Acoustic techniques have always had an important presence in monument conservation. A first evaluation of the formation of cavities and/or inhomogeneities is commonly done by finger rapping and ear listening, to discover acoustical variations in the transmission of sound waves. We have characterized the size of the cavities in several materials, as stones, rocks, mural paintings and stucco, by using 7 kHz sound waves generated by a piezoelectric transducer. The experimental set-up developed for these purposes allows, through the analysis of the waves' delay times, to detect cavities and/or inhomogeneities as small as 4 cm in diameter. The design of the instrument, its capabilities and potential applications are documented.