The appliance (1) with volumetric detector, usable in particular in anti-break-in systems for vehicles, comprises support means (2) associable with the structure of a vehicle, inside the compartment, an electronic control board (3) associated with the support means (2), at least a volumetric sensor (4) connected to the electronic control board (3) and suitable for detecting break-ins inside the compartment, and at least one volumetric sensor (4) associated integral with the electronic control board (3) and at least an acoustic duct (6) having a first mouth (7) associated with the volumetric sensor (4) and at least a second mouth (8) for emitting and/or receiving the sound waves generated by the volumetric sensor (4) and/or reflected by at least an obstacle inside the compartment.