# Bulk modulus

The bulk modulus (K) of a fluid, gas or solid denotes the resistance to change of the material when a uniform pressure is applied. It is given by the ratio of volume stress to volume strain:

${\displaystyle K={\frac {P}{\Delta V/V}}}$

with P being pressure and V being volume. The delta (Δ) represents change.

## In terms of VP and VS

We can express the modulus in terms of acoustic velocity:[1]

${\displaystyle K=\rho \left(V_{\mathrm {P} }^{2}-{\frac {4}{3}}V_{\mathrm {S} }^{2}\right)}$