A sound's noise level is measured in decibels (dB) by comparing the sound's intensity, , to a benchmark sound that has an intensity of watts/cm. In particular,

A single jet engine that is 100 feet away from the listener has a noise level of 140dB.

a) Find the sound intensity of the jet engine in watts/cm.

b) Suppose two jet engines are 100 feet away from a person, each with the same sound intensity as the jet engine from part (a). What is the combined noise level in decibels? [Hint: the combined sound intensity is the sum of the intensities of the two engines.]

c) Suppose a source has sound intensity . Using the definition of decibels, write an expression for the noise level in decibels if two sources with sound intensity are present.

d) Using properties of logarithms, write the expression from part (c) as the sum of two logarithms.