Measuring Room Impulse Responses: Impact of the Decay Range on Derived Room Acoustic Parameters

Standards for measuring room acoustic parameters, such as ISO 3382, in many cases do not, or only partially, specify the requirements that a measured impulse response should meet to allow calculation of a certain parameter. Among other things, it is often left to the user of the standard to find a practical interpretation of the time infinity that appears in the theoretical formulas defining the parameters. Hence, the parameter value may depend on the decay range and measurement time. Under the most adverse conditions this may lead to variations in the calculated parameter values larger than the Just Noticeable Difference (JND). Using the suggested ISO 3382 'infinite' integration time limits for some parameters and otherwise the crosspoint of decay line and noise level as the 'infinite' integration time limit for the other parameters, the influence of the decay range on the calculated parameter values is investigated. This is done for all ISO 3382-1 parameters: EDT, T ₂₀, T ₃₀, C ₈₀, D ₅₀, T_S , G, ST, IACC, LF, LFC and LG, using the INR (room Impulse response to Noise Ratio) as an estimator for the decay range. The result is a proposal for a minimum decay range value for all ISO 3382-1 parameters, based on the JND and the INR.