Predicting speech metrics in a simulated classroom with varied sound absorption

By systematically varying the amount of sound absorption, and the location of the sound-absorbing material in a simulated classroom, it was possible to assess the accuracy of the prediction of speech metrics in quite simple acoustical environments. Predictions of speech level, early-to-late sound ratios ${\mathrm{(C}}_{50})$ and speech transmission index (STI) values were obtained analytically and with two hybrid ray-based computer programs, RAYNOISE 3.0 and ODEON 4.1. The RAYNOISE predictions were accomplished with a purely specular reflection model and also with a calibrated diffuse reflection model. ODEON uses a parameter called transition order, TO, to change the reflection procedure from purely specular to diffuse for reflections that have orders higher than TO. A parametric study was conducted to determine the best transition order for the ODEON prediction of speech metrics. It was found that the analytical predictions of speech level and $C_{50}$ were on average accurate to about 1 just-noticeable difference (jnd), whereas the analytical predictions of STI were on average within 2 jnd’s. ODEON predictions of speech level, $C_{50}$ and STI were on average within 2 jnd’s. RAYNOISE predictions of $C_{50}$ and STI with the specular model were on average within 2 jnd’s. However, the RAYNOISE predictions of speech level, with both types of reflection models, and the RAYNOISE predictions of $C_{50}$ and STI with the diffuse model had average errors greater than 2 jnd’s. The effects of the sound-absorption treatments on the measured speech metric values are also discussed.