An electronic point source emits sound isotropically at a frequency of 4 × 10 3
Hz and a power
of 33 watts. A small microphone has an area of 0.78 cm 2
and is located 248 meters from the
point source.
1) What is the sound intensity at the microphone ?
I =
W/m 2
2) What is the power intercepted by the microphone?
P in =
W
3) How many minutes will it take for the microphone to recieve 0.1 Joules from this sound?
?T =
min
4) What is the sound intensity level at the microphone from this point source?
? =
dB
5) What would be the sound intensity level at the microphone if the point source doubled its
power output?
? =
Solution
(1)

Intensity = P / 4pir^2
l = 33 / 4*3.14*248^2
l = 4.27*10^(-5) W/m^2
(2)
Pin = l * A
Pin = 4.27*10^(-5) * 0.78 *10^(-4)
Pin = 3.33*10^(-9) W
(3)
E = P*t
t = 0.1 / 3.33*10^(-9)
t = 3*10^7 s
(4)
l = 4.27*10^(-5) W/m^2