The ornamental design for a loudspeaker, as shown and described.
FIG. 1 is a front perspective view of a loudspeaker showing my new design;
FIG. 2 is a front elevational view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a side elevational view thereof;
FIG. 5 is an opposite side elevational view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The evenly spaced broken lines in the figures illustrate portions of the design that form no part of the claimed design. The evenly spaced broken lines in FIGS. 1 and 4-6 on the loudspeaker are included to show an unclaimed seam on the article that forms no part of the claimed design. The dash-dot broken lines in the figures represent the boundaries of the claimed design and form no part of the claimed design.