DL2 Requirements for Speech Intelligibility and Privacy in Rooms

Sound pressure level decay per doubling of distance (DL2) is an attractive room acoustic parameter as it, by design, bears easily understandable spatial information. Since DL2 is dependant on local geometry in addition to all room acoustics parameters, it can be viewed as an all-encapsulating metric. The theoretical study discussed in this presentation shall serve as the framework for implementation of DL2 as a universal room acoustics parameter. Target values for DL2 for unamplified talkers were investigated in rooms of varying purposes and sizes (e.g. classrooms, dining establishments, offices, study spaces). These rooms were assumed to be classifiable under one of two use purposes: (1) speech intelligibility is desired throughout the space, (2) speech intelligibility is desired until a specific distance, and speech privacy is desired thereafter. Background noise levels (BNL) and reverberation times (RT) collected in sample spaces were averaged to find representative quantities. The BNL spectra were then adjusted to match A-weighted levels prescribed by ANSI S12.2 for corresponding spaces. As ISO 9921:2002 lacks speech intelligibility index (SII) values for various intelligibility ratings, ratings were translated from the speech transmission index (STI). In turn, the SII values were assigned at typical talker-listener distances depending on intelligibility or privacy objectives. Analysis will be completed over the next month, focusing on finding the frequency-dependant DL2 values required to achieve target SII values, and assessing feasibility based on experimental data.