Abstract: A method and apparatus for determining the compressive strength of masonry units based on airborne sonic signals generated by stimulating the samples with short duration, mechanical impulses are described. The non-destructive testing apparatus consists of a specimen support unit which mechanically isolates the masonry unit from the environment and from the support, a mechanical stimulator, and an acoustic response detector. The signals recorded are analyzed to extract the pertinent features which are then used to compute the compressive strength of the sample. The pertinent signals are deduced through a calibration of the masonry type being analyzed (material and shape) based on physical analysis of the sample's normal modes of vibration, or through Finite Element Analysis of the vibrations, or through direct empirical calibration. This non-destructive testing method is low-cost, readily implemented and provides a reliable technique for compressive strength estimation.