Development of an optimised, standard-compliant procedure to calculate sound transmission loss: numerical measurements

Availability of low-frequency characteristics of sound insulating elements is required in order to achieve efficient control of noise sources and reduced level of annoyance in the low-frequency range. Previous work by the author has addressed the problem of designing an enhanced calculation environment for the estimation of sound Transmission Loss (TL). In this work, numerical prediction of TL of sound insulating structures is performed using a procedure, which is in compliance with the ISO recommendations for acoustic measurements. The room-structure-room finite element representation, employed to solve sound propagation and sound-structure interaction problems, as well as the dynamic coupling of and the sound energy propagation through successive air-structure layers are investigated. Several cases of single-layered plain structures of common sound insulating materials such as steel, glass and aluminium with various thickness values are modelled and the calculated TL is compared with published experimental results. It is shown that although the detailed dynamic response of the structures is not accurately predicted due to uncertain parameters, such as the test-specimens dimension and vaguely known boundary conditions, the octave band averaged TL is sufficiently predicted for the majority of the tested materials. Extension of the method to multi-layered structures is attempted and discrepancies at low frequencies are depicted. Finally, the effect of poor mode distribution of the measurement rooms upon the estimated TL is examined in focus. Comparison is performed between TL values calculated with typical and intensely modified transmission rooms. The low-frequency improvement on measurements, when the second ones are used, is demonstrated.