A method of evaluating surface diffusivity of rooms — I: computer modeling and field measurement

The present study aims to express the possibility of a new criterion to measure the diffusivity of surfaces in rooms through computer modeling and field measurements. The new criterion (Δx) means the phase difference of the values of the room acoustical parameters between two microphones of which, the first microphone (M1) is pointed toward the forward part of a room while the other microphone (M2) is facing the rear wall of the room. To make this microphone layout possible a specially designed device, microphone holder panel, was employed to measure Δx. Computer simulations, as the pilot survey, were undertaken to analyze the change of Δx values according to the different surface diffusivity using two imaged three-dimensional (3D) halls. As the next step, field experiments were carried out to measure sound level (SPL), early decay time (EDT) and clarity index (C80) in the two existing halls which have different surface diffusivity conditions. The results show that difference of three acoustic parameters (SPL, EDT, C50) captured at two microphones, decreases according to the increase of the interior surface diffusivity. Especially, it was found that in the hall of high diffusivity; ΔSPL, ΔC80, ΔEDT decrease in the range of high frequencies (above 500 Hz) but, the same Δx values increase in the range of low frequencies (below 250 Hz). This means that the high frequency sound may easily reflect, diffuse in the hall of high diffusivity but, low frequency sound may diffract on the surface. It was demonstrated that the difference (Δx values) of acoustic parameters are low in the halls of high surface diffusivity. Thus, Δx values could be used as a criterion to evaluate the surface diffusivity of rooms.