This paper relates to a series of measurements undertaken in the reverberation room (25×20×14 ft) at the Indian Institute of Science, Bangalore. It is a common practice with all laboratories install diffusing panels in reverberation rooms for absorption measurements from decay rates. However, it has been observed that diffusers are not entirely satisfactory under all circumstances and hence the need for a method for the proper identification of the initial slope related to the material properties and subjective experience. With this aim in view, measurements have been performed on a large area of a highly absorbing polyurethane foam sample, and the following important features have been observed: (1) Choice of proper writing speed of the high‐speed level recorder is necessary to delineate the first part of the decay curve. (2) The loudspeaker location and directional characteristics will determine the proper excitation of the room modes giving rise to a correct initial slope. (3) The microphone should as far as possible be located close to the sample. From a study of the above factors it has been possible to establish an initial slope related to the sound‐absorption property of the material.