[3] Birch's law applies to rocks that are under pressures of a few tens of gigapascals, enough for most cracks to close.
Birch's law is used in determining chemical similarities in the mantle as well as the discontinuities of the transition zones.
In cases where Birch's law was applied beyond the transition zone, parts of the formula need to be revised.
In the experiment, a pulse of voltage is applied to a circular plate of polarized barium titanate ceramic (the transducer) which is attached to the near end of the material sample.
[clarification needed] The resulting relationship between the density of the material and the discovered velocity is known as Birch's law.
It represents how the change in density, as given in the second column, is related to the velocity of the P wave moving in the material.