Sound insulation characteristics of membranes with additional weights (MAW), developed to improve insulation performance at low frequencies, are investigated by applying a numerical method, and subsequently, the mechanism of the insulation effect of MAW is discussed theoretically through an infinite model. As a result, it is confirmed that the insulation effect occurs independently of the incident angle of a sound wave and the boundedness of MAW, and the effects of properties of MAW on transmission loss (TL) are also clarified. The mechanism is interpreted as an interference phenomenon between two vibration components, and the approximate peak frequency of TL is formulated.