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Acoustic wave Acoustic waves are a type of energy propagation through a medium by means of adiabatic loading and unloading. Important quantities for describing acoustic waves are acoustic pressure, particle velocity, particle displacement and acoustic intensit ...
s emitted by
ultrasonic Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
s transducer crystals exhibit a property known as self-focusing (or ''natural focusing''). Note that this is distinct from the electronically controlled focusing employed in diagnostic
ultrasound Ultrasound is sound waves with frequency, frequencies higher than the upper audible limit of human hearing range, hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hea ...
devices which employ arrays of transducers. The self-focusing effect exists even for a single
crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
. Self-focusing refers to the narrowing of the ultrasonic beam in the near-field. The effect occurs because, if the ultrasound wave generated by the crystal is
coherent Coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference * Coherence (units of measurement), a deri ...
, the edges of the emitting surface generate
wavelet A wavelet is a wave-like oscillation with an amplitude that begins at zero, increases or decreases, and then returns to zero one or more times. Wavelets are termed a "brief oscillation". A taxonomy of wavelets has been established, based on the num ...
s that constructively interfere with the plane wave that is generated. The volume near the transducer which exhibits this phenomenon is called the near field, as opposed to the
far field The near field and far field are regions of the electromagnetic (EM) field around an object, such as a transmitting antenna, or the result of radiation scattering off an object. Non-radiative ''near-field'' behaviors dominate close to the ant ...
where wave intensity decays approximately exponentially. The near-field size is usually approximated by D = \frac where d = crystal diameter and \lambda =
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
of the emitted wave. At the edge of the near-field, where the focusing reaches its maximum, the beam width reaches \frac. However, there exist techniques, such as
apodization In signal processing, apodization (from Greek "removing the foot") is the modification of the shape of a mathematical function. The function may represent an electrical signal, an optical transmission or a mechanical structure. In optics, it is p ...
that may be used to reduce the near field size.


References

Ultrasound Nondestructive testing {{acoustics-stub