A sound insulator for a motor vehicle passenger compartment is disclosed. The aim of the invention is to develop a sound insulator which makes it possible to obtain an improved damping effect within a frequency range less than 300 Hz. In that respect, the invention relates to a sound insulator for a motor vehicle passenger compartment, which comprises an air cushion consisting of a dampingly active envelope which is made of an elastic material and disposed between a sound-generating component and the compartment internal space. The invention provides for a sound damping layer (25) which matches the shape of said component (13), acts as a spring material system and is covered on the compartment side by a heavyweight film (26), thereby forming the first shell of the air cushion (29). The invention also provides for the second shell of the air cushion (29) which is tightly connected to the heavyweight film (26), consists of an elastic, preferably pre-shaped, film (27) which forms the cushion air between the first and second shell.