The exact theory of the acoustic interferometer will be discussed following the three stages of its development. First will be discussed the original approximation introduced to simplify the solution in the case of moderately dissipative media. Second will be discussed the more rigorous solution necessary in the case of highly dissipative media, including the approximation for low acoustic impedance to simplify the solution in the case of gases at low pressures. Third will be discussed the exact solution necessary in the case of very highly dissipative media now being approached inH2and He. This indicates a departure of the maxima and minima of the real part of the acoustic impedance from the simple hyperbolic functions, necessitating corrections to the absorption and reflection calculations. Also at very high dissipation these maxima are no longer a half‐wave‐length apart, requiring corrections to velocity calculations.