The depth of short cracks (70-200 mum surface length) has been measure
d within an acoustic microscope by utilizing the nondestructive time-o
f-flight diffraction technique (TOFD). The depth measurements were fir
st carried out in the transparent polymer polystyrene, thus allowing a
comparison between the acoustical values and direct optical measureme
nts: the agreement in the results was better than 95%. The depth of a
70 mum long crack in an aluminium alloy was then measured, demonstrati
ng the application of the technique to metals.