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## PowerPoint Slideshow about ' Waves' - amelia

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### Practice

### Practice

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

Speed

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

- Mechanical waves require a mediumin order to travel.
- examples:

- electromagnetic waves do not require a medium

water,

earthquakes,

and sound

Electromagnetic Waves

All EM waves travel at 3.0 x 108m/s

Electromagnetic Waves

Radio waves are the longest of the spectrum

Electromagnetic Waves

Visible light is a tiny section

Interference

- Superposition: when two or more waves overlap
- Waves do not affect each others identity

Interference

- waves add together
- overall amplitude is sum of individual wave amplitudes
- Interference Example

Practice

Each wave pulse moves 1 grid at a time

Practice

Each wave pulse moves 1 grid at a time

Practice

sketch where wave would be if alone

Practice

- Waves above rest are positive
- below are negative

Practice

- Move another grid and repeat process
- (remember, if both same sign then they stack/add)

- Now try it yourself

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 at 0 °C

At average temperature:

20 °C

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

Distance

- If you hear lightning 3.0 seconds after seeing it, how far away did it strike?

Characterizing Sound

- Sounds waves are characterized by their:
- speed
- pitch
- loudness
- quality

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

- 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)

- 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

- 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 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

- 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 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

- 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

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:

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

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)?

go to page 129

Doppler Effect

Pig running right

OINK!

λ in front

is smaller λthan behind

Who hears the higher pitch?

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

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 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 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

observer moving away from source (-)

source moving towards observer (-)

car hears a lower pitch!

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 it?

Given:

Now let’s choose signs

moving toward the source

moving toward the observer

Pitch goes up!

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

go to page 140

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