• The speed of sound is the sum of two
nonadiabatic terms
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• Thus, even for a nonzero ∂p/∂r the speed of sound may vanish if the second term on the right-hand
side cancels the first one. This
cancellation will take place if in the course of an adiabatic expansion, the perturbation δφ grows with a in the same
way as δr
. In this case,
it is only a matter of adjusting the initial conditions of δφ with δr to get cs=0.
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