Abstract: A waveguide (1) comprising a shell (11, 12) having an inlet opening (110, 120) intended to receive acoustic energy and an outlet opening (111, 121) for emitting acoustic energy; and a phase plug (13, 14) located into the shell (11, 12) so as to define therebetween a pathway (W) for the acoustic energy from the inlet opening (110, 120) to the outlet opening (111, 121). The acoustic pathway (W) has a longitudinal extension direction (D) between the openings (110, 120, 111, 121) and the directions of all longitudinal splines (S1, S2, S3, S4, S5, S6) of the pathway (W) are perpendicular to the inlet opening (110, 120) at the inlet opening (110, 120) on one ideal longitudinal surface.