We\'re in class with a SIL decibel meter up front, and everyone is screaming at

Question

We're in class with a SIL decibel meter up front, and everyone is screaming at the top of their lungs, hooting and shouting and pounding on their desks, shooting off firecrackers, its Phys 1240 gone wild...and the meter is reading a reasonably steady sound intensity level of 120 dB. At a pre-arranged signal from Professor Parker, seventy-five (75) percent of the class suddenly gets totally quiet, while the remaining students continue making the same noise. What sound intensity level would the meter now show? (Find the answer in dB, but do not enter units.)

For you to think about: Did you find the answer just a little surprising, or counter-intuitive? Most people who haven't taken a course like this would guess the answer would be about 30 dB. Try to explain how someone might come up with that *wrong* answer, and then explain, qualitatively or informally, why *your* (correct) answer does really make more sense. (Imagine you were trying to explain this to someone who didn't know much math and had never taken this course. You're not trying to convince them of your specific numerical result, just why the dB level did not drop nearly so much as one might first imagine.

Explanation / Answer

Intensity level B = 10 dB log (I/Io)

for n no. of equal loud sound sources

n = (10^dL/10)

n = 10^(120/10)

n = 1,000,000,000,000    this is 100%

after 75% gets quiet, rem 25% = 250, 000, 000, 000

dL = 10 dB log(250000000000)

dL = 113.9 dB (ANSWER)

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