Hello - If the sides are straight, I would make two lines at right angles, one as the side and one at the max points of the radius. BlendCrv these, with curvature on the side and tangency on the max point. Adjust the blend inside the BlendCrv command to get the nice graph to taste.
It will never be as crisp and geometric looking as a tangent 180 degree arc - like on the speakers image - with a G2 -to-the-sides curve of course.
-Pascal