Round transducers are often referred to as piston source transducer because the sound field resembles a cylindrical mass in front of the transducer. However the energy in the beam does not remain in a cylinder, but instead spread out as it propagates through the materials. The phenomenon is usually referred to as beam spread but is sometimes also referred to as beam divergence or ultrasonic diffraction. It should be noted that there is actually a difference between beam spread and beam divergence. Beam spread is a measure of the whole angel from side to side of the main lobe of the sound beam in the far field. Beam divergence is a measure of the angle from one side of the sound beam to the central axis of the beam in the far field. Therefore, beam spread is the twice the beam divergence.