Huygen's Principle & Wave Interference Flashcards
What is a sine wave?
A symmetrical wave that describes the movement of something with time
How does amplitude affect a sign wave?
Higher peaks = higher amplitude
How does wavelength affect a sine wave?
Larger wave length = more spaced apart waves
If two sine waves appear to be mirror images, what happens?
The waves are completely out of phase with each other so they will cancel each other out.
What are Huygen sources?
Point sources that emit a wave (the crystals).
What are Huygen wavelets?
The waves emitted from each source (each crystal).
What does the rate of divergence depend on?
The size of the source
What is Divergence?
Divergence is the spreading of the beam as it travels from the source.
How does the source effect divergence?
The larger the source of sound, the less divergence there is
What is the cause of weak echos failing to make it back?
Divergence.
The source of sound is small so there is a large amount of diveregence
What is diffraction?
Diffraction occurs when a beam with planar wave fronts passes through a small aperture and spreads out.
What is an aperture?
An opening
What is a planar wave front?
A wave that is harmonic and homogenous (in phase)
What is Huygen’s principal?
Each point source creates it’s own sound wave (beam) and these many wavelets from multiple sources join together to form a new wave front.
How does the concept of a “new wave front” help us in UT?
By controlling the shape of the beam we can control certain aspects like sensitivity and resolution.
How does Huygen’s principle effect UT?
Probe design.
Each crystal on the interface of the transducer is it’s own separate source and all of the sound sources together create the larger beam, the shape of this beam impacts the resolution and sensitivity of our image.
What is interference?
When two or more waves interact and their motions add together.
What are the types of interference?
1) Constructive
2) Destructive
What is constructive interference?
Constructive interference occurs when two waves are “in phase” and their combination results in an increase in intensity and amplitude.
What is destructive interference?
Destructive interference occurs when two waves are “out of phase” and their combination results in a decrease in amplitude and intensity or they may completely cancel eachother out.
When is a beat frequency produced?
When two waves that are neither completely in phase nor out of phase interact a beat frequency is produced.
How are doppler signals detected in ultrasound?
Through beat frequencies.
How does interference effect the beam?
It effects the uniformity of the beams intensity and amplitude throughout the ultrasonic field.
How do changes in intensity effect the beam?
Intensity effects the sensitivity of the beam
What is responsible for the natural focus of the beam?
Interference.
It gives the beam a shape.
Why is beam shape important?
Beam shape effects image resolution and natural focus.
Why is the ‘focus’ placed at or under a structure?
The beam is narrowest at that point, above the sound diverges but not as much as it does below the focus.
What are standing waves?
Standing waves occur when two waves propagate in the same medium but in different directions.
What happens when standing waves add together?
At some points the amplitude will equal zero and at other points the amplitude will be doubled.
What do areas of no amplitude equal and what do they look like on UT?
They equal zero
They are black
How does an increase in amplitude effect the image?
Brighter
What is the ‘node’ in a standing wave?
A point of minimum fluctuation, low pressure.
What is an ‘anti-node’ in a standing wave?
A point of maximum fluctuation, high pressure.
How do standing waves effect the image?
Create areas of enhanced or decreased amplitude (bright and dark spots on the screen).
