Ch.9 Sound Beams Flashcards
What are the 5 areas of a sound beam?
1) Focus
2) Near zone
3) Focal length or near zone length
4) Far zone
5) focal zone
What is the focus?
is the location where the beam is the narrowest.
For a disc-shaped crystal, the width of the sound beam at the focus is…
1/2 the width of the beam as it leaves the transducer
What is the near zone?
(Near field or Fresnel zone)
is the region from the transducer to the focus
The beam gradually narrows, or converges, within the ___ ____
near zone
Where is the focus or focal point located?
1) at the end of the near zone
2) at the beginning of the far zone
3) the middle of the focal zone
What is the focal length?
(focal depth or near zone length)
is the distance from the transducer to the focus
What is the far zone?
(far field or Fraunhofer zone)
is the region that starts at the focus and extends deeper
-within the far zone, the beam diverges, or spreads out.
When the beam is two near zone lengths from the transducer, the beam is ?
again the same size as the active element
At depths more than two near zone lengths, the beam is ?
wider than the active element
what is the focal zone?
is a region around the focus where the beam is relatively narrow. Half is located in the near field and the other half is in the far field.
Reflections arising from the _____ _____ create images that are more _____ than those from other ____ .
focal zone
accurate
depths
Location Beam diameter
1) at transducer
2) at 2 near zone lengths
3) at the focus
4) deeper than 2 near zone lengths
1) beam diameter = transducer diameter
2) beam diameter = transducer diameter
3) beam diameter is 1/2 transducer diameter
4) beam diameter is wider than transducer diameter
The focal depth
is the distance from the transducer to the narrowest part of the beam (the focus).
Ultrasound systems that adjust the focus location are called what?
phased array
The characteristics of what determine the depth of the focus?
active element
What characteristics of a fixed focus transducer determine the focal depth?
1) Transducer diameter
2) Frequency of the sound
How does the transducer diameter affect the focal depth?
a larger diameter results in a deeper focus
What is the relationship between the transducer diameter and focal depth ?
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 ?
are directly related
What are the two characteristics of shallow focus?
smaller diameter PZT
lower frequency
What are the two characteristics of deep focus?
larger diameter PZT
higher frequency
PHYSICS: Higher frequency sound creates a
deeper focus
How do transducer manufacturers overcome the rule that higher frequencies create deeper focus?
by making very small diameter, high frequency crystals
Focal depth formula
focal depth (mm) = diameter (mm)^2 x frequency (MHz) / 6
or
focal depth (mm) = diameter (mm)^2 / 4 x wavelength (mm)
what characteristics of a transducer determine the spread of the beam in the far field?
1) transducer diameter
2) frequency of sound
How does transducer diameter affect beam divergence in the far field?
crystal diameter and beam divergence are inversely related
(small crystals produce beams that spread out or diverge more in the deep far zone (Fraunhofer) )
Larger diameter crystals create sound beams that diverge ____ in the far field, larger diameter crystals improve ______ _______ in the far field.
less
lateral resolution
How does frequency alter beam divergence in the far field?
frequency and beam divergence are inversely related
High frequency sound beams diverge ____ in the far field, higher frequency sound improves ____ ______ in the far field.
less
lateral resolution
Less Divergence in far field have 2 characteristics?
1) larger diameter
2) higher frequency
More divergence in far field have 2 characteristics?
1) smaller diameter
2) lower frequency
Larger transducer diameters produce a beam with ____ ______
less diverge
Sound waves produced by very small sources (tiny pieces of PZT) diverge in the shape of a ?
V
The V shaped waves are known as ?
spherical waves
diffraction patterns
Huygen’s wavelets
Huygens’ Principle
states that a large active element may be thought of as millions of tiny, distinct sound sources. Each tiny particles is a Huygens’ source and creates a Huygens’ wavelet with a V shape
