For the purposes of this Directive:
the number and type(s) of audible warning devices fitted to the vehicle;
the warning device adapters on the vehicle;
the position of the warning devices on the vehicle;
the stiffness of the structural parts to which the audible warning device(s) is (are) fitted;
the shape and the materials used in the bodywork forming the front of the vehicle which are likely to affect the level of the sounds emitted by the warning device(s) and to mask those sounds.