Travel comfort in airplanes is affected by numerous parameters from which the acoustic environment provides a major contribution, as revealed in measurements of various symptoms of passenger and crew in long-haul flights. Tools known from room acoustics are useful to calculate and estimate an optimal acoustic design of the cabin interior, e.g., background noise levels must match the demand for privacy, or an appropriate STI ensures good public address and communication between the passenger and crew member. Many measures assume a statistical description of the acoustic field. But the strong frequency-dependant absorption leads to different Schroeder frequencies fs, which indicate the transition from well-separated resonances to many overlapping normal modes. In consideration of the extended noise sources as boundary conditions it is useful to determine with fs the regimes of analytical solutions of the wave field within the cabin from a more costly numerical (FEM/BEM) approach.