Phys 1240: Sound and Music
LAST: interference, superposition,
beats
TODAY: diffraction, doppler, and
outdoor sound
NEXT: sound intensity and loudness
READ: 5.1 and 5.2
CAPA 4 due tonight.
CAPA 5 will be “back on
schedule”, in TWO weeks, the
Thursday AFTER the break…
(It’ll be in the bins in a day or so)
CT 4.5.2x
Two waves traveling in the same
direction
Imagine two traveling sin waves (same
frequency, in phase, or “in synch”)
traveling together (same direction,
starting in the same place)
What happens when they “superpose”?
A) A traveling wave, same as either one
individually
B) A traveling wave, twice the amplitude
of either one
C) A traveling wave that gets bigger and
smaller, bigger and smaller, in
amplitude (beats)
D) A standing wave
E) ?? (something else)
CT 4.5.2x
Two waves traveling in the same
direction
Imagine two traveling sin waves (same
frequency, in phase, or “in synch”)
traveling together (same direction,
starting in the same place)
What happens when they “superpose”?
A) A traveling wave, same as either one
individually
B) A traveling wave, twice the amplitude
of either one
C) A traveling wave that gets bigger and
smaller, bigger and smaller, in
amplitude (beats)
D) A standing wave
E) ?? (something else)
CT 4.5.2xb
Two waves traveling in opposite
directions
Imagine two sin waves (same frequency)
traveling towards each other (opposite
directions, passing through each other),
What happens when they “superpose”?
A) A traveling wave, same as either one
individually
B) A traveling wave, twice the amplitude of
either one
C) A traveling wave that gets bigger and
smaller, bigger and smaller, in amplitude
(beats)
D) A standing wave
E) ?? (something else)
Physlets
Wave on a string
Physlet: periodic motion
Beats
Standing wave
CT4.5.4x
Suppose the two speakers in the
room are playing the same steady
frequency, but one is “reverse wired”
(so they’re exactly out of phase) I’m
standing on the “midline”. What do I
hear?
A) Steady, loud pitch
B) Steady pitch, same as either one
of the speakers alone
C) Silence
D) Beats
E) Something else (or ???)
Diffraction
When traveling waves reach a “hole”,
they continue but also bend.
That’s diffraction.
Larger λ => MORE “bending”.
Similarly, larger opening =>
LESS bending.
Decent diffraction if λ > hole size
Traveling wave approaches a
small slit:
Diffraction: it “spreads out”, the
small hole acts like a little point
source of waves on the far side.
(Lots of “bending”, wave goes in
all directions!)
Bigger slit (compared to
wavelength) => less bending. It’s
more just a “shadow” here…
Remember the wavelengths of
typical sounds:
λ * freq = speed of sound,
So λ = (344 m/s) / frequency
Low (34 Hz) => 10 meters
Medium (1000 Hz) => .3 m
High (10,000 Hz) => 3 cm
A doorway (size ~1 meter) will
diffract low sounds a lot, high
sounds much less.
CT 4.2.3
You can hear a sound in your left ear
that came from your right.
There are many physical reasons why
this occurs, but which below is best?
a) Because your head is a relatively
rough surface
b) Because of interference
c) Because the sound just keeps
traveling through your head to your left
ear drum
d) Most sound wavelengths are larger
than your head so they diffract
e) Most sound wavelengths are smaller
than your head so they diffract
CT 4.2.4
Which sounds are more likely to
head out only in the forward
direction from a large speaker?
A) The low pitches
B) The high pitches
C) All pitches
D) None…
(Why might we choose to use
smaller speakers for treble?)
Remember the wavelengths of
typical sounds:
λ * freq = speed of sound,
So λ = (344 m/s) / frequency
Low (34 Hz) => 10 meters
Medium (1000 Hz) => .3 m
High (10,000 Hz) => 3 cm
A small speaker (~couple cm)
will diffract MOST sounds =>
sound goes out in all directions.
Ct 4.2.5
Would light also diffract if you
pass it through a slit?
A) Sure
B) Only if the slit is much
SMALLER than the wavelength
of light
C) Only if the slit is much
LARGER than the wavelength of
light
D) ???
Decent diffraction if λ > hole size
sims
Diffract sims
time=0, freq=1 Hz
time=0, freq=1 Hz
Time=1 sec
distance=344 m
Time=1 sec
distance=344 m
T=2 sec
inner circle at 344 m, outer at 688:
T=2 sec
inner circle at 344 m, outer at 688:
6 sec:
time=0, freq=1 Hz
v(source=172 m/s)
Time=just before 1 sec
distance=344 m
Time=1 sec
distance=344 m
Time=1 sec
distance=344 m
T=2 sec
6 sec:
4.4.3
Given the following picture of a
sound source and its sound waves
in still air, what do you conclude?
s
s
a) The source is moving
b) The source is stationary
c) There is some obstruction to the
right of the source
d) The sound being produced is of
varying pitch and wavelength
e)???
sims
Diffract sims
Doppler shift
Frequency tells you “number of
wiggles per second”.
If a source wiggles the air at
frequency f, but gets closer to you
each cycle, each cycle arrives a little
earlier than it would have.
You perceive MORE wiggles
each second => higher
frequency.
What if it gets farther from you
each cycle?
4.4.1
You are standing stationary and see an
ambulance approaching with its sirens
on. As the ambulance passes you the
pitch of the siren seems to…
a)
b)
c)
d)
Get higher
Get lower
Stays the same
Need more information
The pitch…
a)
b)
c)
d)
e)
Steadily decreases the whole time
Decreases really fast at first then slower as
time goes on
Decreases slowly at first then faster as time
goes on
None of the above really describes it
I’m still not entirely convinced that the pitch
decreases
4.4.4
The picture below shows a traveling
object that is emitting a sound. There is a
person at point A and a person at point
B. Which direction is the sound traveling?
A
a)
b)
c)
d)
B
Right
Left
All directions
Need more information
Who hears the higher pitch?
a)
b)
c)
d)
A
B
They hear the same pitch
Need more information
4.4.2
A child is swinging on a swing
towards a person blowing a whistle.
At which position will the pitch
sound the highest to the child?
start
end
a.
c.
Whistler
b.
d. The pitch is the same at all the
points.
4.3.1
Adding a shell around a
performer at an outdoor show…
a) Improves the music pretty
much no matter where the
performer is positioned
b) Allows the performer to
better hear their own music
c) Creates interference,
making the music less
enjoyable
d) Is mostly for aesthetic
purposes
e) (depends on where the
performer is positioned within
the shell)
Outdoor music
• Sound is generally weaker than
indoors (why?!)
• Shell reflects sound, can improve
– Volume for audience
– Consistency of sound at various spots
– Acoustics for the performers
(Why?!)
4.3.2
Where should the performer stand to
get the maximum benefits out of this
circular shell?
a.
b.
d)Somewhere else
e)There is no difference
c.
Discussion
How/why do each of the
following improve outdoor music
performances?
• Stadium seating
• Shell behind the performers
• Roof above the performers
• Locating the musicians
halfway between a curved wall
and that wall’s center of
curvature