Phys 1240: Sound and Music
LAST: outdoor sound, energy
TODAY: intensity and loudness
NEXT: more on decibels
A
READ:Finish Ch. 5 (can skip 1st part
of 5.5, (p. 83) about amplitudes.)
6.1 Don’t try to memorize - just think
about the physics of the important
pieces of your ear.
6.2
CAPA 5 due Thurs AFTER break.
So… no help sessions this week.
If you want to meet, email us!
Reading q. every Tues, as usual!
Measuring…
FORCE => Newtons
WORK or ENERGY => Joules
Recall:
Work = force * distance
(So, 1 Joule = 1 Newton* 1 meter)
C
B
D
The above pictures represent wave
fronts of a sound wave emitted from
some source. Which source is moving
at the speed of sound?
Mass on a spring
• If pull it twice as far =>
need twice the force
Work = force*distance
Pull it twice as far =>
“twice the force” * “twice the dist.”
=> FOUR times the work!
Energy of oscillating objects
grows like (amplitude)2
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CT 5.1x.1
You pull a mass on a spring 1 cm and
let it go. Suppose it has 2 Joules (2 J)
of energy.
If you now pull it 10 cm and let it go,
how much energy does it have?
A) 2 J
B) 10 J
C) 20 J
D) 100 J
E) 200 J
power
If energy CHANGES, we can ask
“how rapidly”?
POWER tells you this:
Power=Energy transfer / time taken
(1 Joule of energy)/ (1 second)
is called 1 Watt of power
1 W = 1 J/sec
5.1.6
A light bulb has a power of 100 J/s (also
called 100 Watts). After being on for 2
seconds how much energy has the bulb
released?
a)
b)
c)
d)
e)
100 J
50 J
200 J
102 J
None of the above
5.1.6b
If the company had been run by
physics knurds, what should they
have called the Power Bar?
A) The Force Bar
B) The Pressure Bar
C) The Energy Bar
D) The Chemical Bar
E) The Power Bar is fine
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5.2.2
BANG! The sound wave from a firework
carries 1 J of energy (total!). If
the sound burst lasts .2 seconds, what is
the power of this sound wave?
a)
b)
c)
d)
e)
1W
.2 W
5W
2W
??
A lawnmower runs for 10 minutes.
Over that time, a total of 1 J of sound energy is
released. How does the power of the
lawnmower sound compare to that of the
firework sound?
A) Greater than
B) Less than
C) Equal to
D) ??
5.1.6b2
On Monday I walk slowly up the
Gamow tower. This takes a certain
amount of energy.
On Tuesday, I run up as fast as I can.
Compare the ENERGY and POWER
on the two days:
A) Energy expended is same, power
is more on Tuesday
B) Power is same, more energy
expended on Tuesday
C) Energy and power both the same
on both days
D) More energy and more power
expended on Tuesday
E) ??
5.1.8
Energy (J)
The following is a graph of the energy in
a system vs. time.
As time goes on, what can you say about
the power going into the system?
intensity
Energy often more interesting or
important than pressure.
Sound waves carry energy:
energy flows through an AREA
(and it flows over TIME.)
time (s)
a) Power is increasing with time
b) Power is decreasing with time
c) Power is steady
d) ???
3
intensity
Intensity is Energy / (time * area)
I
= power / area
Measured in Joules/(sec m2)
(Which is also W/m2)
It’s about the “flow of energy”, or “flux”
If 1000 Joules of solar energy flows
through a 1 m2 window every
second, we say
“The intensity of light is 1000 W/m2
Related to (not same as) “loudness”
Depends on (but not linearly)
“pressure amplitude”
5.1.9
You and I are standing next to each other,
listening to the exact same sound.
Our ears are identical.
You listen for TWICE as long as I do
How does the intensity received compare?
A)You receive twice what I do
B)You receive half what I do
C)We receive the same
How does the power compare?
How does the total energy received compare?
5.1.10
You and I are standing next to each other,
listening to the exact same sound.
Your ears are TWICE the area of mine!
How does the intensity received compare?
A) You receive twice what I do
B) You receive half what I do
C) We receive the same
Remember, for any oscillation,
energy grows like (amplitude)2.
So
Intensity grows like (amplitude)2
too!
How does the power compare?
How does the total energy received compare?
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