Sound We Don’t Hear Pt.1 Flashcards
Frequency
number of cycles in a wave in 1 sec
Rarefactions
Low pressure and low density
Compressions
High pressure and high density
Longitudinal wave
compressional wave with back and forth particle motion that is parallel to the direction of wave travel
Acoustic variables
Pressure
Density
Particle motion
Frequency units
Hz
kHz
MHz
Infrasound
Less than 20 Hz
Too low for human hearing
Ultrasound
20,000Hz or higher
Human hearing
20-20,000Hz
Period (T)
Time it takes for one cycle to occur
Duty factor
percentage of time that pulsed ultrasound is on
The relationship between period and frequency
Period decreases and frequency increases
Wavelength (cycle length)
the length of space one cycle takes up from front to back
Propagation speed (c)
The speed a wave moves through a medium
What does wavelength depend on?
It depends on frequency and propagation speed.
Detailed resolution improves with…
Increasing frequency because the wavelength decreases
Propagation speed is determined by…
Stiffness of the medium
Stiffness
Resistance of a material to compression
Propagation speeds are highest in..
Solids
Propagation speed is lowest in..
Gases
Harmonics
Frequencies that are even and odd multiples of the fundamental frequency.
Continuous wave
Nonlinear propagation
Nonlinear propagation
Higher pressure portions of the wave travel faster than lower pressure portions
Average longitudinal propagation speed in soft tissues
1540 m/s
Fundamental frequency
Original frequency
Continuous wave
Cycles repeat indefinitely
Pulsed ultrasound
Pulses separated by gaps in time
Pulses
a few cycles that travel together, separated by gaps in time
2 components of pulsed US
Transmit time (send,on)
Receive time (listening, off)
Pulse Repetition Frequency (PRF)
number of pulses that occur in 1 sec
Pulse Repetition Period (PRP)
the time from the beginning of one pulse to the beginning of the next
Includes pulse duration and listening time
The PRP ____ while PRF increases. Why?
decreases. When more pulses occur in a second, the time between them decreases.
Pulse duration
the time it takes for one pulse to occur 
How does the number of cycles and frequency affect pulse duration?
Pulse duration decreases when number of cycles in a pulse is decreased or if the frequency is increased (reducing T)
Which pulse durations improve image quality?
Shorter
What affects the duty factor?
Longer pulses increase DF because the sound is on more of the time.
What increases DF? Why?
Higher PRFs because there is less listening time between pulses
What happens when DF increases?
Pulse duration increases
What happens when PRF increases?
PRP decreases and DF increases
What happens when wavelength decreases?
Frequency increases
What happens when frequency is increased?
Period is decreased, reducing pulse duration
Spatial pulse length
the length of a pulse from front to back
What increases spatial pulse length?
wavelength and the number of cycles in a pulse
With SPL what improves resolution?
Shorter pulse lengths
Bandwith
a range of frequencies
Which bandwidth has a large range of frequencies?
Broad
Which pulses have a broad bandwidth?
Short pulses
Fractional bandwidth
bandwidth divided by operational frequency
Wavelength is the length of ____ over which one cycle occurs.
space
The ___ and ___ of a medium determine propagation speed.
density, stiffness
Propagation speed increases if ____ is increased.
stiffness
Classifications of matter in order of increasing sound propagation speed
gas, liquid, solid
Wavelength in soft tissues ___ while frequency increases.
decreases
Propagation speed in bone is ____ that in soft tissues.
higher than
Sound travels faster in ____.
solids
Solids have higher propagation speeds than liquids because they have greater _____.
stiffness
Sound travels most slowly in _____.
gases
Sound is a ______ _______ wave.
mechanical longitudinal
