Using ultrasonic backscatter from a specimen containing a cluster of cavities, the size of a single cavity in the cluster has been extracted. A time-domain response obtained by the extraction procedure compares favorably with the analytically calculated response for a single cavity. The procedure removes distortions of the measured response caused by the measuring system, by the specimen, and by other cavities in the cluster. This method should be useful for obtaining similar results from different types of flaw clusters in various types of materials.