Recording of ultrasonic signals from reflections at boundaries of different materials and analysis regarding their phase position
If a plane ultrasonic wave from a medium with the characteristic acoustic impedance Z1 strikes a plane boundary to a second medium with the impedance Z2, it is partially or completely reflected on this. The reflected part of the sound energy depends on the ratio of the impedances and is described by the reflection factor. The reflected wave generally also has another phase. In the case of perpendicular sound incidence, the phase change only takes two values: 0° for Z1 < Z2 and 180° for Z1 > Z2. Based on such a phase shift, the impedance ratio of two adjacent materials can be qualitatively described. Particularly interesting effects occur when sound passes through thin layers, the thicknesses of which are a quarter or a half of the sound wavelength. λ/4 layers are used e.g. as matching layers in order to minimise reflections and to transfer the largest possible share of sound energy from one medium into the other medium.
The first graphic shows echo signals from an acrylic-brass and brass-acrylic boundary with perpendicular sound incidence. The characteristic acoustic impedance Z = c ρ, determined from the sound velocity c and the material density ρ, was 36.8 Mrayl for brass and 3.25 Mrayl for acrylic. The second graphic shows measurements in transmission at an acrylic cylinder. Each time, a thin aluminium plate (cL ≈ 6309 m/s) with a thickness of λ/4 (approx. 1.5 mm) and/or λ/2 (approx. 3.1 mm) was arranged between the transmitting probe and the cylinder.