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Title. Elementary Principles. What is sound and how is it produced? Audible sound vs. ultrasound Waves, “wavelength” Pressure, intensity, power Frequency and period Acoustic impedance Reflection Review metrics. Production of sound. “Clink”. “Clink”. “Clink”. Particle vibrations.

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  1. Title

  2. Elementary Principles • What is sound and how is it produced? • Audible sound vs. ultrasound • Waves, “wavelength” • Pressure, intensity, power • Frequency and period • Acoustic impedance • Reflection • Review metrics

  3. Production of sound “Clink” “Clink” “Clink”

  4. Particle vibrations

  5. Talking Air vibrations Voice box Ear drum

  6. Sound • A mechanical disturbance propagating through a medium • Mechanical: particle motion is involved • Particle vibrations • Energy is transmitted through the medium • Particles themselves do not propagate through the medium.

  7. Bell Jar Experiment

  8. Generation of ultrasound Piezoelectric ‘element’

  9. Generation of ultrasound Piezoelectric ‘element’ (vibrates when driven with an electrical signal)

  10. Sound travels in “waves” • A wave is an oscillating disturbance that travels through a medium • Many forms of energy travel in waves • Sound travels as a wave

  11. Two Types of Waves Mechanical ocean waves seismic waves sound waves Electromagnetic radio waves x-rays light waves

  12. Mechanical Waves: • characterized by physical motion of particles in the medium • cannot travel through a vacuum • (Electromagnetic waves CAN travel through a vacuum.)

  13. Longitudinal Particle motion (vibration) parallel to direction of wave travel Particle motion (vibration) perpendicular to direction of wave travel

  14. Picture of slinky • “Compressional (or longitudinal) wave traveling along a slinky • Simply snap one end back and forth • Transverse wave obtained by jerking up and down

  15. Ultrasound waves in tissue • Sound waves used for medical diagnosis are LONGITUDINAL. • Transverse waves are not involved at all (at least not until recently … “supersonic imaging” and ARFI imaging involve transverse waves, though these are not produced by the transducer).

  16. Other types of elasticity imaging • Acoustic radiation force imaging (ARFI) • Tissue displacement created by energetic acoustic pulses from the transducer • SuperSonic Shear wave Imaging • Energetic pulse • =>shear wave • Create shock front • High speed imaging • Tracks shear wave • Reconstruct speed • Related to elasticity (Supersonic Imagine white paper, Jeremy Bercoff www.supersonicimagine.fr)

  17. Compression and rarefaction Continuous Transmission

  18. Schlieren Photography Water Light beam This is a way to view sound waves. The compressions and rarefactions disturb light propagating through the beam. One can view these disturbances.

  19. Compression and rarefaction Compression: density is higher than normal Rarefaction: density is lower than normal

  20. Compression and rarefaction Pulsed Transmission

  21. Pressure amplitude Amplitude Amplitude: measure of the amount of change of a time varying quantity.

  22. Pressure amplitude • pascals (Pa) • 1 Pa = 1N/m2 • megapascals (MPa) (mega = 1,000,000) • Other units • Pounds/square inch (32 lb/in2 ~ 220 kPa) (kilo = 1,000) • mm of mercury (blood pressure) • cm of water

  23. Pressure of the atmosphere pascals (Pa) megapascals (MPa) (mega = 1,000,000) Other units Pounds/square inch (32 lb/in2 ~ 250 kPa) (kilo = 1,000) mm of mercury (blood pressure) cm of water

  24. Ways we describe amplitude • High vs. low • Loud vs. soft • Strong echoes vs. weak echoes • Bright dots vs. dim dots

  25. Ways we describe amplitude • High vs. low • Loud vs. soft • Strong echoes vs. weak echoes • Bright dots vs. dim dots

  26. Frequency • Number of oscillations per second • By the source • By the particles • Called “pitch” for audible sounds • Expressed in hertz (Hz) • 1 Hz = 1 cycle/s • 1 kHz = 1,000 cycles/s • 1 MHz = 1,000,000 cycles/s

  27. Frequency • Number of oscillations per second • By the source • By the particles • Called “pitch” for audible sounds • Expressed in hertz (Hz) • 1 Hz = 1 cycle/s • 1 kHz = 1,000 cycles/s = 103 cycles/s • 1 MHz = 1,000,000 cycles/s = 106 cycles/s • 2.5 MHz = 2,500,000 cycles/s = 2.5 x 106 cycles/s • 7.5 MHz = 7,500,000 cycles/s = 7.5 x 106 cycles/s

  28. Frequency

  29. Supersonic vs. Ultrasonic • Supersonic = faster that sound • Ultrasonic = sound whose frequency is above the audible (greater than 20 kHz)

  30. Pressure amplitude Amplitude Amplitude: measure of the amount of change of a time varying quantity.

  31. Wave Period T Distance Wave motion at a specific point in space. The wave variable (pressure in this case) varies over time. Period = time for 1 cycle. Pressure vs. distance at two different times.

  32. Period vs. frequency period period

  33. Wave Period T • Amount of time for 1 cycle • Equal to the inverse of the frequency • What is the period for a 10 Hz wave?

  34. Wave Period Amount of time for 1 cycle Equal to the inverse of the frequency What is the period for a 10 Hz wave? T

  35. Wave Period Amount of time for 1 cycle Equal to the inverse of the frequency If the period is 0.01 s, what is the frequency? T

  36. Dividing fractions • To divide 1 fraction (1/2) by another (1/4) • Invert the denominator • Multiply the numerator by the inverted denominator

  37. Wave Period T • Amount of time for 1 cycle • Equal to the inverse of the frequency

  38. Metric System Unit Prefixes Please note: the sound emitted from your 3.5 MHz transducer is 3.5 MHz, not 3.5 mHz or 3.5 mhz!

  39. Wave Period T • Amount of time for 1 cycle • Equal to the inverse of the frequency

  40. Pressure fluctuations Wavelength • Wavelength is the distance between any two corresponding points on the waveform. l

  41. Wavelength vs. frequency • As frequency increases, wavelength decreases. • Wavelength is inversely proportional to frequency. • If you double the frequency, the wavelength is halved. • If you triple the frequency, wavelength is cut to 1/3 of the original.

  42. Wavelength depends on speed of sound and Frequency Wavelength is “directly proportional” to sound speed. (For a given frequency, if 1 medium’s sound speed is 2 times that of another, the wavelength for any frequency will also be two times that of the other.)

  43. Suppose the speed of sound is 330 m/s. For a 1 kHz sound wave, what is the wavelength?

  44. Suppose the speed of sound is 330 m/s. For a 1 kHz sound wave, what is the wavelength?

  45. Suppose the speed of sound is 330 m/s. For a 1 kHz sound wave, what is the wavelength?

  46. The average speed of sound in soft tissue is 1,540 m/s. What is the wavelength for a 3 MHz sound beam?

  47. The average speed of sound in soft tissue is 1,540 m/s. What is the wavelength for a 3 MHz sound beam? 1 meter=1,000 millimeters; 1 mm = 0.001 m

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