Phys 1240: Sound and Music
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www.colorado.edu/physics/phys1240
LAST: Intro, course logistics
Production, propagation, perception
TODAY: pitch/frequency
Speed of sound
NEXT: waves and pressure waves
READ: Hall 1.5, 2.1, 2.2
Online homework Tues - see web.
(CAPA next Thursday… pick one up
in basement)
A) Yes, I do!
B) Not yet…
(Don’t forget to register it, there’s a
link in the “announcement box”
on our home page)
CT 1.1.2
CT 1.0.1
How do you think you will learn physics the
best?
a)
b)
c)
d)
e)
Listening to lecture
Reading the textbook
Doing the homework
Discussing with others
Other…
The following sound would be classified as:
a)
b)
c)
d)
e)
Musical
Speech
Noise
Organized sound
Transient
We cannot teach you physics (!)
Physics is not a collection of facts - it’s a
way of thinking.
Only you can teach yourself to think!
Analyzing, sense making, applying
concepts , solving problems
1
Pretest survey from Tues
•
•
•
Broad spread around 50%.
Helpful for me (what to target)
We’ll be covering all that
material, so no worries!
Except maybe…
What is 103?
103= 10*10*10
= 1,000
Calculators (or CAPA!)
= 1E3
CT 1.2.2
If I whack a ruler hanging off the
desk, I can make a “note”.
What happens if I move it back so
less hangs off the edge, but I
whack it just as hard?
A) Pitch , loudness similar
B) Pitch similar, loudness
C) Pitch , loudness
D) Pitch , loudness similar
E) Something else
WHY?
Pitch & loudness
(Perception)
Pitch: “how low or high”
For musicians: bass or treble
Loudness: sensation of strength
CT 1.2.3
What determines pitch?
Which is the most important feature
of the instrument/source of
sound?
A) Length/size
B) “Bendability”/rigidity
C) Heaviness/density of material
D) Amplitude (how hard it is struck)
E) More than one of the above
A-B: very (equally) important
C: Depends!
D is a special case… but usually
does NOT matter (much).
2
CT1.2.4
Let’s fill the Tibetan singing bowl with
water. What happens to the pitch of
the tone it makes?
A)
B)
C)
D)
goes up
goes down
pretty much unchanged
no way to know this without
trying
Propagation:
• Speed.
(How could we measure it?)
• Medium
(Does it matter? Come back to
this very soon)
Why?
CT 1.4.1
CT 1.4.7
Speed of sound
Which sounds travel the fastest through air?
a)
b)
c)
d)
e)
Higher pitch
Lower pitch
Louder sounds
Quieter sounds
They all travel at the same speed
And a darn good thing…
(344 m/s in room temperature air.)
Approximately how close is a thunderstorm
if you see lightning flash and hear a clash
of thunder after counting up to 6 seconds?
(the speed of sound is 344 m/s)
a) 0 km
b) 1 km
c) 2 km
d) 6 km
e) None of these
3
CT 1.4.2
Units:
1 meter is 3.3 feet
344 meter
= 344 m " 1
!
!
= 344 m " 3.3 feet
1m
You are traveling on a commercial airplane (say
a Boeing 757). Relative to the speed of sound
you are traveling:
a)
b)
c)
d)
e)
much slower than sound
a little slower than sound
the same speed than sound
a little faster sound
much faster sound
= 1135 feet
!
So, 344 m/s is the same as
1135 feet/s
(more than 3 football fields/s)
= 770 mi/hr
CT 1.4.4
In which of the following venues can you notice
a difference between the music you perceive
and the movements of the musicians (i.e. can
you tell that the sound has traveled a distance?)
a)
b)
c)
d)
The rear of the Fox Theater
Pepsi Center
Red Rocks amphitheater
All of the above
Textbook says about 20 meters (about 60
feet, about the width of the grass field in the
stadium) is where most people start to
notice «lag».
CT 1.4.5
In a 32 o C room sound travels…
a) > 344 m/s
b) 344 m/s
c) < 344 m/s
Speed depends on the temperature
Depends on the medium
(Faster through solid metal than
through air, e.g!)
What if the room is 32o F? Does the sound
travel faster or slower than a 32o C room?
a) Faster
b) Slower
c) Same speed
4
CT 1.4.3
(speed is 344 m/s at 20 °C)
Temperature dependence?
• Speed of sound goes up by
about .6 m/s for every degree
Celsius increase.
Sound travels at a speed of 344 m/s (770 mph).
How fast does the air flow out of my mouth as I
speak?
a)
b)
c)
Much slower than 344 m/s
344 m/s
Much faster than 344 m/s
Tune clarinet in an air conditioned
room (20 °C)
Go outside (HOT) 40 C (>100°F!)
Speed is (40-20)*.6 m/s faster
= 12 m/s
(12/344 = 3% faster) Noticeable!
CT 1.5.1
CT 1.4.6
When sound travels from a source to a listener,
what do air molecules never do?
a)
b)
c)
d)
e)
Travel from the source to the listener at
344 m/s
Move back and forth with the frequency of
the sound wave
Bump into other air molecules
Move in the direction of wave propagation
Air molecules do all of the above
If there is really a force of 105 N (that’s
100,000 N!) on each square meter of a glass
window, why is it that the window does not
shatter?
a)
b)
c)
d)
e)
That is such a small force it does not
matter
The total force exerted on the window is
not equal to zero
Glass is stronger than you think
There is an equal but opposite force
pushing against the window from the other
side
Gravity counteracts the force
5
CT 1.5.2
I stand on a scale with both feet and measure
My weight. If I stand on one foot, does the
reading:
a)
b)
c)
Increase
Decrease
Remain the same
I weigh 500 N, what is the force on each foot?
a)
b)
c)
CT 1.5.2
What is the area of my feet in m2 (Hint: my feet
are very roughly 25 cm x 8 cm)?
a)
b)
c)
d)
e)
200
20
2
.2
.02
500 N each
250 N each
It depends on the area of my foot
CT 1.5.2
What is the approximate pressure on each
foot?
a)
b)
c)
d)
e)
N/m2
.125
125 N/m2
1250 N/m2
1.25x105 N/m2
Not enough information
Convert this to atmospheres
a)
b)
c)
d)
e)
CT 1.5.3
I stand on both feet, when I lift one foot up the
pressure on the other foot (the one that
remains
on the ground)…
a)
b)
c)
Increases
Decreases
Remains the same
1.25x10-5 atm
1.25x10-3 atm
1.25 atm
125 atm
1.24x105 atm
6
CT 1.5.4
CT 1.5.5
Who is more likely to damage a hardwood
dance floor?
When the air is sucked out of this barrel, what
is going to happen to the barrel?
a)
a)
b)
c)
d)
b)
c)
d)
A 250 lb cowboy wearing boots with 6 cm
x 6 cm heels
A 100 lb waif-like woman wearing 1 cm x 1
cm high heels
They will do equal damage
Not enough information to make an
informed choice
e)
The barrel will explode
The barrel will completely collapse
The barrel will collapse halfway
Absolutely nothing, it’s a reinforced steel
barrel!
I have no idea
CT 1.3.1
On the picture shown, what is the wavelength?
a
d
b
c
e) None of these
7