An ultrasonic inspection system comprising: • an ultrasonic emitter sensor configured for sending an ultrasonic beam through a medium to be inspected, and • an ultrasonic receiver sensor configured for receiving the ultrasonic beam, ∘ each ultrasonic sensor (1, 2) comprising a piezoelectric crystal and a sole, the sole being able to be arranged on the surface of the medium to be inspected, and the sole having an inclined plane with respect to the surface of the medium on which the piezoelectric crystal is supported, wherein the piezoelectric crystal has a depth and a width, wherein the depth of the piezoelectric crystal is defined as dimension extending along the slope length of the inclined plane of the sole, and wherein the width of the piezoelectric crystal is defined as dimension extending perpendicular to the slope length of the inclined plane of the sole, wherein the inclined plane of the sole on which the piezoelectric crystal is supported has a concave shape for defocusing the ultrasonic beam, the piezoelectric crystal being configured to adapt to said concave shape of the sole, wherein the piezoelectric crystal has: • a first area (6.1) for directing the ultrasonic beam according to an upper critical angle (β), said first area (6.1) having a first width extending parallel with the width of the piezoelectric crystal, • a second area (6.2) for directing the ultrasonic beam according to a central angle (α), said second area (6.2) having a second width extending parallel with the width of the piezoelectric crystal, and • a third area (6.3) for directing the ultrasonic beam according to a lower critical angle (γ), said third area (6.3) having a third width extending parallel with the width of the piezoelectric crystal, wherein the second area (6.2) of the piezoelectric crystal is arranged between the first area (6.1) and the third area (6.3), and wherein the second width of the second area (6.2) is smaller than the first width of the first area (6.1) and the third width of the third area (6.3). • an ultrasonic emitter sensor configured for sending an ultrasonic beam through a medium to be inspected, and • an ultrasonic receiver sensor configured for receiving the ultrasonic beam, ∘ each ultrasonic sensor (1, 2) comprising a piezoelectric crystal and a sole, the sole being able to be arranged on the surface of the medium to be inspected, and the sole having an inclined plane with respect to the surface of the medium on which the piezoelectric crystal is supported, wherein the depth of the piezoelectric crystal is defined as dimension extending along the slope length of the inclined plane of the sole, and wherein the width of the piezoelectric crystal is defined as dimension extending perpendicular to the slope length of the inclined plane of the sole, wherein the inclined plane of the sole on which the piezoelectric crystal is supported has a concave shape for defocusing the ultrasonic beam, the piezoelectric crystal being configured to adapt to said concave shape of the sole, wherein the piezoelectric crystal has: • a first area (6.1) for directing the ultrasonic beam according to an upper critical angle (β), said first area (6.1) having a first width extending parallel with the width of the piezoelectric crystal, • a second area (6.2) for directing the ultrasonic beam according to a central angle (α), said second area (6.2) having a second width extending parallel with the width of the piezoelectric crystal, and • a third area (6.3) for directing the ultrasonic beam according to a lower critical angle (γ), said third area (6.3) having a third width extending parallel with the width of the piezoelectric crystal, • a first area (6.1) for directing the ultrasonic beam according to an upper critical angle (β), said first area (6.1) having a first width extending parallel with the width of the piezoelectric crystal, • a second area (6.2) for directing the ultrasonic beam according to a central angle (α), said second area (6.2) having a second width extending parallel with the width of the piezoelectric crystal, and • a third area (6.3) for directing the ultrasonic beam according to a lower critical angle (γ), said third area (6.3) having a third width extending parallel with the width of the piezoelectric crystal, wherein the second area (6.2) of the piezoelectric crystal is arranged between the first area (6.1) and the third area (6.3), and wherein the second width of the second area (6.2) is smaller than the first width of the first area (6.1) and the third width of the third area (6.3).