resident physics lectures 02: sound properties and parameters
TRANSCRIPT
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Resident Physics LecturesResident Physics Lectures
02:02:Sound Properties and Sound Properties and ParametersParameters
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Sound Wave Definition?Sound Wave Definition?
• Sound is a WaveWave• WaveWave is a propagating (traveling)
variation in a “wave variablewave variable”
• “An elephant is big, gray, and looks like an elephant.”
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Sound Wave VariableSound Wave Variable
• Examples– pressure (force / area)– density (mass / volume)– temperature
• Also called acoustic variableacoustic variable
Sound is a propagating (moving) variation in a “wave variablewave variable”
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Sound Wave VariationSound Wave Variation
• Freeze time• Measure some acoustic variable as
a function of position
Position
AcousticVariableValue
PressureDensityTemperature
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MOREMORE• Make many measurements of an
acoustic variable an instant apart• Results would look the same but
appear to move in space
1
2
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MOREMORE• Track acoustic
variable at one position over time
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Sound WavesSound Waves• Waves transmit energy• Waves do not transmit matter• “Crowd wave” at sports event
– people’s elevation varies with time– variation in elevation moves around stadium
» people do not move around stadium
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Transverse WavesTransverse Waves
• Particle moves perpendicular to wave travel
• Water ripple– surface height varies with time– peak height moves outward
» water does not move outward
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Compression (Longitudinal) Waves
Compression (Longitudinal) Waves
• Particle motion parallel to direction of wave travel
1
2
1
2
Wave Travel
Motion ofIndividual Coil
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Sound Waves are Compression Waves
Sound Waves are Compression Waves
• Regions of alternating low and high pressure move through air
• Particles oscillate back & forth parallel to direction of sound travel
• Particles do not move length of sound wave
Wave Travel Motion of IndividualAir Molecule
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MediumMedium
• Material through which wave moves
• Medium not required for all wave types
– no medium required for electromagnetic waves» radio
» x-rays
» infrared
» ultraviolet
– medium is required for sound» sound does not travel through vacuum
Talk louder! I can’t hear
you.
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Sound WavesSound Waves
• Information may be encoded in wave energy
– radio– TV– ultrasound– audible sound
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Sound FrequencySound Frequency
• light frequency corresponds to color
• sound frequency corresponds to pitch
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Sound FrequencySound Frequency# of complete variations (cycles) of an
acoustic variable per unit time
• Unitscycles per second
1 HzHz = 1 cycle per second
1 kHzkHz = 1000 cycles per second
1 MHzMHz = 1,000,000 cycles per second
• Human hearing range 20 - 20,000 Hz
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Sound FrequencySound Frequency
• Ultrasound definition> 20,000 Hz
– not audible to humans» dog whistles are in this range
• Clinical ultrasound frequency range
1 - 10 MHz
1,000,000 - 10,000,000 Hz
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PeriodPeriod
• time between a given point in one cycle & the same point in the next cycle
– time of single cycle
• Units– time per cycle (sometimes expressed
only as time; cycle implied)
period
Magnitude of acoustic
variable
time
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PeriodPeriod
• as frequency increases, period decreases
• if frequency in Hz, period in seconds/cycle
1Period = ------------------- Frequency
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PeriodPeriod
• if frequency in kHz, period in msec/cycle• if frequency in MHz, period in sec/cycle
1 kHz frequency ==> 1 msec period
1 MHz frequency ==> 1 sec period
Period = 1 / Frequency
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Reciprocal UnitsReciprocal Units
Frequency Units
Period Units
Hz (cycles/sec) seconds/cycle
kHz (thousands of cycles/sec)
msec/cycle
MHz (millions of cycles/sec)
sec/cycle
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Period / FrequencyPeriod / Frequency
If frequency = 2 MHz then sound period is 1/2 = 0.5 sec
If frequency = 10 kHz then sound period is 1/10 = 0.1 msec
If frequency = 50 Hz then sound period is 1/50 = 0.02 sec
If sound period = 0.2 sec then frequency = 1/0.2 = 5 MHz
If sound period = 0.4 msec then frequency = 1/0.4 = 2.5 kHz
If sound period = 0.1 sec then frequency = 1/0.1 = 10 Hz
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Sound Period & Frequency are
determined only by the sound source. They are independent of medium.
Who am I?
Burt Mustin
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Propagation SpeedPropagation Speed
• Speed only a function of medium
• Speed virtually constant with respect to frequency over clincial range
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WavelengthWavelength• distance in space over which single cycle
occurs OR
• distance between a given point in a cycle & corresponding point in next cycle
• imagine freezing time, measuring between corresponding points in space between adjacent cycles
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Wavelength UnitsWavelength Units
• length per cycle– sometimes just length; cycle implied
• usually in millimeters or fractions of a millimeter for clinical ultrasound
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Wavelength EquationWavelength Equation
Speed = Wavelength X Frequency [ c = X (dist./time) (dist./cycle) (cycles/time)
• As frequency increases, wavelength decreases
– because speed is constant
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WavelengthWavelengthSpeed = Wavelength X Frequency
[ c = X (dist./time) (dist./cycle) (cycles/time)
mm/sec mm/cycle MHz
Calculate Wavelength for 5 MHz sound in soft tissue
Wavelength = 1.54 mm/sec / 5 MHz
Wavelength = 1.54 / 5 = 0.31 mm / cycle
5 MHz = 5,000,000 cycles / sec = 5 cycles / sec
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Wavelength is a function of both the
sound source and the medium!
Who am I?
John Fiedler
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Pulsed SoundPulsed Sound• For imaging ultrasound, sound is
– Not continuous
– Pulsed on & off
• OnOn Cycle (speak)– Transducer produces short duration sound
• OffOff Cycle (listen)– Transducer receives echoes
– Very long duration
ON OFF ON OFF
(not to scale)
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Pulse CyclePulse Cycle
• Consists of– short sound transmission
– long silence period or dead time» echoes received during silence
• same transducer used for– transmitting sound– receiving echoes
sound silence sound
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Pulsed Sound ExamplePulsed Sound Example
• ringing telephone– ringing tone switched
on & off
– Phone rings with a particular pitch
» sound frequency
sound silence sound
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Parameters Parameters
• frequency
• period
• wavelength
• propagation speed
• pulse repetition frequency
• pulse repetition period
• pulse duration• duty factor• spatial pulse
length• cycles per pulse
Sound Pulse
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Pulse Repetition FrequencyPulse Repetition Frequency
• # of sound pulses per unit time
• # of times ultrasound beam turned on & off per unit time
– independent of sound frequency
• determined by source
• clinical range (typical values)– 1 - 10 KHz
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Pulse Repetition PeriodPulse Repetition Period
• time from beginning of one pulse until beginning of next
• time between corresponding points of adjacent pulses
Pulse Repetition Period
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Pulse Repetition PeriodPulse Repetition Period
• Pulse repetition period is reciprocal of pulse repetition frequency
– as pulse repetition frequency increases, pulse repetition period decreases
• units– time per pulse cycle (sometimes simplified to just time)
• pulse repetition period & frequency determined by source
PRF = 1 / PRP
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Higher FrequencySame PulseRepetition Frequency
Pulsed SoundPulsed Sound
• Pulse repetition frequency & period independent sound frequency & period
Same FrequencyHigher PulseRepetition Frequency
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Pulse DurationPulse Duration• Length of time for each sound pulse• one pulse cyclepulse cycle =
– one sound pulse and one period of silence
• Pulse duration independent of duration of silence
Pulse Duration
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Pulse DurationPulse Duration
• units– time per pulse (time/pulse)
• equationpulse duration = Period X # cycles per pulse
(time/pulse) (cycles/pulse) (time/cycle)
Pulse Duration Period
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Pulse DurationPulse Duration
Longer Pulse Duration
Shorter Pulse Duration
Same frequency; pulse repetition frequency,period, & pulse repetition period
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Pulse DurationPulse Duration
Pulse duration is a controlled by
the sound source, whatever
that means.
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Duty FactorDuty Factor• Fraction of time sound generated• Determined by source• Units
– none (unitless)
• EquationsDuty Factor = Pulse Duration / Pulse Repetition Period
Duty Factor = Pulse Duration X Pulse Repetition Freq.
Pulse Duration
Pulse Repetition Period
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Spatial Pulse LengthSpatial Pulse Length
• distance in space traveled by ultrasound during one pulse
HEYH.......E.......Y
Spatial Pulse Length
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Spatial Pulse LengthSpatial Pulse Length
So, can you like show me an example?
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Spatial Pulse LengthSpatial Pulse Length
• depends on source & medium
• as wavelength increases, spatial pulse length increases
Spat. Pulse Length = # cycles per pulse X wavelength
(dist. / pulse) (cycles / pulse) (dist. / cycle)
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Spatial Pulse LengthSpatial Pulse Length
• as # cycles per pulse increases, spatial pulse length increases
• as frequency increases, wavelength decreases & spatial pulse length decreases
– speed stays constant
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
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Why is Spatial Pulse Length Important
Why is Spatial Pulse Length Important
Spat. Pulse Length = # cycles per pulse X wavelength
Wavelength = Speed / Frequency
Spatial pulse length determines axial resolution
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Acoustic ImpedanceAcoustic Impedance• Definition
Acoustic Impedance = Density X Prop. Speed
(rayls) (kg/m3) (m/sec)
• increases with higher– Density
– Stiffness
– propagation speed
• independent of frequency
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Why is Acoustic Impedance Important?
Why is Acoustic Impedance Important?
• DefinitionAcoustic Impedance = Density X Prop. Speed
(rayls) (kg/m3) (m/sec)
• Differences in acoustic impedance determine fraction of intensity echoed at an interface