Computer model studies of RASTI values in classrooms

Computer models of classrooms were used to study the effects of room shape, furniture arrangement and interior finish materials at various background noise levels. The background noise levels used in the computer models were obtained in field measurements of actual classrooms and varied from approximately Noise Criterion (NC) 25 to NC 65. A sound source was configured to approximate the level, frequency and directionality of the normal speaking voice of a teacher in the room. RASTI, reverberation time, loudness, and early energy ratios were estimated for multiple seating locations in each room for each background noise level. RASTI was related to background noise level (NC rating), distance and room finish materials with a second order equation with a model r² of 0.96. The background noise level had the primary effect on RASTI values in the rooms. Once a background noise level of NC 35 or less was achieved, the amount and distribution of absorbent materials in the room became important factors. Reflective materials in parts of the ceiling of the rooms contributed to increased loudness and higher RASTI values at most seating locations.