An apparatus (10,10') for redistributing acoustic energy, the apparatus comprising a surface with: - a first part having a shape of a first surface of revolution, within a first angular interval from a first angular position to a second angular position and around an axis, A, of a first line extending through a point, P, on the axis, a first border being defined by the first part at the first angular position and a second border being defined by the first part at the second angular position, - a second part having a shape of a second surface of revolution, within a second angular interval from a fifth angular position to a sixth angular position and around the axis, of a second line extending through the point P, the first part being defined at longitudinal positions, along the axis, on one side of the point P and the second part being defined at longitudinal positions on an opposite side of the point P, a fifth border being defined by the second part at the fifth angular position and a sixth border being defined by the second part at the sixth angular position, - a back surface positioned, within a third angular interval from a third angular position to a fourth angular position and around the axis, a third border being defined by the back surface at the third angular position, a fourth border being defined by the back surface at the fourth angular position, the back surface being positioned further away from the axis than the first part, - a first side portion extending from the first border to the fifth border and to the third border and - a second side portion extending from second border to the sixth border and to the fourth border, wherein the second part has an outer edge portion shaped as a polygon all parts of which are positioned at or within a circle having a centre at the axis and a radius defined by a distance between at the axis and the second line at the one or both of the third and fourth angular positions, respectively, in a plane perpendicular to the axis and comprising the outer edge (26,26"), wherein a distance between the axis and the third and fourth borders, respectively, in the plane perpendicular to the axis, decreases with increasing distance, along the axis, to the point P. - a first part having a shape of a first surface of revolution, within a first angular interval from a first angular position to a second angular position and around an axis, A, of a first line extending through a point, P, on the axis, a first border being defined by the first part at the first angular position and a second border being defined by the first part at the second angular position, - a second part having a shape of a second surface of revolution, within a second angular interval from a fifth angular position to a sixth angular position and around the axis, of a second line extending through the point P, the first part being defined at longitudinal positions, along the axis, on one side of the point P and the second part being defined at longitudinal positions on an opposite side of the point P, a fifth border being defined by the second part at the fifth angular position and a sixth border being defined by the second part at the sixth angular position, - a back surface positioned, within a third angular interval from a third angular position to a fourth angular position and around the axis, a third border being defined by the back surface at the third angular position, a fourth border being defined by the back surface at the fourth angular position, the back surface being positioned further away from the axis than the first part, - a first side portion extending from the first border to the fifth border and to the third border and - a second side portion extending from second border to the sixth border and to the fourth border,