Acoustic design of spaces for speech or music is needed for new buildings. Architects and acousticians collaborate for a common cause to create an adequate space where sound is of primary importance in the design procedure. In that respect, acoustics is considered to be part of the design process since it creates something from the beginning and it can be considered as positive thinking. On the other hand, changing the utilization of an existing space to a space for speech or music, acoustic design is focusing in reducing or eliminating the acoustic faults rather creating an optimum space. This is a fact that is totally dependant on the shape of the space, it's geometry, it's volume and a number of existing construction solutions which cause difficulties for adaptations of new requirements. The transformation of a space, in order to fulfill acoustic requirements, must follow certain criteria related to the basic acoustic design of rooms. These are: a) statistic acoustics by means of optimum reverberation time, calculation of the surfaces of sound absorptive materials and b) geometric acoustics by means of reflecting surfaces, diffusing elements and absorptive surfaces. The final shape of the room depends on the proportions of the dimensions, the use of the room (speech or music), the volume of the room, the number of the audience and the type of seats. The acoustic qualities that take part in the design are the directivity of human voice, the directivity of musical instruments, the angle of direct sound to the audience, the enforcement of sound through reflecting surfaces, the introduction of diffusing elements near the sound source, the positioning of porous absorptive materials as far as possible from the sound source, the positioning of low frequency resonators near the sound source and the elimination of acoustic faults such as echo, long delayed reflections, flatter echoes, focusing, acoustic shadows e.t.c. This paper shows the stages that a designer follow in order to reach that goal from a simple rectangular space to an appropriate shape for acoustic performance. It will be shown step by step through sketches, in plan and in section, how the final shape is a transformation of a rectangular simple shape. An example of an existing rectangular space with the transformation stages will be also shown without changing the overall shape of the outside shell. The following topics will be discussed: a) sound directivity and optimum shape of plans, elimination of parallel side walls to avoid flatter echoes, b) direct sound angle and optimum shape of sections, c) use of a small part of side walls and a small part of the ceiling as reflecting surfaces, d) elimination of back corners to avoid echoes, e) elimination of near corners to avoid delayed reflections and f) introducing reflectors and diffusers.