An investigation of sound pressure level predictions in large lecture halls

Abstract

Measurements of steady‐state sound pressure levels above the audience in large lecture halls show that the classical equation for predicting the sound pressure level is not accurate. The direct field above the seats was studied on a 1:10 scale model of a large lecture hall using the removable audience from the scale model. It was found to be dependent on the incidence angle and direction of sound propagation across the audience. The reverberant field above the seats was studied by subtracting the direct field from the measured field in the model. It was found to be dependent on the magnitude and particularly on the placement of absorption in the room. The decrease of sound pressure level versus distance in the total field depends on the angle (controlled by absorption placement) at which the strong reflections are incident upon the audience area. Sound pressure level decreases at a fairly constant rate with distance from the sound source in both the direct and reverberant field, and the decrease rate strongly depends on the absorption placement. The lowest rate of decay occurs when the side walls are absorptive, and both the ceiling and rear wall are reflective. The consequences are discussed with respect to prediction of speech intelligibility, and modifications are suggested for three current speech intelligibility prediction schemes.