Clicker question
A string of beads are connected by a set of taut, ideal springs.
At the instant the clock starts (t=0), a pulse is moving to the right on the beads
and the shape looks like this.
Which of the following graphs looks like a graph of the position for the bead
marked with a red arrow as a function of time?
A
B
C
D
Phys 1240: Sound and Music
Reminder: lecture attendance is not
required. If you do attend, please
create a good learning environment!
Please opt-in to reduce
distractions:
Turn off cell phones
Put away laptops
Put away newspapers or other reading
Stop side conversations
Phys 1240: Sound and Music
www.colorado.edu/physics/phys1240
LAST TIME: Interference, superposition, beats.
TODAY: Diffraction.
NEXT TIME: Outdoor sound, Doppler shift, sound
intensity and loudness.
READ: Ch 5.1 and 5.2
• Homework 4 due TONIGHT.
• Homework 5 and Reading Question 6 due
Thursday.
Clicker question
The pulse on the left is moving right, the pulse on
the right is moving left. What do you see at the
“central moment” they pass through one another?
A
B
D
C
E
Pulse interference animations
www.physics.nyu.edu/~ts2/Animation/waves.html
Diffraction
When traveling waves reach a “hole”, they
continue but also bend. (They “spread out” from
the hole, like ripples on a pond.)
That’s diffraction.
Larger => MORE “bending”.
Similarly, larger opening => LESS bending.
Get decent diffraction if
> hole size
(If it does NOT diffract, it just goes “straight on
through.”)
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…
Simulations
Ripple tank Diffract simulations
http://www.falstad.com/ripple/
More diffraction simulations
http://www.falstad.com/mathphysics.html
Remember the wavelengths of typical sounds:
f = speed of sound = v
So = (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.
Clicker question
You can hear a sound in your left ear that came from
your right side. There are many physical reasons why
this occurs, but which below is best? (And, can you
come up with more?)
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
Clicker question
Would light also diffract if you pass it through a
slit?
A) Sure, always
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
Decent diffraction if
> hole size
Remember the wavelengths of typical sounds:
f = speed of sound = v
So = (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.
Propagation of sound
Reflection (echoes)
Can be diffuse (every which way)
or specular (like light off a mirror, or
tennis balls off a smooth wall)
When does sound “bounce” and when does it “scatter”?
Specular (mirror-like) reflections happens if
the wall is smooth
( not too bumpy, or small bumps)
HOW small is small?
sets the scale!
f = v = speed of sound
So = v/f = 344 m/s / frequency
• Low (34 Hz) => 10 meters
• Medium (1000 Hz) => .3 m
• High (10,000 Hz) => 3 cm
Clicker question
Which kind of reflection is far and away the
most common for light, in ordinary life?
A) Specular
B) Diffuse
Clicker question
I play a bass note facing an indoor climbing wall. The
sound is reflected off the surface. What will happen?
(The note has a wavelength of about 10 meters.)
a)
b)
c)
d)
The sound will largely reflect diffusely because the
surface is rough
The sound will largely reflect specularly because the
surface is smooth
The sound will all be absorbed because the surface
is rough
The sound will all be absorbed because the surface
is smooth
Clicker question
If the bumps and irregularities on the wall are
about 30 cm wide (or smaller), which frequencies
of sound are most likely to reflect diffusely? (i.e.
what’s the “cutoff”?)
A) Frequencies above 10 Hz
B) Frequencies below 10 Hz
C) Frequencies above 1000 Hz
D) Frequencies below 1000 Hz
E) ???