Chapter
10 & 11
Elasticity,
Oscillations,
Waves,
and Sound
Superposition
of Waves
Superposition
Principle – In the region where two or more waves overlap, the
resultant is the algebraic sum of the various contributions at
each point.
Example
19
Two
speakers separated by a distance of 4.30 m emit sound of frequency
221 Hz. The speakers are in phase with one another. A person
listens from a location 2.80 m directly in front of one of the
speakers. Does the person hear constructive or destructive interference?
Solution:
Find
the wavelength of the sound:
Find
distance:
Find
the diference of the distances:
Find
the number of wavelengths in the distance:
This
shows that there is destructive interference.
Example
20
Two
speakers are opposite in phase. They are separated by a distance
of 5.20 m and emit sound with a frequency of 104 Hz. A person
stands 3.00 m in front of the speakers and 1.30 m to one side
of the center line between them. What type of interference occurs
at the person's location?
Solution:
Calculate
the wavelength:
Find
the distance from the speakers:
Find
the difference in the paths:
Divide
by the wavelength:
The
person will experience constructive interference.
Sound
Intensity
The
Intensity (I) of a wave is the average energy delivered per unit
area per unit time. Or the average power divided by the perpendicular
area.
(18)
Units:
Example
21
A
loudspeaker puts our 0.15 W of sound through a square area 2.0
m on each side. What is the intensity of this sound?
Solution:
Example
22
Two
people relaxing on a deck listen to a warbler sing. One person,
only 1.00 m from the bird, hears the sound with an intensity
of . (a) What intensity is heard by the second person, who is
4.25 m from the bird? Assume that no reflected sound is heard
by either person. (b) What is the power output of the bird's
song?
Solution:
(a)
Find the intensity at the second person:
(b)
Find the power:
IntensityLevel
(19)
This
equation gives you the decibel level (dB) of a sound.
Note:
To double the loudness of a source, its intensity must be increased
by a factor of ten. An increase of 10 dB in soundlevel corresponds
to a sound that's twice as loud.
Example
23
A
crying child emits sound with an intensity of .
Find (a) the intensity level in decibels for the child's sounds,
and (b) the intensity level this child and its twin, both crying
with identical intensities.
Solution:
(a)
(b)
On
to Sound Part 4
