New method for the computation of acoustic parameters according to the updated Italian Legislation

Standard acoustic parameters such as reverberation time T20 and clarity C50 are measured or calculated according to ISO 3382-1. The additional parameter STI (Speech Transmission Index) is measured or calculated according to the IEC 601268-16 standard, recently upgraded in 2020. In Italy, it is a legal requirement to verify the values of these acoustic parameters for new public buildings, both in the design phase (calculated values) and in the official assessment of the building performance (measured values). The relevant legislation was drafted in 1997, updated in 2017 with the introduction of the minimum environmental criteria (Decree of the Ministry of the Environment of 11/10/2017)), and recently further updated with the Decree by Ministry of Ecological Transition No. 256 of 23/06/2022. This decree became legally effective in December 2022 and is still very innovative at the time of writing. It requires that the values of three “advanced” acoustic parameters (T20, C50 and STI) be evaluated in all new public buildings, with particularly severe prescriptions in the values to be achieved in special categories of buildings, such as schools. These advanced acoustic parameters are additional to the standard ones, such as the sound reduction index of partitions and facades, the impact sound level and the noise level generated by HVAC systems, which are mandatory in Italy for all types of buildings since the introduction of DPCM 5/12/1997. This paper deals with these three advanced acoustic parameters, which have only recently become mandatory, and describes the update of the Ramsete and Aurora software packages, widely used in Italy to perform predictive calculations and experimental verification of these “room acoustic” parameters. Recent updates to these software tools make them compatible with the newer version of the standards, and hence with the new legislation. In particular, these developments now include a new method for calculating the room impulse response, and hence the STI value, taking into account the different orientations of the speaker, synthesizing the corresponding voice directivity balloon. This new STI(θ) measurement allows the evaluation of the speech intelligibility index for the case when the speaker is not facing the listener. This result can be obtained both experimentally and in computer simulations by evaluating the MIMO Room Impulse Response, and then synthesizing a virtual source with the standardized human voice directivity and proper aiming.