Abstract: A procedure for nondestructively determining a physical characteristic of a test object (10) involves introducing an electromagnetic energy pulse into the test object, and measuring elapsed time before the signature of a time-dependent function of the energy thereby propagated through the test object (10) reaches a value larger than a pre-set threshold value. For example, the volume density of graphite fibers in a graphite-epoxy composite material can be determined by introducing a pulse of radio-frequency electromagnetic energy into the composite material, and by measuring the elapsed time before the voltage signature of the energy propagated through the composite material reaches a value larger than the threshold value.