Acoustic Properties of Fluid‐Filled Chambers at Infrasonic Frequencies in the Absence of Convection

The general equation of heat conduction inside a fluid‐filled chamber takes two forms, which correspond to zero and infinite acoustic impedance of the driving source, respectively. By means of a simple transformation, it is possible to convert this equation to the standard diffusion equation with simple initial and boundary conditions. Special solutions to this equation are used to derive transfer functions E, which relate the impressed temperature to the resultant average temperature. The equation of state of the fluid is then used to determine the relationships between pressure and volume changes in terms of E. These relationships take the form of two different acoustic impedances Zp and Zv which correspond to zero and infinite source impedance, respectively. In general, the chamber impedance is a function of Zp, Zv, and the external impedances. The above results can be used to extend the validity of acoustic circuit diagrams to zero frequency. The infrasonic acoustic impedance is an R C network, and numerical values of the components can be calculated in a simple manner.