Logarithms and decibels Flashcards
What occurs as sound strikes a boundary?
It creates a small/weak reflection.
Reflected sound waves received by the transducer are _________.
Amplified
What happens to sound as it travels?
It attenuates, weakens
In diagnostic ultrasound, we are often interested in the _____ _ _______ or the _____ _ _________.
The changes.
Degree of attenuation
Extent of amplification
What is a standard measurement tool used to note changes in attenuation or amplification?
Decibel notation
Decibels are based on a mathematical construct called ________.
Logarithms
What does the logarithm, or log, of a number represent?
The number of 10’s that are multiplied to create the original number.
What is a simple way to determine the logarithm of an even power of ten?
Count the number of zeroes.
Ex: There are six zeroes in a million, so the log of a million is 6.
Decibel (dB) notation does not measure absolute numbers; rather, decibels report _____ ______.
Relative changes
Ex: the signal strength has doubled, or is now one tenth as large as it once was.
Which two intensities are required for comparison when using decibel notation?
The reference (or starting level) and the actual level at the time of measurement.
Decibels are a ratio: _______________.
The measured level divided by the starting level.
What do positive decibels report?
Signals that are increasing in strength, or getting larger.
Every 3dB change means that the intensity will ______.
Double
Every 10 dB change means that the intensity
will _______ __ _____.
Increase ten times
What do negative decibels describe?
Signals that are decreasing in strength, or getting smaller.
A reduction in the intensity of a sound beam to one-half of its original value is ____ dB.
-3 dB
A reduction in the intensity of a sound beam to one-quarter of its original value is ____ dB.
-6 dB
-10 dB means that the intensity is reduced to _____ of its original value.
One-tenth
dB is a mathematical representation with
a _______ _ ______scale.
Logarithmic and relative scale
True or false:
We need one intensity to calculate decibels.
False. We need two intensities.
A wave’s intensity is 2 mW/cm2.
There is a change of +9 dB.
What is the final intensity?
16 mW/cm2
If the final intensity of a sound beam is more than the initial intensity, then the gain in dB is _______ (+ or -).
Positive. The beam’s intensity is increasing.
If the initial intensity of a sound beam is less than the final intensity, then the gain in dB is ______ (+ or -).
Positive. The beam’s intensity is increasing.
dB are used to express changes in:
____, ____, ____, and _____.
*Amplitude. *Intensity. *Sound pressure levels.
*Range of gray scale (log compression, dynamic range)
In the equation dB = 10 log I2/I1,
what does I2 and I1 represent?
I1 is original intensity
I2 is changed intensity
