This dissertation describes the development of an evaluation tool that can be used by an acoustican during the design stage of enclosures used for unassisted speech. Enclosures include lecture theatres, lecture halls and speech auditoriums. The tool is designed to enable Acousticians to be able to manipulate various acoustical parameters such as the geometry and the materials or construction selection to gauge the impact on speech performance. The tool can also be used to evaluate the performance of speech privacy within spaces using the Speech Transmission Index. Computer simulation tools have a number of advantages over existing methods such as physical scale models for this type of evaluation. Typical advantages are in the elimination of the difficult selection of materials with appropriate scale model acoustic performance, resolution of air absorption at scale model frequencies, reduced cost in development of the model, no storage space problems, ease of modifying and duplicating the model. Scale models also present difficulties in measuring some of the indices such as Speech Transmission Index. Whilst equipment can be purchased for the measurement of STI, scale model equivalents and the impact of the change in frequencies and modulations have not been researched or published. / Currently, there are only two methods of evaluating the Speech Transmission of an enclosure: Build a full size enclosure and test; or simulate mathematically to derive the performance. At the time this thesis was commenced there were no commercial simulation programs available that could derive Speech Transmission Index information. The evaluation tool has been implemented as a computer program, based on IBM PC type computers running Microsoft WINDOWS 3.1 or later. The implementation uses the image method for the 'ray trace' algorithm. This basic image method utilises the enhancements made by a number of authors. In particular the Transformation Matrix method and homogenous coordinates have been used to improve the speed of the algorithm. Pre-computation of mutually invisible planes allows trimming the number of possible combination of rays that need to be computed. Results of physical measurement from two case studies have been compared to results of the simulation. Good correlation between the simulations and the case studies were achieved for the Speech Transmission Index and RASTI values. The accuracy of the simulation,in terms of decay based indices, is limited by the lack of sufficient tail to the calculated number of rays. Further research and implementation of hybrid techniques utilising both the image method and more traditional ray-tracing algorithms to improve the quality of the calculated decay data are required. Investigation of techniques used in photo-realism 'ray-tracing' may result in far more realistic data which is the basic input to the Speech Transmission Index calculations.