The definition for the bulk modules is as follows:
The resulting bulk modulus of the oil - air mix is:
where, 
is the air volume
fraction of the mix. The air and the oil states are calculated separately.
In isothermal case, assume that the bulk module of the air formulated as:
In isentropic case, assume that the bulk module of air formulated as:
where, 
is the ratio of
specific heat.
Assume that the bulk modulus of oil formulated as:
where,
•
is the pressure coefficient
•
is the bulk modulus at the
reference pressure
The air volume can be calculated as follows:
where, is the ratio of
specific heat.
The oil volume can be calculated as follows:
If 
,
If 
,
can be calculated as
follows:
