Logarithms and decibels

Question 1

What occurs as sound strikes a boundary?

Answer 1

It creates a small/weak reflection.

Question 2

Reflected sound waves received by the transducer are _________.

Answer 2

Amplified

Question 3

What happens to sound as it travels?

Answer 3

It attenuates, weakens

Question 4

In diagnostic ultrasound, we are often interested in the _____ _ _______ or the _____ _ _________.

The changes.

Answer 4

Degree of attenuation

Extent of amplification

Question 5

What is a standard measurement tool used to note changes in attenuation or amplification?

Answer 5

Decibel notation

Question 6

Decibels are based on a mathematical construct called ________.

Answer 6

Logarithms

Question 7

What does the logarithm, or log, of a number represent?

Answer 7

The number of 10’s that are multiplied to create the original number.

Question 8

What is a simple way to determine the logarithm of an even power of ten?

Answer 8

Count the number of zeroes.

Ex: There are six zeroes in a million, so the log of a million is 6.

Question 9

Decibel (dB) notation does not measure absolute numbers; rather, decibels report _____ ______.

Answer 9

Relative changes

Ex: the signal strength has doubled, or is now one tenth as large as it once was.

Question 10

Which two intensities are required for comparison when using decibel notation?

Answer 10

The reference (or starting level) and the actual level at the time of measurement.

Question 11

Decibels are a ratio: _______________.

Answer 11

The measured level divided by the starting level.

Question 12

What do positive decibels report?

Answer 12

Signals that are increasing in strength, or getting larger.

Question 13

Every 3dB change means that the intensity will ______.

Answer 13

Double

Question 14

Every 10 dB change means that the intensity

will _______ __ _____.

Answer 14

Increase ten times

Question 15

What do negative decibels describe?

Answer 15

Signals that are decreasing in strength, or getting smaller.

Question 16

A reduction in the intensity of a sound beam to one-half of its original value is ____ dB.

Answer 16

-3 dB

Question 17

A reduction in the intensity of a sound beam to one-quarter of its original value is ____ dB.

Answer 17

-6 dB

Question 18

-10 dB means that the intensity is reduced to _____ of its original value.

Answer 18

One-tenth

Question 19

dB is a mathematical representation with

a _______ _ ______scale.

Answer 19

Logarithmic and relative scale

Question 20

True or false:

We need one intensity to calculate decibels.

Answer 20

False. We need two intensities.

Question 21

A wave’s intensity is 2 mW/cm².

There is a change of +9 dB.

What is the final intensity?

Answer 21

16 mW/cm²

Question 22

If the final intensity of a sound beam is more than the initial intensity, then the gain in dB is _______ (+ or -).

Answer 22

Positive. The beam’s intensity is increasing.

Question 23

If the initial intensity of a sound beam is less than the final intensity, then the gain in dB is ______ (+ or -).

Answer 23

Positive. The beam’s intensity is increasing.

Question 24

dB are used to express changes in:

____, ____, ____, and _____.

Answer 24

*Amplitude. *Intensity. *Sound pressure levels.

*Range of gray scale (log compression, dynamic range)

Question 25

In the equation dB = 10 log I₂/I₁,

what does I₂ and I₁ represent?

Answer 25

I₁ is original intensity

I₂ is changed intensity