.
kinetic energy/unit volume: .
The potential energy in compressed gas is an average of 0, to first
order. To second order it is 
. so the kinetic and potential energy
terms are the same.
Energy is propagated at the group velocity, 
. Energy flux = 
where
Z is the acoustic impedance.
microphones detect pressure variation rather than power variation.
The pressure fluctation is 
where P is the power.
A decibal is 
.
. displacement: 
. displacement of 
= 30dB. 
. 