Clicker question
The picture below shows a traveling
object that is emitting a sound. There is a
person at point A and a person at point B.
A
Who hears the higher pitch?
A) Person A
B) Person B
C) They hear the same pitch
B
Phys 1240: Sound and Music
www.colorado.edu/physics/phys1240
LAST TIME: Outdoor sound, Doppler shift.
TODAY: Sound intensity and loudness: energy,
power, and intensity
NEXT TIME: Decibels.
READ: Ch 5.4
• Homework 6 and Reading Question 7 due
Thursday.
Clicker question
A
B
C
D
The above pictures represent wave fronts of a sound wave emitted from some
source. Which source is moving at the speed of sound?
Describing sound strength
pressure
wavelength:
(above normal)
Amplitude:
(below normal)
position,x
time
Amplitude tells you “overpressure” =>
extra force/area
Can be very useful…
But sometimes we are more interested in something
else…
Clicker question
Over Pressure (N/m2)
Over Pressure (N/m2)
Which sound is louder?
Time (s)
a)
b)
c)
d)
The one on the left
The one on the right
They are the same strength
You can’t tell
Time (s)
Intensity, Energy, and Power
• Sound intensity I describes energy flow
per unit area due to the wave.
– Intensity is used to describe how loud sound
is
– 100 decibel sound -> corresponds to a certain
value of sound intensity
• What does intensity mean?
– Intensity = Energy/(time area) = Power/area
• Need to develop physics terms: energy
power, and intensity
Measuring Force and Energy
FORCE => Newtons
WORK or ENERGY => Joules
Recall:
Work = force times distance
So: 1 Joule = 1 Newton * 1 meter
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” and “twice the distance”
=> FOUR times the work!
Energy of oscillating objects grows like
(amplitude)2
Clicker question
You pull a mass on a spring 1 cm and let it go.
If you now pull it 10 cm and let it go, how much
more energy does it have (compared to when you
pulled in 1 cm)?
A) 5 times more energy.
B) 10 times more energy.
C) 50 times more energy.
D) 100 times more energy.
E) 500 times more energy.
Clicker question
A)
B)
C)
D)
I.
Time (s)
Over Pressure (N/m2)
Over Pressure (N/m2)
Wave I has three times the amplitude.
What can you say about the energy (each second)
carried by wave I compared to wave II?
Energy(I)=Energy(II)
Energy(I)=2*Energy(II)
Energy(I)=4*Energy(II)
None of the above.
II.
Time (s)
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
Clicker question
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
Clicker question
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
Intensity
Energy often more interesting or important than
pressure.
Sound waves carry energy:
Energy flows through an AREA (and it flows
over TIME.)
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 “pressure amplitude”(but not linearly)
Intensity
Intensity is Energy / (time * area)
I
= power / area
Intensity is how we describe the “strength” of
sound waves.
It's a property of the wave (like its wavelength, or
frequency, or amplitude)
It's associated with “loudness”
(sort of like frequency <=> pitch)
Clicker question
You and I are standing next to each other, listening to the
exact same sound for the same amount of time.
Your ears are TWICE the area of mine!
How does the total energy received compare?
A) You receive twice what I do
B) You receive half what I do
C) We receive the same