The divergence is an ultrasound beam characteristic of the far field. The beam divergence angle q, depends on the transducer frequency and diameter according to the following approximation:
sin q 1.22 ld
where l is the wavelength of the ultrasound in the medium of propagation and d is the diameter of the transducer element.

Attenuation is the reduction of power, for example due to the passage through a medium or electrical component. In ultrasound imaging, attenuation means the decrease in amplitude and intensity as a sound wave travels through a medium. In ultrasound attenuation is often characterized as the half-value layer, or the half-power distance. These terms refer to the distance that ultrasound will travel in a particular tissue before its energy is attenuated to half its original value.

Attenuation originates through:
A thick muscled chest wall will offer a significant obstacle to the transmission of ultrasound. Non-muscle tissue such as fat does not attenuate acoustic energy as much. The half-value layer for bone is still less than muscle, that's why bone is such a barrier to ultrasound.

See also Attenuation Coefficient, and Derated Quantity.

The far field (also called Fraunhofer zone) is the distal part of an ultrasound beam characterized by a diverging shape and continuous loss of ultrasound intensity with distance from the transducer. The angle of divergence increases with lower transducer frequency and with smaller transducer diameter.

See also Sonographic Features.

Linear array transducer elements are rectangular and arranged in a line. Linear array probes are described by the radius of width in mm. A linear array transducer can have up to 512 elements spaced over 75-120 mm. The beam produced by such a narrow element will diverge rapidly after the wave travels only a few millimeters. The smaller the face of the transducer, the more divergent is the beam. This would result in poor lateral resolution due to beam divergence and low sensitivity due to the small element size.
In order to overcome this, adjacent elements are pulsed simultaneously (typically 8 to 16; or more in wide-aperture designs). In a subgroup of x elements, the inner elements pulse delayed with respect to the outer elements. The interference of the x small divergent wavelets produces a focused beam. The delay time determines the depth of focus for the transmitted beam and can be changed during scanning.
Linear arrays are usually cheaper than sector scanners but have greater skin contact and therefore make it difficult to reach organs between ribs such as the heart. One-dimensional linear array transducers may have dynamic, electronic focusing providing a narrow ultrasound beam in the image plane. In the z-plane (elevation plane - perpendicular to the image plane) focusing may be provided by an acoustic lens with a fixed focal zone.
Rectangular or matrix transducers with unequal rows of transducer elements are two-dimensional (2D), but they are termed 1.5D, because the number of rows is much less than the number of columns. These transducers provide dynamic, electronic focusing even in the z-plane.

See also Rectangular Array Transducer.