The present invention relates to an acoustic baffle device for sealing a cavity of a vehicle body at a predetermined cross section of the cavity, comprising a piece of sealing material that is formed in a shape corresponding to but being smaller than the shape of the cross section of the cavity. The acoustic baffle device of the invention is characterized in having a flap assembly having a lower neck defining a passage for a flexible tube or a similar device, said lower neck having a flap secured in closed position by a locking device and being connected preferably to the lower end of the lower neck through means that permit the reversible opening of the above-defined passage. Furthermore, the present invention relates to a flap assembly useful for such an acoustic baffle device, and to a process for sealing a cavity of a vehicle body at a predetermined cross section of the cavity.