Lecture 2: Waves

Question 1

What is a transverse wave?

Answer 1

A wave that oscillates in a perpendicular direction to the wave motion

Question 2

What is a longitudinal wave?

Answer 2

A wave that oscillates in a parallel direction to the wave motion

Question 3

What type of wave is ultrasound?

Answer 3

Longitudinal (parallel), compressional waves that can be periodic or pulsed, propagate at roughly 1500 m/s in water, can leave the medium unchanged (except at high intensity ultrasound).

Question 4

Propagating waves?

Answer 4

are a means by which energy can be transferred from one point to another without transfer of matter

Question 5

What is a sound wave?

Answer 5

Sound - a disturbance in pressure that propagates through a compressible medium.

Question 6

Infrasonic waves frequency?

Answer 6

<20Hz

Question 7

Audio frequency

Answer 7

20Hz- 20kHz

Question 8

Ultrasonic frequency

Answer 8

> 20kHz

Question 9

Wavelength, lambda

Answer 9

It is the adjacent distance between two adjacent peaks or troughs.

Question 10

Wavenumber, k

Answer 10

k is defined as k=2*pi/wavelength

Question 11

Frequency, f

Answer 11

If a periodic pattern propagates then the rate at which the peaks pass a fixed point is called the frequency of the wave.

Question 12

Wave speed, c.

Answer 12

Propagating waves often travel at characteristic speeds in a given medium.

Question 13

Why do we ignore the shear waves in ultrasound imaging?

Answer 13

• Shear waves are not generated efficiently by ultrasound transducers which send short compressive pulses into the tissue
• Shear waves are strongly absorbed by tissue so don’t travel far
• Shear waves travel much more slowly than compressional waves

Question 14

What is the Continuum hypothesis

Answer 14

The medium through which the ultrasound wave is traveling is continuous.

Question 15

Particle displacement

Answer 15

Fluid elements or particles of a medium move - oscillate about their mean position - as a sound wave passes through.

Question 16

Particle velocity

Answer 16

In general the fluid velocity vector will include a non-oscillatory net flow velocity u0 as awell as the acoustic velocity vector, which is the time derivative of the acoustic particle displacement. The total particle velocity uT is therefore uT=u0+u.

Question 17

Acoustic pressure, density and temperature fluctuation equations?

Answer 17

pT=p0+p
roT= ro0+ro
Tt=T0+T

Question 18

Instantaneous acoustic intensity

Answer 18

In a time varying acoustic field is a vector defined as I(x,t)=pressure(x,t)u(x,t). Where u is the acoustic particle velocity.

Question 19

Acoustic intensity

Answer 19

is also used sometimes to refer to the time-averaged acoustic intensity. Average intensity = pu = 1/T integral(I(x,T))dt

Question 20

Sound power, P

Answer 20

in watts, is the rate at which acoustic energy is flowing across a surface S. P = integral ( averageintensity.n) dS

Question 21

Decibel: ratio of powers

Answer 21

10log10(power/ reference power)

Question 22

Sound power level P can be written in decibels how?

Answer 22

sound power level in db= 10log10(P/Pref)

Question 23

Sound pressure level

Answer 23

Sound pressure level = 20log10(p/pref)