- #1
stevenbhester
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Homework Statement
A rock group is playing in a bar. Sound
emerging from the door spreads uniformly in
all directions. The intensity level of the music
is 116 dB at a distance of 5.77 m from the
door.
At what distance is the music just barely
audible to a person with a normal threshold
of hearing? Disregard absorption.
Answer in units of m.
So,
Given-
I1(dB) (the intensity level 5.77 meters from the door)=116 dB
r1 (distance from door when intensity is 116 dB)= 5.77 m
Io (Intensity at threshold of hearing)= 1e-12
Unknown-
r2 (Radius at threshold of hearing)
P (power of sound source)
I1(w/m^2) (intensity 5.77 meters from door in watts/meters squared)
Homework Equations
dB=10log(I/Io)
P=4*I*π*r^2
r=√(P/4πI)
The Attempt at a Solution
First, I changed the given Intensity into W/m^2 instead of hertz.
dB=10log(I1/Io)
116=10log(I1/1e-12)
11.6=log(I1/1e-12)
10^11.6=I1/1e-12
(1e-12)(10^11.6)=I1
.3981071706=I1
So that's the Intensity at the spot from the door mentioned, so now I calculated the power source.
P=4*I1*π*r1^2
P=4*.3981071706*π*5.77^2
P=166.5564633
So, now that I had the power source, I calculated the radius needed to achieve threshold of hearing
r2=√(P/4πIo)
r2=√(166.5564633/4*π*1e-12)
r2=√(166.5564633/1.256637061e-11)
r2=√1.325414222e13
r2=3640623.878
Doesn't seem right... 3.6 million miles seems overkill.
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