In long spaces, such as road or railway tunnels, underground/railway stations, corridors, concourses and urban streets, classic room acoustic theory is not applicable since the assumption of a diffuse field does not hold with the extreme dimensional condition. This paper presents theories and computer models for long spaces, including formulae based on the image source method, the radiosity method, a method for predicting train noise in underground stations, and an integrated model for practical prediction of speech intelligibility in long enclosures. The theories have been validated by on-site and scale model measurements. Based on parametric studies using the theoretical/computer models, as well as a series of 1:16 physical scale model measurements, effectiveness of strategic architectural acoustic treatments and design guidelines are presented. Particular attention is given to the improvement of the speech intelligibility of multiple loudspeaker PA systems.