Clicker question
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
Phys 1240: Sound and Music
Meredith Betterton
[email protected]
www.colorado.edu/physics/phys1240
LAST TIME: pitch/frequency
TODAY: Speed, pressure and the nature of sound
NEXT: Waves and oscillations
READ: Hall 2.4, chapter 2 lecture notes
•  Homework 1 and Reading question 1 due Thursday
night
•  We start collecting clicker points today
•  Physics help room hours on web page
Physics of Sound
•  Production
  Vibrations
•  Propagation
•  Perception
Sound Propagation
•  Speed
(How could we measure it?)
•  Medium
(How do sound waves disturb the medium?
Come back to this very soon)
Make the Wave!!!
Speed
Speed is the distance traveled divided by the
time it takes to travel that distance.
In math symbols we let:
d stand for the distance traveled
t stand for the time to travel d
v stand for the speed
v = d/t
Speed has units of distance per time
Clicker question
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)  Fox Theater
b)  Pepsi Center
c)  Red Rocks amphitheater
d)  None of these - you will not notice the finite
speed of sound in any of these venues.
e)  Depends on the music
Speed of sound
•  Sound waves are disturbances which
move with a characteristic speed
•  The characteristic speed does depend on
the medium through which the
disturbances passes.
•  Sound in different gases moves with
different speeds
•  Sound in solids or liquids move with
different speeds from gases
Clicker question
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
Speed of sound
•  Sound waves in gases are special; the
speed does not depend on the frequency
(remember frequency is the equivalent of
pitch)
•  Often other types of waves do have
speeds which depend on frequency; the
most common example is light waves
passing through a glass prism. Different
colors are different frequencies
Light waves with different
Speeds in Glass
Speed of sound
•  The speed of sounds also depends on the
temperature of the medium!
•  At room temperature (about 73 degrees F
or 20 degrees Celsius), speed of sound in
air is 344 meters/second
•  We use abbreviations for units!
–  m is for meters
–  s is for seconds
•  The speed of sound is 344 m/s at 20 deg C
Notes
Scientific Units: The Metric System
1 meter is 3.3 feet
344 meter
So, 344 m/s is the same as 1135 feet/s
(more than 3 football fields/s !)
= 770 mi/hr
Clicker question
You are traveling on a commercial airplane
(say a Boeing 757). Relative to the speed of
sound you are traveling:
A.  much slower than sound
B.  a little slower than sound
C.  the same speed as sound
D.  a little faster than sound
E.  much faster than sound
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
Temperature Dependence of Air Sounds
A general formula for determining the speed of
sound v as a function of temperature T is
v = 344 m / s + 0.6 *(T − 20 C)

Warnings about this equation!
•  T must be a temperature in Celsius!
•  v will be in units of m/s
Clicker question
In a 32 o C room sound travels…
a)  > 344 m/s
b)  344 m/s
c)  < 344 m/s
v = 344 m / s + 0.6 *(T − 20 C)

v (32o C) = 344 + 0.6(32-20) = 351.2 m/s
Nature of Sound Disturbances
•  Sound starts with something vibrating!
•  How do the vibrations make the sound
disturbance?
•  How does the sound disturbance at one
location move to another location?
–  To answer these questions we need the
concept of forces!
Force = push or pull
1 lb is a measure of FORCE.
Hold up a pint of water.
The force (push) you feel is a force of 1 lb
Metric system:
measure force in Newtons (N)
1 N is a gentle push
= light touch of a finger
(5 Newtons is about 1 lb).
Clicker question
What is the weight (force on the
floor) of the boulder on the forklift?
A. 
B. 
C. 
D. 
E. 
About 1N
About 10 N
About 100 N
About 1000 N
About 10000 N
Hint: A rock that size might weigh, say, a ton, which is about
2000 pounds. Each pound is about 5 N of force.
Air Pressure
•  Pressure is not force… it’s force divided by
area.
•  Pressure = Force/Area
Some force SPREAD OUT over bigger area =>
less pressure
Lots of pressure from the air in this room!
Call it “1 atm” of pressure
Atmospheric pressure
At sea level, the pressure of the atmosphere is
1 atmosphere = 1 atm
= 14.7 pounds/square inch (psi)
= 100,000 Newtons/square meter (N/m2)
= 1.013 bar
= 760 torr
Metric Unit is Pascal (Pa) = 1 Newton/square meter
Notes
Is 15 psi a big pressure? Yes!!! We are normally unaware of this because forces on us are balanced. Here is a video example! hAp://www.youtube.com/watch?v=LbmMnRGSdno 22
Normal sound => pressure at your ear goes from
(perhaps)
100,001 N/m2 down to
99,999 N/m2 (and back)
This is ~ ±1 N/m2 overpressure
Pressure
100,001
100,000
99,999
time
(That cycle can happen 100’s of times every second)
“Amplitude of the pressure wave” is the amount of
over (or under) pressure. Picture above, Amplitude is
1 N/m2