The ornamental design for a loudspeaker, as shown and described.
FIG. 1 is an upper perspective view of the loudspeaker design;
FIG. 2 is a top view of the loudspeaker design;
FIG. 3 is a bottom view of the loudspeaker design;
FIG. 4 is a right side view of the loudspeaker design;
FIG. 5 is a left side view of the loudspeaker design;
FIG. 6 is another left side view of the loudspeaker design;
FIG. 7 is another right side view of the loudspeaker design; and,
FIG. 8 is a lower perspective view of the loudspeaker design.
The broken lines shown represent unclaimed subject matter and form no part of the claimed design. The shade lines and/or shaded surfaces shown represent surface shading and not surface ornamentation, color, or material type.