Selecting linear diffusers are like sleeting rectangular diffusers and the other type of diffusers. For selecting diffuser, three factors need to be considered, Airflow, air Velocity and NC (noise level). The airflow is based on design airflow and number of diffusers in each room or space. For office space the air velocity should not exceed 700 fpm but the best reasonable velocity is about 500 fpm and the NC shouldn’t exceed 35. In ASHRAE fundamental the NC level for the different application is given. In manufacturers catalogue for the linear diffusers, the airflow is indicated by CFM/ ft. therefore the length of the linear diffuser will be determined by dividing the required airflow by the airflow per foot. Linears are made in one slot or 2 slots and in different width depend on the width of the slot (1”, 1.5”, 2”), for example the 1 slot with 1” would be 4” wide.