Module 2, Lecture 5: Sound
Sound: Pitch, Intensity and Loudness
Sound: ___________________________
Speed of longitudinal wave in a solid: v 
Speed of longitudinal wave in a gas:


Modulus of
E 
Elasticity:
 L / L0
Thermodynamics
and Ideal Gas Las
Approximation:
v
Tangent to curve at T=20°C
B

E 
OPTIONAL:
Download a sound
(decibel) meter app to
your smartphone for
lecture today
•
v B 
p
Bulk
B
Modulus:
V /V0

RT
M
_______: related to frequency
 20.05 T
v  331  0.60TC 
•
Sound wavelength:
_________: sensation in human beings
•
_________: Power per unit area perpendicular to wave
For T = 20°C, v = 343 m/s = 767 mph
EF 152 Lecture 2-5
Humans can respond to frequencies from about ____ Hz to
_________ Hz
•
v = speed of sound in air, m/s
γ = adiabatic constant
R = universal gas constant
T = absolute temperature (K)
M = molecular mass, 29g/mole
Approximately how long does it take sound
to travel the length of a football field?
•
•
Humans can detect sounds approximately in range
of _______ to ____ W/m2
•
Higher intensity levels can be heard, but are painful
•
Loudness _____________ proportional to intensity
•
To produce a sound twice as loud requires about
____ times the intensity
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EF 152 Lecture 2-5
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EF 152 Lecture 2-5
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Sound Level
•
Sound levels are usually specified on a __________ scale
•
Common unit: bel, after Alexander Graham Bell
More common is
•
__________, or 1/10 bel
 in dB  10 log
I0 is a reference intensity, usually taken as the
threshold of hearing, 1x10-12 W/m2
I
I0
Sound level (dB)
•
•
Threshold of hearing:
Level of pain:
EF 152 Lecture 2-5
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Example: Trumpet Players
Displacement and Pressure
A trumpeter plays at a sound level of 75 db.
Three other trumpet players join in. What is
the new sound intensity? What is the new
sound level?
a.) 75 db
Amplitude of a wave in air:
A
b.) between 75 db and 100 db
c.) between 100 db and 150 db
d.) between 150 db and 300 db
e.) 300 db
f.) greater than 300 db
1
f
Pressure amplitude of wave:
I
2v
f = 1000 Hz
I =1x10-12W/m2
p  2vfA
A=
∆p =
Standing waves (pressure and displacement)
Boundary
Condition
Pressure
Displacement
Open End
Closed End
EF 152 Lecture 2-5
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Wind Instruments (vibrations of air columns)
Closed
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Example: Flute
A flute is an open pipe designed to play middle C (262
Hz) when all the holes are covered. Assume the
temperature is 20°C. Find the distance from mouthpiece
to far end of flute.
Displacement
Open
Displacement
EF 152 Lecture 2-5
1.
Review:
The speed of sound is 767 mph in 20°C air.
What is the speed of sound at 30°C air?
a. Faster
First
harmonic
(fundamental)
  4L
4L
n 
n
Third
Fifth
harmonic harmonic
4L
4L


3
5
nv
f n  nf1 
4L
n = 1, 3, 5, …
EF 152 Lecture 2-5
First
harmonic
(fundamental)
  2L
2L
n 
n
2.
Third
Second
harmonic harmonic
2L
2L


2
3
nv
f n  nf1 
2L
4.
n = 1, 2, 3, …
b. 1 db
EF 152 Lecture 2-5
c. 10 db
d. 100 db
e. Depends on freq
How many decibels are in one bel?
a. 1/100
b. 1/10
c. 1
d. 10
e. 100
How must the intensity of a sound be
changed to cut the loudness of it in half?
a. 1/100
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c. Same
What is the threshold level for human
hearing?
a. 0 db
3.
b. Slower
b. 1/10
c. 1/2
d. 1/√2
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