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# Waves - PowerPoint PPT Presentation

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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## PowerPoint Slideshow about ' Waves' - amelia

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Presentation Transcript

the transfer of energy without the transfer of matter

• 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

What is the period?

What is the frequency?

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

• Mechanical waves require a mediumin order to travel.

• examples:

• electromagnetic waves do not require a medium

water,

earthquakes,

and sound

All EM waves travel at 3.0 x 108m/s

Radio waves are the longest of the spectrum

Visible light is a tiny section

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

v

v

• Superposition: when two or more waves overlap

• Waves do not affect each others identity

• overall amplitude is sum of individual wave amplitudes

• Interference Example

Each wave pulse moves 1 grid at a time

Each wave pulse moves 1 grid at a time

sketch where wave would be if alone

• Waves above rest are positive

• below are negative

+2

0

+2

+2

0

0

+2

-2

0

+2

-2

-2

0

-2

• Move another grid and repeat process

• (remember, if both same sign then they stack/add)

• Now try it yourself

• 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 at 0 °C

At average temperature:

20 °C

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

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

• Sounds waves are characterized by their:

• speed

• pitch

• loudness

• quality

• 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

• 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

• 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

• 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

• 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

• 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

• threshold = 0 db = 1 x 10-12 W/m2

• I2 = heard sound

• I1 = sound you compare to

• (usually 1x 10-12 W/m2)

• 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

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

but this time, we want I2

Energy rate produced by a sound

units are Watts (W)

Power depends only on source, not distance

• this can be used to find I at different distances

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

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)?

### Practice

go to page 129

Pig standing still

OINK!

λ in front

is same as λbehind

Pig running right

OINK!

λ in front

is smaller λthan behind

Who hears the higher pitch?

• 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

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

Given:

So,

Now to pick signs. This requires logic.

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

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

observer moving away from source (-)

source moving towards observer (-)

car hears a lower pitch!

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

Given:

Find the speed (T = 33°C)

Given:

Now let’s choose signs

moving toward the source

moving toward the observer

Pitch goes up!

• 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

Which plane is moving faster?

Mach 6

Mach 3.5

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