Transcript Slide 1

Wave Energy Transfer
& Sound Wave Energy
If a vibrational disturbance occurs,
energy travels out in all directions
from the vibrational source.
Ripple demo
All points on a wave that are in phase
comprise a wave front.
Wavefronts join points in phase.
Rays – a ray is an arrow sketched
through the wave fronts (perpendicular)
to show direction of wave propagation.
Rays
Waves transfer energy
Rate of E transfer is proportional
to the A .
Less energy
More energy
For light increased amplitude
increases brightness.
For sound: increased amplitude
increases volume.
What does wave frequency (f)
determine?
• Wave type for EM waves.
• Color for light.
• Pitch for sound
Sound
Hear different frequencies
• http://www.fearofphysics.com/Sound/soun
ds.html
Sound is a Mechanical
Longitudinal/Compressional Wave
Sound velocity
solid
liquid
gas
In gas
hot faster.
cold slower.
Increasing velocity
The Doppler Effect
Stationary Source Emitting Waves all
Directions. Circular wavefronts have = l & f.
Doppler Effect from Moving Source
In front of source l is less, behind l is
longer.
In front of source -short l higher f:
hear higher pitch soundsee shorter l light (blue).
behind source - longer l lower f:
hear lower pitch sound
see longer l light (red).
Doppler Effect
When objects are in relative motion:
a) Toward each other, f received
increases.
b) Away from each other, f received
decreases.
Doppler Clip 3.1 minutes
• http://www.youtube.com/watch?v=Kg9F5
pN5tlI
• https://www.youtube.com/watch?v=h4On
BYrbCjY
Resonance & Sympathetic Vibration
*
Fact *
All objects have a natural
frequency of vibration.
Resonance - the inducing
of vibrations of a natural
rate by a vibrating source
having the same frequency
“sympathetic vibrations”
Push at natural frequency, amplitude
increases
Resonance:
• An oscillatory system that is driven by a force
with a f = to its natural f.
• System will resonate – amplitude will increase.
• The natural frequency of objects is the frequency
that produces a standing wave in the object.
• Can occur from reflection causing a standing
wave. Amplitude is increased.
Sympathetic Vibration
when a wave is near an object & is vibrating at
the natural frequency of the object. Object
vibrates sympathetically at same frequency –
they resonate.
Causes the amplitude to increase.
Break the glass 20 sec.
http://www.youtube.com/watch?v=
17tqXgvCN0E
Standing Waves on Guitar String
½ l.
hwk Rd Text 13 – 1 and 13-2
Wksht Review concepts sound.
hwk Rd 13 – 1 & pg 491 – 493
p.507 #1, 3, 6-8, 11-13, 15, 16
The three components of sound
are:
• Pitch (how high or low)
• Loudness (volume)
• Timbre (tone color)
• Vibration patterns are
also called
waveforms.
• Each repetition of a
waveform is called a
cycle.
• We can hear
frequencies between
20 hertz to 20 kHz i.e.
20,000 Hz.
Pitch
• When the
frequency of a
sound doubles we
say that the pitch
goes up an octave.
• We can hear a
range of pitches of
about ten octaves.
• Many animals can
make sounds and
hear frequencies
that are beyond
what we can hear.
Loudness
• To create vibration
energy is used.
• The greater amount
of energy used the
louder the sound.
• The strength of the
changes in air
pressure made by
the vibrating object
determines
loudness.
• As the distance
from the source
increases the
amount of power
is spread over a
greater area.
• The amount of
power per square
meter is called the
intensity of the
sound.
Humans do not perceive sound intensity linearly.
• For us to perceive a
sound as twice as loud
its intensity must be ten
times greater.
• The perceived intensity
level of sound is
measured in a
logarithmic scale using
a unit called the decibel
(dB).
The scale begins (0
dB) on the softest
sound that a person
can hear. This is
called the threshold
of hearing.
The scale ends at
the volume that
causes pain (120
dB) and is
therefore called
the threshold of
pain.
Tacoma Narrows Bridge
mechanical universe
resonance
“Timbre” (TAM-ber) or tone color
is the specific
property of
sound that
enables us to
determine the
difference
between a
piano and a
harp.
A broad variety of tone
colors exist because most
sounds we perceive as
pitch contain many
frequencies.
• The predominant pitch
is called the
fundamental
frequency. It is the
longest l that forms a
standing wave.
Standing Wave patterns form
notes.
Each string or pipe vibrates with
particular frequencies of standing
waves.
Other frequencies tend to die out.
Although we would perceive a string
vibrating as a whole,
it vibrates in a pattern that appears erratic
producing many different overtone pitches. What
results are particular tone colors or timbres of
instruments and voices.
Waveform with overtones.
Frequencies which occur along with
the primary note are called the
harmonic or overtone series.
When C is the fundamental the pitches
below represent its first 15 overtones.
Harmonics
There are several standing
waves which can be produced by
vibrations on a string, or rope.
Each pattern corresponds to
vibrations which occur at a
particular frequency and is
known as a harmonic.
The lowest possible frequency at which
a string could vibrate to form a
standing wave pattern is known as the
fundamental frequency or the first
harmonic.
2nd Harmonic
Which One??
String Length L, l & Harmonics
Standing waves can form on a string of length L,
when the l can = ½ L, or 2/2 L, or 3/2L etc.
Standing waves are the overtones or harmonics.
L = nln. n = 1, 2, 3, 4 harmonics.
2
Harmonic Frequencies
form where ½ l can fit the string exactly.
To calculate f:
v  fl
nl
L
2
v
l
f
nv
L
2f
Substitute v/f for l.
nv
f 
2L
1st standing wave forms when l = 2L
First harmonic frequency is when n = 1
as below.
nv
f1 
2L
When n = 1 f is fundamental frequency
or 1st harmonic.
Other standing waves with smaller
wavelengths form other frequencies that
ring out along with the fundamental.
nv
f2 
2L
For second
harmonic n = 2.
f2 = v/L
In general,
nv
fn 
2L
The harmonic frequencies can be found
where n = 1,2,3… and n corresponds to the
harmonic. v is the velocity of the wave on
the string. L is the string length.
It is helpful to note that the distance
between nodes on a standing wave
is ½ l.
½l
Pipes and Air Columns
A resonant air column is
simply a standing longitudinal
wave system, much like
standing waves on a string.
closed-pipe resonator
tube in which one end is open
and the other end is closed
open-pipe resonator
tube in which both ends
are open
Open Pipe –
open end has antinode.
Standing Waves in Open Pipe
Both ends must be antinodes.
How much of the wavelength is the
fundamental?
The 1st harmonic or fundamental can fit ½ l into
the tube.
Just like the string
L = nl
2
fn = nv
2L
Where n, the harmonic is an integer.
Closed pipes must have a node at closed
end and an antinode at the open end.
How many wavelengths??
Here is the next harmonic.
How many l’s?
There are only odd harmonics
possible.
L = 1/4l.
L = 3/4l.
L = 5/4l.
fn = nv
4L
where n = 1,3,5 …
Beats – caused by constructive
& destructive interference from 2
frequency sounds interacting.
• Beat Frequency heard is the
difference between 2 frequencies.
• If a 50 Hz wave and a 60 Hz wave
overlap, you hear beat of 10 Hz.
• hear beat frequencies
Traveling Waves Beats
Holt read 13 - 3
pg 499 #1 – 4
Start in class finish for hwk.
Hwk read 491 – 503
do 499 and 503