The wave reflection coefficients of damage such as cracks, notches and slots in otherwise uniform beams depend on frequency and on the size of the damage. Experimental results are presented for the wave power reflection coefficients of transverse slots of various depths sawn into a number of beam specimens. These results are compared with a conventional spring model to estimate the depth of the slot. The method appears to work well for larger slot depths (greater than about 30% of the thickness of the beam) and at higher frequencies, allowing their existence to be inferred and their size to be estimated. This is due to the fact that the reflection coefficients are larger in these regimes. For smaller slots or at low frequencies, noise and experimental errors, such as miscalibration errors and ill-conditioning, become more significant.