Biomedical Engineering Reference
In-Depth Information
a
ka >> 1
(a)
ka ~ 1
(b)
ka << 1
(c)
FIGURE 16.19
(a) Specular scattering from an object much greater than a wavelength. (b) Diffractive scatter-
ing from an object on the order of wavelengths. (c) Diffusive scattering from an object much smaller than a
wavelength.
factor appropriate for the angle of incidence. At the other extreme, when an object is
much smaller than a wavelength, its reflections are diffusive. Here, the object is so small
relative to a wavelength that its features no longer contribute to the reflected wave.
If the object is a sphere, then the reflected pressure is proportional to the frequency
squared, inversely with the radius cubed and a term related to the differences in elastic
constants and densities between the sphere and the surrounding medium.
The intermediate range between these two extremes, diffractive scattering, occurs
when the dimensions of the object are on the order of wavelengths. If the surface of the
object is divided into infinitesimal points and lines are drawn from these points to an
observationpoint,thenthedifferencesinpathlengthsalongtheselinescanbesignificant
in terms of wavelengths. Contributions from these points could cause constructive and
destructive interference effects at the observation point because of phase differences along
the paths. Depending on the shape of the object and its orientation and distance to an
observation point, complicated scattering patterns can result. Even for simply shaped
objects, such as spheres or cylinders, the scattering pattern or directivity of these objects
is highly frequency dependent as the object shape and dimensions change relative to a
wavelength.
Human optical vision depends mainly on specular reflections (objects much greater than
wavelengths of light). An acoustic image is formed from pulse-echoes along acoustic lines
from an observation point to parts of the object. Unlike optics, the acoustic appearance of
an object can change both with the orientation of the observer relative to the object and with
the insonifying frequency.
