Attenuation & Decibels

Question 1

___ is How the strength of the ultrasound changes as is moves through the medium

Answer 1

attenuation

Question 2

____ is the Rate that work is done, energy

in US it is a wave of energy

Some energy is released into the medium during propagation

Measured in _____

Determined by the _____

Answer 2

power

Watts (mW)

source

Question 3

_____ is the Power of the wave per area over which it is spread. what is it determined by?

Answer 3

intensity

source

Question 4

what is the formula for intensity? what is it measured in?

Answer 4

I (W/cm2) = Power (W)/ Area (cm2)

Question 5

what is intensities relationship to power and area?

Answer 5

Increasing power increasing Intensity

Decreasing area increasing Intensity - focusing

Example: Sun on dry leave, focused through a lens will increase the intensity and cause leaves to burn – power is unchanged, only area changed

Question 6

what increases w/ increasing power and intensity?

what improves the image quality?

Answer 6

potential bioeffets.

Image quality – Will generally be improved by increasing power because you get stronger reflections back

Question 7

_____ is the Maximum variation of an acoustic variable

Unit depends on the variable being measured

Answer 7

amplitude

Question 8

what is the relationsihp b/t amplitude and intensity?

Answer 8

Intensity = amplitude2

Amplitude = square root of Intensity

If amplitude increased by 2x, then intensity increased 4x

Question 9

what are power and attenuation described in ?

Answer 9

decibels

Question 10

what is a decibel?

what does it represent?

what do you need to compute a decibel change? why?

Answer 10

Bel – logarithmic ratio of the relative powers of 2 beams.
Decibel is 1/10 of a bel, it is a unit of measurement used in sound applications

Represents the smallest degree of difference in loudness the normal human ear can hear

Relative values, not absolute – need 2 powers or intensities to compare in order to compute a decibel change

*50% off of $100 = $50 it has changed by 50$. 
if it’s 3 decibels, it has increased 2x from where it started

Question 11

what is a logarithms

Answer 11

Different way to look at numbers, called log base 10

The log of a number is the amount of times 10 has to multiplied together to get the number.

Also described as the power to which 10 must be raised to get the number

Look at the chart

Question 12

what is

Log10 of 10 =
Log10 of 100 =
Log10 of 1000 =

Log of a fraction is –
Log10 of .01 = -2

why are these used?

Answer 12

1

2

3

Logs are used when dealing with very large numbers (to many 0). In sound application we are comparing 2 like quantities, such as power, intensity or amplitude

Question 13

When comparing power, intensity or amplitude change
if there is an increase, it is a ____
If there is a decrease, it is an _________

Answer 13

gain

attenuation

Question 14

Input power is 10W, output power is 100W. What is the decibel change? Is it a gain or attenuation?

Answer 14

Question 15

Input power is 100W, output power is 10W. What is the decibel change? Is it a gain or attenuation? What is the power ratio?

Answer 15

Question 16

Beginning intensity is 100 mW/cm2, ending intensity is 10 mW/cm2. What is the decibel change? Is it a gain or attenuation? What is the intensity ratio?

Answer 16

Question 17

Beginning intensity is 100 W/cm2, ending intensity is 10 mW/cm2. What is the decibel change? Is it a gain or attenuation? What is the intensity ratio?

Units must match.

Answer 17

Question 18

When comparing voltage and amplitude, you must use ___in the equation

Because intensity = amplitude2

Because Power = voltage2

Answer 18

20

Question 19

Beginning voltage is 100 mV, ending voltage is 10 mV. What is the decibel change? Is it a gain or attenuation? What is the voltage ratio?

Question 20

The input intensity is 10 times less than the output intensity. What is the intensity ratio? What is the “relative intensity?”

This is asking for dB change.

Answer 20

Question 21

The transmitted intensity is 1000 times greater than the reflected intensity. What is the relative intensity?

This is asking for dB change.

Answer 21

Question 22

Power one is 100 W and power two is 50 W. What is the decibel change?

Power one is 100 W and power two is 200 W. What is the decibel change?

Answer 22

Question 23

what are the intensity power ratios for 3, 6, 10

Answer 23

Question 24

Power one is 100 W and power two is 25 W. What is the decibel change?

Power one is 100 W and power two is 400 W. What is the decibel change?

Answer 24

Question 25

3 dB change is increasing quantity\_\_times -3 dB change is decreasing quantity to \_\_ 6 dB change is increasing quantity \_\_times -6 dB change is decreasing quantity to \_\_\_ 10 dB change is increasing quantity\_\_\_\_ times -10 dB change is decreasing quantity to 1\_\_\_\_ 20 dB change is increasing quantity ___ times -20 dB change is decreasing quantity to \_\_\_

Answer 25

3 dB change is increasing quantity 2 times -3 dB change is decreasing quantity to ½ (.5) 6 dB change is increasing quantity 4 times -6 dB change is decreasing quantity to ¼ of original (.25) 10 dB change is increasing quantity 10 times -10 dB change is decreasing quantity to 1/10 of original (.1) 20 dB change is increasing quantity 100 times -20 dB change is decreasing quantity to 1/100 of original (.01)

Question 26

what are the casues of attenuation?

Answer 26

(RR SAID) Absorption (conversion to heat) Reflection Scatter Interference Refraction Diffraction

Question 27

what does the amnt of attenuation depend on ? how is it measured?

Answer 27

Medium, Frequency  Measured in decibels (dB)

Question 28

\_\_\_\_ is Secondary to internal friction of molecular vibration.. it Converts US energy to heat what is it's relationship to frequency?

Answer 28

absorption higher frequency, increased absorption

Question 29

\_\_\_\_ is affected by Viscosity – related to adhesiveness of molecules in medium. Increased viscosity increased absorption  Relaxation time – time required for molecules to return to equalibrium position after wave passed. Increased relaxation time, increased absorption

Answer 29

absorption

Question 30

\_\_\_\_ is When 2 waves combined together Combinations of positive and/or negative pressures (Kremkau) Variation with distance or time of the amplitude of a wave, which results from the addition of two or more waves (Zagzebski)

Answer 30

interference

Question 31

what are the 3 types of interference?

Answer 31

Constructive – combine in phase, make greater amplitude wave  Destructive – combine out of phase, decrease amplitude of wave  Combination – complex wave formed when waves of different frequencies combine

Question 32

\_\_\_\_ is 2 waves with same frequency  Combine in-phase, they interfere constructively and the result has twice the amplitude of the individual waves.

Answer 32

constructive interference

Question 33

\_\_\_ is 2 waves with same frequency  When the two waves have opposite-phase (180o out of phase), they interfere destructively and cancel each other out

Answer 33

destructive interference

Question 34

\_\_\_ is Wave with different frequencies  Combine to form a complex wave

Answer 34

complex interference

Question 35

\_\_\_ is the Spreading out of the beam as it moves away from the source. when it increases what happens to intensity? area?

Answer 35

Increasing diffraction causes loss of intensity.  Area becomes larger

Question 36

Beginning intensity is 100 mW/cm2 End intensity is 50 mW/cm2 What is the dB change? What is the intensity ratio?

Answer 36

IE = II x Intenisty ratio 1. Calculate attenuation coefficient 2. Calculate total attenuation 3. Find intensity ratio for dB of attenuation 4. IE = II x Intenisty ratio

Question 37

\_\_\_\_ is a numerical value used to express how different materials attenuate sound. what is its unit? what is it affected by?

Answer 37

Attenuation coefficient dB/cm Frequency and Tissue

Question 38

wht is the attenuation coefficient for soft tissue? what is its formula? what happens to it if frequency increases?

Answer 38

Rule of thumb for soft tissue… Ac = .5 dB/cm for each MHz of frequency Ac (dB/cm) = .5 dB/cm x F (MHz) 1 MHz transducer will attenuate .5 dB/cm 2 MHz transducer will attenuate 1 dB/cm Increase frequency, increase Ac

Question 39

what is the formula for attenuation?

Answer 39

Attenuation (dB) = Ac (dB/cm) x distance (cm) we can calculate total attenuation by knowing Ac and the distance traveled in the medium

Question 40

If Ac is 3 dB/cm and the sound travels 1 cm, what is the amount of attenuation?

Answer 40

Attenuation (dB) = Ac (dB/cm) x distance (cm) 3 dB

Question 41

Using a 4MHz transducer in soft tissue, what is the amount of attenuation after 3 cm of travel?

Answer 41

Ac (dB/cm) = .5 dB/cm x F Attenuation (dB) = Ac (dB/cm) x distance (cm) 6dB

Question 42

Once we know attenuation in dB, determine by using the chart what the decimal value (Intensity ratio). so... 3dB attenuation = 6 dB attenuation = 10 dB attenuation = 20 dB attenuation =

Answer 42

3dB attenuation = .5 6 dB attenuation = .25 10 dB attenuation = .1 20 dB attenuation = .01

Question 43

what is the formula for end intensity?

Answer 43

End intensity = Initial intensity x Intensity ratio

Question 44

 What is the intensity of a 100 W/cm2 , 2 MHz beam after traveling through 10 cm of soft tissue?

Question 45

what is 1/2 intensity depth formula?

Answer 45

Depth at which the intensity is decreased by ½ (3dB) ½ Int Depth (cm) = 3(dB) / Ac(dB/cm) Not a new equation, same only rearranged

Question 46

Question 47

what is ½ Intensity depth is also known as:  What is the depth at which half intensity occurs in soft tissue for a 4 MHz transducer?

Answer 47

 Depth of penetration  ½ value layer thickness  ½ boundary layer