We present experimental and numerical results on the propagation of Lamb waves in duraluminum elastic waveguides whose section is varying linearly from 5 mm to 2 mm. The Lamb mode is generated in the area of 5 mm thickness and is propagating down slope. If the incident Lamb wave propagating down slope reaches its thickness cut-off in the varying section area, it is totally reflected. However, a small part of its energy is transmitted by tunnel effect to the area of 2 mm thickness, and is converted into other Lamb waves. The tunnel length and the energy distribution on the transmitted waves are given as function of frequency. Experimental and numerical results are in good agreements.