The noise from subsonic coannular jets with normal velocity profiles (to represent typical high-bypass-ratio turbofan engines) is studied experimentally and theoretically. The 'source alteration' effects and the "flow-acoustic interaction" effects are isolated as far as possible for all jet conditions. In the acoustic experiments, the effects of fan-to-primary velocity ratio and static temperature ratio are quantified explicity. Furthermore, the coannular jet noise levels are compared directly with the equivalent single jet noise levels for fixed aerodynamic performance of the engine. Previous theoretical work on single-stream jet noise conducted at Lockheed is used to developed an analytical model, based on geometric acoustics, to link the acoustic and flow characteristics of coannular jets. The predictions based on this model provide very good agreement with measured results in most cases.