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 Also called acoustic variable
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 of Individual 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 Individual Air Molecule

11. Talk louder! I can’t hear you.
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
,000 Hz

15. Sound Frequency Ultrasound definition
> 20,000 Hz
not audible to humans
dog whistles are in this range
Clinical ultrasound frequency range
MHz
1,000, ,000,000 Hz

16. Magnitude of acoustic variable
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 = ------------------- Frequency1
Period =
Frequency
as frequency increases, period decreases
if frequency in Hz, period in seconds/cycle

18. Period = 1 / Frequency Period
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 Frequency Units Period Units Hz (cycles/sec)
seconds/cycle
kHz (thousands of cycles/sec)
msec/cycle
MHz (millions of cycles/sec)

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
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 MHz Calculate 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 On Cycle (speak)
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 same transducer used for
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 pulse repetition frequency frequency period
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)
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 Frequency Higher Pulse Repetition Frequency
Higher Frequency Same Pulse Repetition Frequency

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

37. Pulse Duration units equation time per pulse (time/pulse)
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. So, can you like show me an example?
Spatial Pulse Length
So, can you like show me an example?

43. Spat. Pulse Length = # cycles per pulse X wavelength
Spatial Pulse Length
Equation
Spat. Pulse Length = # cycles per pulse X wavelength
(dist. / pulse) (cycles / pulse) (dist. / cycle)
depends on source & medium
as wavelength increases, spatial pulse length increases

44. Spat. Pulse Length = # cycles per pulse X wavelength
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 increases with higher
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