Flashcards in Sound beams Deck (53):
What happens to the width of the beam as sound travels
At the starting point what size is the beam width as compared to the transducer diameter?
They are exactly the same
What are the five terms that describe the shape and regions of a sound beam?
focal length or near zone length
What is the focus or focal point?
the location where the beam is the narrowest
for a disc-shaped crystal, the width of the sound beam at the focus is ________ the width of the beam as it leaves the transducer.
What are all of the names for the near zone?
near zone length (NZL)
What is the near zone?
the region from the transducer to the focus
what happens to the beam as it get closer to the focus?
the beam gradually narrows or converges
where is the focus located?
at the end of the near zone
What is the focal length?
the distance from the transducer to the focus
what is the far zone?
the region that starts at the focus and extends deeper
within the far zone the beam diverges (spreads out)
How far does the beam have to be from the transducer to be the same size as the active element again?
two near zone lengths
what happens to the beam when it is farther than two near zone lengths from the active element?
the beam is wider than the active element
What is the focal zone?
a region around the focus where the beam is relatively narrow, resulting in superior image detail
what kind of images are reflected back from the focal zone?
more accurate than from other depths
How is the focal zone split up between near field and far field?
half in the near, half in the far
At these locations:
transducer, focus, 2 near zone lengths, deeper than 2 near zone lengths...
what is the beam diameter?
transducer = beam diameter equals transducer diameter
focus = 1/2 transducer diameter
2 near zone lengths = beam equals transducer diameter
deeper than 2 nzl's = beam is wider than transducer diameter
what is an adjustable focus system called?
with a fixed focus transducer, what two factors combined determine the focal depth?
frequency of sound
How does transducer diameter affect the focal depth?
a larger diameter results in a deeper focus
what is the relationship between transducer diameter and focal depth?
they are directly related
how does frequency affect focal depth?
higher frequency sound results in a deeper focus
what is the relationship between frequency and focal depth?
so what do you need if you want a shallow focus?
smaller diameter PZT
what do you need if you want a deep focus?
larger diameter PZT
higher frequency sound creates a ________ focus
What is the equation to find focal depth?
focal depth (mm) = diameter (mm)2 x frequency (MHz) / 6
focal depth (mm) = diameter (mm)2 / 4x wavelength (mm)
what is the formula for wavelength?
wavelength = speed/frequency
what is aperture?
diameter of the PZT or single element
active element diameter and near zone length are ___________related
frequency and near zone length are ________ related
wavelength and near zone length are _________related
Where is the diameter of the sound beam the smallest
at the end of the near zone (the focus)
What is beam divergence?
the gradual spread of the ultrasound bean in the far field
what two factors combine to determine beam divergence?
frequency of the sound
how does transducer diameter affect beam divergence in the far field?
smaller diameter crystals produce beams that spread out or diverge more in the deep far (Fraunhofer) zone.
crystal diameter and beam divergence are __________ related
What happens to the divergence of a beam when it has larger diameter crystals?
it diverges less in the far field
since larger diameter crystals create sound beams that diverge less in the far field, larger diameter crystals do what to lateral resolution in the far field?
how does frequency alter beam divergence in the far field?
lower frequency sound beams spread out or diverge more in the deep far (fraunhofer) zone.
how are frequency and beam divergence related
low frequency more divergence
if you want less divergence what do you need?
what do you need if you want more divergence
what improves lateral resolution in the far field?
higher frequency sound because it diverges less in the far field.
Transducer frequency and beam divergence are ________ related
active element diameter and beam divergence are _______ related
what are spherical waves?
sound waves produced by very small sources (tiny pieces of PZT) diverge in the shape of a "V"
When is the V shaped wave created?
when the source is about the size of the sound's wavelength
What are the other names for V-waves?
What are huygens' sources
the small sources of sound producing these wavelets
What does Huygens' principal state?
a large active element may be thought of as millions of tiny, distinct sound sources.
each of these tiny particles is a Huygens' source and creates a Huygens' wavelet with a V-shape.
they interfere creating an hour-glass shaped beam
what produces the hourglass shape of the beam?
produced by a large crystal and is the result of interference of the many Huygens' sound wavelets emitted from these numerous sound sources
some are IN PHASE and interfere CONSTRUCTIVELY