Phys 1240: Sound and Music
www.colorado.edu/physics/phys1240
LAST: pitch/frequency
Speed of sound
TODAY: pressure and the nature of
sound
NEXT: waves and oscillations
READ: Hall 2.4
CAPA Thursday… Question 1 got
chopped, fixed it ~1 hr after class
last Thurs,
Grab a corrected copy if you want, or
just read the screen!)
Online participation: (little break,
this week only!)
Periodic motion
⇒ motion that repeats itself…
(vibrations, waves, …)
There’s a time ( “period”) (“P”)
that it takes to get back to where
you started.
There’s a frequency (“f”) that tells
you how often you repeat (each
second)
1 wiggle/sec
= 1 oscillation/sec
= 1 cycle/sec
= 1 Hertz (1 Hz)
CT 1.2.3
What determines pitch?
Length/size?
“Bendability”/rigidity ?
Heaviness/density? (maybe) ?
The above may influence it,
but it’s really all about the
frequency of vibration!
What about amplitude?
This impacts the loudness, not
(generally) the pitch.
Period and frequency
Period is “seconds for one cycle”.
Frequency is “cycles per second”
LARGE (long) period (time per cycle)
=> SMALL (low) frequency.
P = 1/f
Ex: If wiggle at 2 Hz (=2 cycles/sec)
⇒each cycle takes
1/(2 Hz) = 1/(2 cycles/sec)
= 0.5 seconds / cycle
We say “the period is 0.5 seconds”
1
CT 2.1.2
When the amplitude of an oscillating
object is doubled, the period becomes:
a) twice as big
b) 1/2 as big
c) Stays the same
d) 1/4 times as big
e) Not enough information to decide
When the frequency is doubled,
the period becomes…
CT 2.1.4
A vibration has a frequency of 100
cycles/second, what is the period
(or the amount of time for one
cycle)?
a) 100 sec
b) 1 sec
c) 10 msec
d) 100 msec
e) None of these
Is this tone audible?
If so, is it high or low?
CT 2.1.1
How fast (frequently) can you
tap the desk?
You get the strobe timed so an
object appears stationary.
What if we speed up the strobe’s
frequency just a little?
a) The object still staying stationary
b) The object alternating back and
forth between two extremes
c) The object moving forwards
d) The object moving backwards
e) Can’t really guess
2
CT 2.1.3
How fast does a tuning fork
vibrate?
How fast can your vocal cords
vibrate?
What frequencies can your ear
detect?
Production,
propagation
perception
• Instrument (string/tube/etc)
wiggling (at some frequency)
=> air pressure a little high,
then a little low, over and over.
• Air pressure “wiggles” too!
• Frequency of pressure
changes is the SAME as the
frequency of the instrument
• You detect varying pressure at
your ear: frequency => pitch
If I lower the frequency of this tone what
happens to the pitch?
a)
b)
c)
d)
It decreases
It increases
It remains the same
Not enough information
What if I make the same sound louder
(keeping the same pitch),
the frequency…
Brief detour: Air pressure
• Pressure is not force… it’s
force divided by area.
• P = Force/Area
Lots of pressure from the air in
this room!
Called “1 atm” of pressure
= 14.7 pounds/in2 (sea level)
= 100,000 N/m2 (sea level)
(P ~ 12.5 pounds/in2 in Boulder,
about .85 atm)
3
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
time
99,999
(If it’s 100 Hz, that cycle happens
100 times every second)
CT 1.5.1
If there is really a force of 105 N (that’s
100,000 N, about 11 tons!) on each
square meter of a glass window, why is
it that the window does not shatter?
a) That is such a small force it does not
matter
b) Other forces (e.g. from the frame)
counteract and balance this
c) Glass is stronger than you think
d) There is an equal but opposite force
of air pushing against the window from
the other side
e) Gravity counteracts the force
“Amplitude of the pressure wave” is the
amount of over (or under) pressure.
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) Increase
b) Decrease
c) Remain the same
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) Increases
b) Decreases
c) Remains the same
I weigh 500 N, what is the force on
each foot?
a) 500 N each
b) 250 N each
c) It depends on the area of my foot
4