Given that the energy of the vibration is dissipated to the surroundings.
The energy of a vibration is related to its frequency as,
Where h is the Planck's constant.
From the above equation, we can see that the energy of the vibration is directly proportional to its frequency.
Thus if the energy decreases, the frequency decreases and if energy increases, the frequency increases.
Therefore as the energy of the vibration dissipated, the frequency of the vibration decreases.