Taking two-dimensional double-beam structural for example to study on the method of dealing with the sound filed of the sandwich structure, from different physical perspectives, The transverse acoustical modal expansion (TAME) method and acoustical waveguide modal expansion (AWME) method are presented to express the velocity potential of the air gap sound field of the sandwich structure and compare the sound transmission loss of the various parameter combined sandwich structure with the acoustical modal expansion (AME) method and structural modal expansion (SME) method. The calculation results show that the TAME method is equivalent to the SME method and the AWME method is equivalent to the AME method in the aspects of computing the sound transmission loss of the structure. The sound insulation dips are results of the combination of the structural resonances, ‘beam-air-beam’ resonance and standing-wave resonances. The four computing methods can reflect the influence of the structural resonances, ‘beam-air-beam’ resonance and standing-wave resonances with vertical to the structural surface on the structure of transmission loss. Besides, the AME method and AWME method can also reflect the influence of the standing-wave resonance with parallel to the structural surface and other acoustical modal frequencies on the structure of transmission loss, based on this reason, the sound transmission loss curve have much more dips than the former two methods.