22 at first ends 24 a 26 a thereof and to the air intake duct 30 at the opposite second ends 24 b 26 b thereof respectively the overall shape of the cavity 22 is unimportant and the designer thus has a large degree of freedom in shaping the cavity as needed or as dictated by the space constraints of the area where the resonator is required thus in the embodiment of fig2 the cavity is curved into a hook shape such that it extends around the air intake duct 30 this particular cavity shape effectively uses available space while at the same time having a relatively long cavity since resonator 20 has a length l which exceeds the maximum allowable length of conventional side branch resonators a standing wave is formed in the cavity 22 this standing wave has a high pressure zone or anti node 28 that forms adjacent an end 22 a of the cavity one neck 24 is thus positioned adjacent the anti node 28 and acts to eliminate the standing wave the position of the other neck 26 may be almost anywhere along the length of the cavity as long as the neck ends 24 b 26 b join the air intake duct along the same flow path plane if neck ends 24 b 26 b will not be positioned along the same flow path plane they preferably are no greater than the wavelength u03bb divided by 16 along.