The ornamental design for an on-vehicle acoustic amplifier, as shown and described.
FIG. 1 is a front, bottom perspective view of an on-vehicle acoustic amplifier showing my new design;
FIG. 2 is a front, top perspective view thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a left side elevational view thereof; and,
FIG. 8 is a right side elevational view thereof.
The broken lines in the drawings are for the purpose of illustrating portions of the on-vehicle acoustic amplifier that form no part of the claimed design.