Im having trouble with the last two questions and can you explain to me why you
ID: 1486532 • Letter: I
Question
Im having trouble with the last two questions and can you explain to me why you need to multiply the denominator with 10^-4 in question 2, i was able to find a formula and get an answer but i dont understand where it came from.
Thank you soo much!!
An electronic point source emits sound isotropically at a frequency of 2 × 103Hz and a power of 29 watts. A small microphone has an area of 0.72 cm2 and is located 3.04 meters from the point source.
1)
What is the sound intensity at the microphone ? .2497
2)
What is the power intercepted by the microphone? .00001798
3)
How many minutes will it take for the microphone to receive 0.2 Joules from the sound waves? 185.391
4)
What is the sound intensity level at the microphone from this point source?
5)
What would be the sound intensity level at the microphone if the point source doubled its power output
Explanation / Answer
1)
Sound intensity at mic:
I = P / (4*pi*R^2)
=29/(4*pi*3.04^2)
= 0.2497 W/m2
2)
Power intecepted = I*A
here A = 0.72 cm2 = 0.72*10^-4 m2 <------- converting into SI units
So, P = 0.2479*(0.72*10^-4)
= 1.78*10^-5 W
3)
P = E/t
So, t = E/P
= 0.2/(1.78*10^-5)
= 1.124*10^4 s
= 187.3 min
4)
Sound intensity level = 10*log(I/Io)
Here Io = threshold of hearing = 10^-12 W/m2
So, Sound intensity level : = 10*log(0.2497/(10^-12)) = 114 dB
5)
If power is doubled, the change is 10*log(2) = 3 db
So, sound intensity will be increased by 3 dB <----- as multiplying 2 times equates to increase in 3dB in log decibel scale
So, answer wiil be : 114 +3 = 117 dB