What exactly is Time Infinity for Acoustical Parameters?

Standards for calculating room acoustical 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. For instance, to determine the reverberation time, clear requirements are stated with regard to the exponential shape of the decay curve and the associated decay range. The T30 can only be calculated if the INR > 45 dB. For T20 the requirement is INR > 35 dB. To calculate the Strength (G) from an impulse response, ISO 3382 requires a minimum decay range (INR) of 30 dB, and sets the integration time limit to the point were the decay curve has decreased by at least 30 dB. For the Inter Aural Cross Correlation (IACC) an integration time limit in the order of the reverberation time is suggested. For the Support (ST) the integration time limit for the late energy is set to 1 s. For calculating energy related parameters such as Clarity (C80) and Definition (D50) no pratical value is specified for the late energy integration time limit. It is therefor often left to the user of the standard to find a pratical interpretation of the time infinity that appears in the theoretical formulas defining the parameters. Under the most adverse conditions this may lead to variations in the calculated parameter values larger than the JND. To investigate the influence of the INR and the integration time limit on the calculated parameter values, noise of different levels was added to practical impulse responses with a high INR. The parameters were then calculated using different integration time limits. The result of this investigation is a proposal for practical values of the integration time limits based on the INR and the reverberation time.