Ultrasound Physics

Method: When aperture is concave, the uniform excitation on the aperture will generate a concave wave front, and it will converge to a point when propagating. As shown in the following figure: The concave wave front that is necessary to achieve the focused beam can be obtained by mechanically shape the aperture into concave, or the using an array transducer and control the transmit delay of each element. Theoretically, the round concave surface gives the best focus effect. The focal effect from of transducer can be improved with more element, smaller element size, and high resolution delay control.
F number: is the ratio of focal depth over aperture equivalent diameter, it is the diameter for round aperture, and the total aperture element length for array transducer. Focus effect is greatly related with frequency, focal depth and aperture size. When focal depth and aperture size are combined into f number, and so the focus effect will be determined by f number and frequency.
Focal zone characteristics

Beam width: The beam width is narrowest at focal point, and it is governed by the f number and center frequency:
Focal distance (length): For a focused beam, it is narrowest at focal depth, and spread on both sides away from the focal depth.
Maximal Intensity: Beam also get maximal intensity at focal point.