Waves and Sound
Name
Physics
Wÿa ve.s
Test Date:
"You can't stop the waves, but you can learn to surf."
-Jon Kabat-Zinn
RM UPS'
Waves and Sound
What is a wave?
An oscillation that travels is a
A point in space
A wave oscillates up and down over time at a given point in
space.
An instant in time
_ The wave's oscillations extend in space at any instant in time.
The 0,,"ÿÿ ÿ A of a-wave is the maximum amount the water rises or falls compared to its
average resting level.
The amplitude of different types of waves may have different units
Water wave amplitude is a ÿ'ÿi°tÿ,ÿ;.:ÿfÿ'!;:ÿ ÿ"ÿ .... ÿ
, in meters.
Sound wave amplitude is a __b '#ÿ_ÿ:,(ÿ, ÿ ÿ"ÿ" ÿ'ÿÿ
, in pascals.
The wavelength A is the
ÿ!,A,
a wave travels before it begins to repeat itself.
The wavelength can be measured from ÿ.,ÿ iÿ
t
to :ÿ ÿ iÿ
ÿ
ÿ ÿ
ÿiÿ ÿ: ÿ
The 3r4ÿlÿ' ÿi ÿ,i,,ÿ ÿ,,:ÿ'i,Jÿ ,ÿ % f of a wave is a measure of how quickly it oscillates. The unit for frequency
,ÿ ° - ÿiÿ ÿ'
ÿ
is the liertz, er Hz. 0ÿ'ÿ ertz equals one cycle per second.
Low frequency (slower oscillations)
High frequency (faster
oscillations)
__
""
Waves and Sound
Frequency
When a wave has a frequency of 10 Hz = 10 cycles/second, then 10 waves travel past a given
point each second.
What is the frequency of the wave shown below?
Star
L !:;.
7;i.
ÿy-,, j+iD ....
Three
seconds
Two
seconds
One
second
i i ! ÿi, iÿiÿ!i' 117!1!, iÿ i i !i i ÿ!i !i !i !ÿi iÿi if! !ÿil ÿi! iÿ!i ÿi !i i !i !ÿi i! ! ÿ i!ÿ i !i ÿi ÿ, :iÿiÿi i i !ÿi !i !iÿi! ÿ!i i! i ÿi! i!?i ÿ!ÿ! iÿ ÿi !i !ÿ! ÿi i! i ÿi il,iÿiÿi! ÿ ÿi i i;ÿi !iÿi l ÿ ! i i! ÿil i i i !i !i' i i !i i!?i ! i!ÿ i i! i! ! ! i li ÿi! i ! i! !i i ÿ ÿ!i !i
Frequency remains the same even if the wave amplitude decreases as it spreads out.
@
@
the frequency of a light wave determines its ÿ<ÿ ÿ {
the frequency of a sound wave determines its ÿ\ÿ7 t
\
Wave speed
The speed of a wave depends on the ÿÿ=
Examples"
• speed of typical waterwaves" 5 m/s
• speed of sound in air: 343 m/s
speed of light: 300,000,000 m/s (in a vacuum)
As a wave moves forward, it advances one wavelength With each complete cycle.
speed" distance
distance"2
time
frequency:
.
wave speed: v-f/;
f=--
T
Height vs. Time
Height vs. Distance
,20 iiÿiÿ;?:: .... .iiÿÿiÿi,i::.:ÿÿ:.i.:ÿ.: ;:;ÿii¢iÿi:i i:iÿ.ÿ.ii:;l fi!ii;i;iiii-;:ÿ:.ÿ?:-ÿ; .:i;X;=! :i-.... ::
:;iU:?!!7 '::iiÿ!:;? ÿ:%r7; :ÿi;:ii:!7:iiUYUi:? i:7!i!iiii',:ii!T:?:77:'i '::?-17i71ÿi;? ':i:i
0
1
2
3
4
Time (s)
5
0
5
10
!5
20
Distance (cm)
This wave's motion is graphed as a function of time and distance.
a. What is the wave frequency?. IWÿ'ÿ4ÿ !
ÿ ÿ:
b. What is the wavelength? ÿ Utÿ
c. What is the amplitude? l 5 ÿ!ÿ,¢ÿ,_
d. Calculate the speed of the wave.
..........
<,-'c =ÿ,' .t.L.;ÿ'
, , =--/@
-r, Z,ÿ..'ÿ+ 'ÿ.
-ÿ
2
25
Z
T
Waves and Sound
Two students use a 10-meter-long spring to create a standing wave. The wavelength is 2.0 m and
the frequency is 2.0 Hz. How fast is the wave traveling along the spring?
Wave energy
A wave is an organized mechanism for transferring ÿ!)/-ÿÿ> ÿ' ÿ'*ÿ -ÿ ÿ' .... d .
• As a wave moves through matter, its energy causes the matter to respond.
• After the wave passes, the matter returns to .ÿ:!'t)' .... &ÿi, s,: .
The energy of a wave increases with .ÿEs. ,._ÿ:ÿ', ÿJi!iÿ" ÿ'_ ..ÿ.ÿ ,;',ÿ
.
The energy of a wave also increases with ÿ"ÿ ÿ'ÿ'" ÿ'
_.'.!]ÿ:ÿ:>: .
<'ÿ<ÿ'!,.-:.%ÿ..
.,:::.... 7"::
,:!_...
zy
j:
%%,..I. _.Y/C:":
%h.,ÿ.,,
j.,!
• .
Trough
•
Trough
m
Transverse waves
A transverse wave causes oscillations that are ÿ '/i::P !?ÿi!:" d:y to the forward motion of the wave.
Examples:
• waves in a string
• light waves
t
.
,, .iÿ)
COnx.:pressiOn.
RarelÿaCti.on.
: Co.ÿ.ÿpressioÿ.a
t
Rare-ft.iciion
'ÿ"a:velengt!ÿ ...... >ÿI
Longitudinal waves
A longitudinal wave causes oscillations that move back and forth in the same direction as the
traveling wave.
Examples:
° sound waves
- the waves in a spring as shown in this figure
Waves and Sound
Longitudinal compression wave
:.;: ,ÿ:i: )iÿ-ÿ i::! !'i iÿ: ii ÿ!i 7 ÿi':: !ii
.
........
Describing waves in
three dimensions
....... ;' describes t directiohof the'J
oscillation in a plane perpendicular
to the wave velocity. The wave in this figure is polarized, it
is traveling in the z-direction and its oscillations occur only
in the y-directionÿnot in the x-direction.
_ÿ_!i<'
What kind of waves can be polarized?--!,
:/. ÿ)ÿ ,ÿ,ÿ, ,ÿ ,ÿ
'
ÿ
Height vs. Distance
._m-o.ÿ
•
.....
0
te
2e
i
"
......
i
3ÿ
4ÿ
ÿ0
ÿO
70
:
.
ÿO
"
90
100
Distance (cm)
1. These graphs show the oscillation of a point on a wave as a function of time, and the
oscillation of the extended wave in space at a moment in time.
1. What is the frequency?.
ÿ,ÿ y,,ÿ4 + ÿ, ..... ÿ .:,
2 What is the wavelength? ÿ,
3. What is the amplitude? 0 ÿ-" ÿ',tYÿ
• 2. Provide an example of a transverse wave and a longitudinal wave. Describe how they are
similar and how they are different.
•
,.\
....ÿ
Waves and Sound.
Wave interactions
A ÿJ?ÿ,
represents all the high points in a wave.
..... .-ÿ--ÿ
.
A ÿ.ÿ'ÿ:,,ÿ.ÿ ÿ,ÿ ÿ is all the low points in the wave.
To _ÿ ÿ3[ ÿ!,,ÿ ,J,,.ÿ ......... Is to spread out a d g
.
Waÿ}es propagate outwards from their source, carrying both energy and information.
.
. ,. U;ÿ?ÿ,
occurs for both longitudinal and
transverse waves.
Reflection causes a wave to change direction, and may also
change its shape.
A fixed boundary does "ÿ ,'i:): move in response to a wave. The wave pulse reflects on the
"ÿ'ÿ ÿ side of the spring
Incident pulse
,.
Reflected pu Ise
An open boundary allows the end of the spring to move freely.
The wave reflects on the b J_'Pf ÿ
side of the spring as the incident wave.
"
Refl echoed pulse
? 'ÿ-,'V;ÿ
•.ÿ ÿ. .ÿiÿ
1
occurs when a wave changes speed at a boundary, resulting in a change of
direction. Water waves refract if the depth changes. They refract because they move slower in
shallow water than in deep water. ' 't
Refraction changes the ÿ, 4ÿ'IÿDIÿ'!, ÿ,,'J
of a wave.
Refraction also changes the ÿ?,l ?ÿ,,14. i:,,ÿ ÿi,¢\ 4ÿ,ÿ,.J of a wave.
When wave velocity changes during refraction, the wavelength also changes.
But frequency CAN'T changeÿ
Every wave that enters the boundary must exit the boundary. Therefore, the number of waves per
second must stay constant.
Refraction occurs for both ?ÿ .ÿ:, "ÿÿ"ÿ-ÿ' ÿ ...... ÿ
ÿ
,},
o:,
'
Waves and Sound
Diffraction
is a property of waves that allows them to ÿ ÿL ÿ
around obstacles and
pass through gaps.
Diffraction often changes the ,!, 7Lt,:.'ÿ, ,,.:ÿ Jÿ: andL :ÿ'li:;,L,ÿÿ, of a wave. i :: ÿÿ
°
1. Define the following events as fitting one of the wave-boundary interactions. Use each term
(reflectiOn, refraction, absorption, and diffraction) once.
A. Tarmac heats up on a sunny day.
ÿj,;ÿ' "ÿ,ÿ':ÿ÷ÿ -'ÿ> ÿ- ,"ÿ" '.ÿ; ....
B A magnifying glass enlarges an image.ÿ4-!ÿ-"M,,ÿJ ÿ" ÿ .,ÿP
C. Waves curve around a boulder in the water. ÿ,ÿ ÿ,-
D. A yell echoes off a building.ÿÿ :ÿt ÿP"÷,,-)
2. A water wave moves from deep to shallow water. Describe changes that occur to the following
characteristics of the wave as it crosses the boundary from deep to shallow water
A. wave speed ; ,.ÿ. b" Z.: °.. "_.°
B wavelength 4 ÿ," " " ;'ÿ ÿ' '" .....
, v.Jÿ-' ..=
.......
.
Superposition principle
The superposition principle saÿs that the total ;:ÿ' ........,ÿ,ÿ,ÿ,,;,,ÿ,
"ÿ: ÿ--ÿ 'ÿ; ,,-.,ÿ
''ÿ ÿ " .,;,. at any point equals the sum of the
amplitudes of all of the ÿtÿ>ÿ?2<J },
that occur at that same place and time.
i
When more than one wave is present'ÿthey
can" ÿ"
sum
to" make
larger or smaller
amplitude wave. If
ÿ .....
'ÿ'ÿ"
. ..ÿ.ÿ a
ÿntofforence
has occurred.
the result is a larger amplitude wave, ÿj)ÿ! ÿ,ÿ-¢ÿ ÿ :;ÿ-"° ÿ: ÿ 'ÿ "
i : : ! i i : : i :i .
:i :! i i .i !: il ; i ' i . i ÿ. i i: ! i
1 +2 ÿiI i ÿi!' ÿi'!:i ÿ!
V V b
ii .... ÿ\' : !f
....
....
.......... ;......:...: ........... :.. .................................. . . . . ....; .....
Waves and Sound
Two wavescan also add up to make a smaller wave.
When two or more waves add up to make a smaller amplitude ÿaye,ÿ, destructive interference has
occurred. If the amplitudes are exactly matched, there can be , .... u ÿ ÿ
, ddvÿ, .ÿ,-<J
0
1. Describe how two wave pulses can combine destructively.
Sound waves
Sound is a longitudinal wave, like this compression wave on a Slinky@.
The difference" it is O, ÿ ÿ¢" that is being compressed.
Sound is a tiny ÿ,ÿ,ÿ "ÿ ,!.ÿ: ÿ! >:ÿ ÿ ÿ
of ÿ> ?{-
pressure.
Imagine a cymbal vibrating up and down when struck.
When the surface moves up, the air above it is slightly compressed (slightly higher pressure).
When it moves down, the air is drawn out (slightly lower pressure).
The result is an oscillation of air pressure - a sound wave!
Sound waves are traveling oscillations of air pressure. Sound waves can interfere.
Sound obeys a WÿSo4ÿ'ÿ ÿ ?,ÿÿ,ÿ and ;?ÿ ..... , • ,ÿ
relationship like a wave.
Properties of Sound
Loudness
Pitch - the perception of high or low
Timbre - the property that makes a piano note sound different from the same note produced by a
guitar or a vocalist.
The loudness of a sound wave depends on its ÿ,tÿ,,ÿ ÿ,-', },..ÿ,.ÿ-
Louder sounds waves have t Lÿ,A.,:ÿi!:.x/
amplitude pressure variations.
A stereo's speakers move back and forth a greater distance when producing a loud sound than when
producing a soft sound.
The ÿ ÿbÿ:}ÿ. ,
of a sound depends on the 7Tÿ "?, ÿ,,ÿ.,ÿ ''°'. ''ÿ ÿ f"
of the sound wave
• ÿLow-pitched sounds have lower frequencies. "
° High-pitched sounds have higher frequencies.
Humans can hear sound frequencies from about 20 Hzto 20, 000 Hz.
<U
Audible frequencies
}
Whales
f
.11
1
I.
1 Hz
Humans
Bats ÿt
t
"-
"
Medical
ultrasound
,. .......... ÿ ......... . ............. f"... ........... , .... ,-,, I ,,"",. ./", .... ", ..'q /:'., ?,..,", /", ;', .,",, t",, ,', ÿ ÿ:ÿ'.....ÿ.,,,ÿ!ÿ:ÿ..@,:.,!ÿ!ÿa "
10 Hz
100 Hz
1,000 Hz
10,000 Hz
7
100,000 Hz
1,000,000 Hz
Waves and Sound
Speed
&
Sound waves are
,:'ÿ ÿ
The speed of sound in air is
A ÿ, ÿ, ÿ 'ÿ.
ÿ,,ÿ
..... ÿ'
.. ÿ tÿ.ÿ.ÿ,ÿ, (767 mph!)
Sound travels even faster in water, or ice, or steel. The stiffer the medium, the faster the sound
speed tends to be.
Wh!n sound passes from one mediu to another g
_ <::ÿY' ,I::ÿ,ÿL,'!,,:>"':, stays the same'--ÿ
A 1000 Hz sound in ...
.... ,: S
has a speed of 343 m/s and a wavelength of 34 cm.
:ÿS"
:,, ,ÿ'ÿ",Jÿ,
has a speed of 1480 m/s and a wavelength of 1.5 m.
has a speed of 3500 m/s and a wavelength of 3.5 m.
Sound
travel in a vacuum. Sounds needs a medium.
The loud explosions from space battles in science fiction movies are not realistic.
If you were actually watching a space battle from a distant space ship, you would hear total silence.
Sound waves have small ÿ":ÿ..'?ÿ:' ÿ,,":\jÿ;?:::ÿLi{ÿ" .
:!::.0,,'rib liA..whiÿsp..er; 1:.meter. away:.,
30:ÿidB- Baÿkgr0und-":" ":":"::: : ' '":"
The .G".;I"ÿ!:':.ÿ, }!) ;: '., ÿ :"..: ,l:," ;!f,ÿ!: of a sound wave determines its loudness. Larger
amplitude" means ioude{ sound.
:50![dB l:NOis:ei ;::}avg!:::i;ÿta.u.ranÿ :: ÿ:i: :.,
BUT, to a human ear,. ÿ"g,:ÿ ÿ,..j),ÿ.ÿ,..ÿ, ,:2ÿjÿ_. also matters.
:65 dB"F--ÿn.versatie-n; !:maWay ...... "
A high amplitude sound at a frequency of 40,000 Hz is silent to a human
ear but quite loud to a bat!
90 CiB !;Jackhammer; 3:maway :: :;
12O:dB ,PhYSical pain"i.. - ':: : i: :. ÿ : .:. ........ ÿ
a) What is the frequency of the sound
i ....... i: ...... !ÿ .............. T ....... r ....
wave?
b) is this a transverse or longitudinal
wave, and w
:.iiZ ,':ÿ - 5,. ,.
-e3
.....
c) What can you say about the loudness
of this
sound? ,,
¢ ,<
0.000
o.o05
Tim e (s):
Waves and Sound
DALLY
WORK
STARTS
HERE
Check falconphysics.pbworks.com for keys.