that the ear can distinguish tones (pitches)
  • at <1000 Hz, JND ≈ 1Hz
  • this means you can tell the difference between 500 Hz and 501 Hz
  • at 2,000 Hz, JND ≈ 2 Hz
  • at 4,000 Hz, JND ≈ 10 Hz
  • Test it
loudness of sound
Loudness of Sound
  • depends on amplitude of a sound wave
  • new unit decibel (db)
  • decibel is based on human hearing
  • 0 decibels is the threshold of hearing
  • 140 db is the sound of a jet on runway
loudness of sound1
Loudness of Sound
  • decibel scale is logarithmic
  • So, 60 db  70 db means 10x louder
  • 60 db  80 db means 100x louder
  • 140 db is 100,000,000,000,000x louder than 0 db
  • Test your range
decibel details
Decibel Details
  • Sound Intensity Level (L) = db
    • measures how loud you perceive sound
  • Sound Intensity (I) = W/m2
    • measures physical intensity of sound
    • threshold I = 1 x 10-12 W/m2
decibel details1
Decibel Details
  • threshold = 0 db = 1 x 10-12 W/m2
  • I2 = heard sound
  • I1 = sound you compare to
  • (usually 1x 10-12 W/m2)
example
Example
  • How many decibels would sound have if it had an intensity of 1 x 10-10 W/m2?
  • given:

for normal problems, use 1 x 10-12 W/m2 for I1

example1
Example
  • What is the sound Intensity of a girl yelling at 86 db?

but this time, we want I2

power
Power!

Energy rate produced by a sound

units are Watts (W)

power1
Power!

Power depends only on source, not distance

  • this can be used to find I at different distances
  • Start with two instances of the sound (2 P’s)
power example
Power Example:

What is the power of a jet engine when standing 5.0 m away from it?

given:

r = 5.0 m

L = 140 db

need I first

frequency sensitivity
Frequency Sensitivity

The ear is much more sensitive to some pitches than others. How much more sensitive are we to a crying baby (3,000 Hz) than to normal talking (1,000 Hz)?

practice10

Practice

go to page 129

doppler effect
Doppler Effect

Pig standing still

OINK!

λ in front

is same as λbehind

doppler effect1
Doppler Effect

Pig running right

OINK!

λ in front

is smaller λthan behind

Who hears the higher pitch?

calculating doppler effect
Calculating Doppler Effect
  • Doppler Effect occurs if either source or listener is moving
  • ƒ’ = frequency the observer hears
  • ƒ = actual frequency of source
  • ν = speed of sound (remember T)
  • νo = speed of observer
  • νs = speed of source
picking signs
Picking Signs

Ex: On a day when the speed of sound is 350 m/s, a police car with a 2,000 Hz siren and a speed of 35 m/s is chasing a car moving at 50. m/s. What frequency does the driver of the car hear?

  • what is the source?

police car (it has the siren)

  • what is the observer?

chased car

Given:

moving away from source

moving toward the observer

picking signs1
Picking Signs

Given:

So,

Now to pick signs. This requires logic.

picking signs2
Picking Signs

Numerator(observer)

If observer travels away from source, pitch goes down.

  • for pitch to go down, vo must be negative

If observer travels towardsource, pitch goes up.

  • for pitch to go up, vo must be positive
picking signs3
Picking Signs

Denominator(source)

If source travels away from observer, pitch goes down.

  • for pitch to go down, vs must be positive

If source travels towardobserver, pitch goes up.

  • for pitch to go up, vs must be negative
put it together
Put it together

observer moving away from source (-)

source moving towards observer (-)

car hears a lower pitch!

do you get it
Do you get it?

Ex: It’s 33°C and you’re moving at 45m/s on a head-on collision course with another car moving at 35m/s. You lean on your 1,800Hz horn. What frequency does the other driver hear?

Given:

moving toward the source

moving toward the observer

do you get it1
Do you get it?

Given:

Find the speed (T = 33°C)

do you get it2
Do you get it?

Given:

Now let’s choose signs

moving toward the source

moving toward the observer

Pitch goes up!

faster than the speed of sound
Faster than the Speed of Sound
  • Objects moving faster than sound make shock waves from the pushed air and sound.
  • An observer would hear this as a crack!
  • See the sounds waves
faster than the speed of sound1
Faster than the Speed of Sound

Which plane is moving faster?

Mach 6

Mach 3.5

practice11

Practice

go to page 140

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