physics (sound)

Since dB = 10.log(I/Io)
where I is the intensity of sound, and Io is the threshold of hearing (i.e. the mimumum sound intensity that an average human can hear).
When A talks at 65 dB, we have
65 = 10.log(I(A)/To)
where I(A) is the intensity of sound wave emitted by A during talking
6.5 = log(I(A)/Io)
i.e. I(A) = (Io) x 10^(6.5)
Similarly, when B talks at 70 dB, we have I(B) = (Io) x 10^(7)
When both A and B are talking together, by the Law of Conservation of Energy, sound intensity is conserved.
Thus, total intensity I(t) = (Io) x 10^(6.5) + (Io) x 10^(7)
i.e. I(t) = (Io) x 1.316x10^7
hence, the sound intensity level when both of them are talking
= 10.log[ (Io) x 1.316x10^7/Io] dB = 71.19 dB


2009-03-24 20:36:57 補充：
Notice that sound intensity level (dB) cannot be added directly as it is expressed by a log scale. However, sound intensity, which is the rate of flow of sound energy, can be added.