Ultrasonic attenuation spectra between 20 kHz and 3 GHz of the nitrobenzene-isooctane mixture of critical composition have been analyzed to show that they contain noncritical relaxation terms in addition to the critical term. The parameter values of the noncritical contributions obtained thereby are used in a reevaluation of smallband attenuation data from the literature. These data, measured at a large number of temperatures near the critical, are most suitable for the determination of the scaling function in the critical dynamics. The procedure allows to verify the empirical scaling function of the Bhattacharjee-Ferrell dynamic scaling theory without an adjustable parameter. (C) 2008 Elsevier B.V. All rights reserved.