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Resident Physics Lectures

Resident Physics Lectures. 02: Sound Properties and Parameters. Sound Wave Definition?. Sound is a Wave Wave is a propagating (traveling) variation in a “ wave variable ” “An elephant is big, gray, and looks like an elephant.”. Sound Wave Variable. Examples pressure (force / area)

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Resident Physics Lectures

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  1. Resident Physics Lectures 02: Sound Properties and Parameters

  2. Sound Wave Definition? • Sound is a Wave • Wave is a propagating (traveling) variation in a “wave variable” • “An elephant is big, gray, and looks like an elephant.”

  3. Sound Wave Variable • Examples • pressure (force / area) • density (mass / volume) • temperature • Also called acoustic variable Sound is a propagating (moving) variation in a “wave variable”

  4. Sound Wave Variation • Freeze time • Measure some acoustic variable as a function of position Pressure Density Temperature Acoustic Variable Value Position

  5. MORE • Make many measurements of an acoustic variable an instant apart • Results would look the same but appear to move in space 1 Instant #1 Instant #2 2

  6. MORE • Track acoustic variable at one position over time

  7. Sound 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

  8. Transverse Waves • Particle moves perpendicular to wave travel • Water ripple • surface height varies with time • peak height moves outward • water does not move outward

  9. Compression (Longitudinal) Waves • Particle motion parallel to direction of wave travel 1 1 Motion ofIndividual Coil 2 2 Wave Travel

  10. 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

  11. Medium • 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.

  12. Sound Waves • Information may be encoded in wave energy • radio • TV • ultrasound • audible sound

  13. Sound Frequency • light frequency corresponds to color • sound frequency corresponds to pitch

  14. Sound Frequency # of complete variations (cycles) of an acoustic variable per unit time • Units cycles per second 1 Hz = 1 cycle per second 1 kHz = 1000 cycles per second 1 MHz = 1,000,000 cycles per second • Human hearing range 20 - 20,000 Hz

  15. Sound 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

  16. Period • 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) Magnitude of acoustic variable period time

  17. Period 1 Period = ------------------- Frequency • as frequency increases, period decreases • if frequency in Hz, period in seconds/cycle

  18. Period Period = 1 / Frequency • if frequency in kHz, period in msec/cycle • if frequency in MHz, period in msec/cycle 1 kHz frequency ==> 1 msec period 1 MHz frequency ==> 1 msec period

  19. Reciprocal Units

  20. Period / Frequency If frequency = 2 MHz then sound period is 1/2 = 0.5 msec 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 msec 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

  21. Sound Period & Frequency are determined only by the sound source. They are independent of medium. Who am I? Burt Mustin

  22. Propagation Speed • Speed only a function of medium • Speed virtually constant with respect to frequency over clincial range

  23. Wavelength • 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

  24. Wavelength Units • length per cycle • sometimes just length; cycle implied • usually in millimeters or fractions of a millimeter for clinical ultrasound

  25. Wavelength Equation Speed = Wavelength X Frequency [ c = l X n ](dist./time)(dist./cycle) (cycles/time) • As frequency increases, wavelength decreases • because speed is constant

  26. Wavelength Speed = Wavelength X Frequency [ c = l X n ](dist./time)(dist./cycle) (cycles/time) mm/msec mm/cycle MHzCalculate Wavelength for 5 MHz sound in soft tissue Wavelength = 1.54 mm/msec / 5 MHz 5 MHz = 5,000,000 cycles / sec = 5 cycles / msec Wavelength = 1.54 / 5 = 0.31 mm / cycle

  27. Wavelength is a function of both the sound source and the medium! Who am I? John Fiedler

  28. Pulsed Sound • For imaging ultrasound, sound is • Not continuous • Pulsed on & off • On Cycle (speak) • Transducer produces short duration sound • Off Cycle (listen) • Transducer receives echoes • Very long duration ON OFF ON OFF (not to scale)

  29. Pulse 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 sound silence

  30. Pulsed Sound Example • ringing telephone • ringing tone switched on & off • Phone rings with a particular pitch • sound frequency sound sound silence

  31. Parameters Sound Pulse • pulse repetition frequency • pulse repetition period • pulse duration • duty factor • spatial pulse length • cycles per pulse • frequency • period • wavelength • propagation speed

  32. Pulse 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

  33. Pulse Repetition Period • time from beginning of one pulse until beginning of next • time between corresponding points of adjacent pulses Pulse Repetition Period

  34. Pulse 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

  35. Pulsed Sound • Pulse repetition frequency & period independent sound frequency & period Same FrequencyHigher PulseRepetition Frequency Higher FrequencySame PulseRepetition Frequency

  36. Pulse Duration Pulse Duration • Length of time for each sound pulse • one pulse cycle = • one sound pulseand one period of silence • Pulse duration independent of duration of silence

  37. Pulse Duration • units • time per pulse (time/pulse) • equation pulse duration = Period X # cycles per pulse(time/pulse) (cycles/pulse) (time/cycle) Pulse Duration Period

  38. Pulse Duration Longer Pulse Duration Same frequency; pulse repetition frequency, period, & pulse repetition period Shorter Pulse Duration

  39. Pulse Duration Pulse duration is a controlled by the sound source, whatever that means.

  40. Duty Factor • Fraction of time sound generated • Determined by source • Units • none (unitless) • Equations Duty Factor = Pulse Duration / Pulse Repetition Period Duty Factor = Pulse Duration X Pulse Repetition Freq. Pulse Duration Pulse Repetition Period

  41. Spatial Pulse Length • distance in space traveled by ultrasound during one pulse H.......E.......Y HEY Spatial Pulse Length

  42. Spatial Pulse Length So, can you like show me an example?

  43. Spatial Pulse Length Equation • 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)

  44. Spatial Pulse Length Spat. Pulse Length = # cycles per pulse X wavelength Wavelength = Speed / Frequency • as # cycles per pulse increases, spatial pulse length increases • as frequency increases, wavelength decreases & spatial pulse length decreases • speed stays constant

  45. Why is Spatial Pulse Length Important Spat. Pulse Length = # cycles per pulse X wavelength Wavelength = Speed / Frequency Spatial pulse length determines axial resolution

  46. Acoustic Impedance • Definition Acoustic Impedance = Density X Prop. Speed(rayls) (kg/m3) (m/sec) • increases with higher • Density • Stiffness • propagation speed • independent of frequency

  47. Why is Acoustic Impedance Important? • Definition Acoustic Impedance = Density X Prop. Speed(rayls) (kg/m3) (m/sec) • Differences in acoustic impedance determine fraction of intensity echoed at an interface

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