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Why Should I Use Decibels to Display My Data?



Primary Software: LabVIEW Development Systems>>LabVIEW Full Development System
Primary Software Version: 7.1
Primary Software Fixed Version: N/A
Secondary Software: N/A

Problem: Why should I use decibels to display my data?

Solution: On some instruments, you will see the option of displaying the amplitude in a linear or decibel (dB) scale. The linear scale shows the actual voltage amplitudes, whereas the decibel scale is a transformation of the linear scale into a logarithmic scale.  The decibel scale is used to compare the voltage or power of one signal to a reference signal.

Suppose that you want to display a signal with very large and very small amplitudes. Let us assume your display is 10 cm high, and you will utilize the entire height of the display for the largest amplitude. So, if the largest amplitude in the signal is 100 V, a height of 1 cm on the display corresponds to 10 V. If the smallest amplitude of the signal is 0.1 V, this corresponds to a height of only 0.1 mm. This will barely be visible on the display.

To see all the amplitudes, from the largest to the smallest, you need to change the amplitude scale. Alexander Graham Bell invented a unit, the Bell, which is logarithmic, compressing large amplitudes and expanding the small amplitudes. However, the Bell was too deemed too large, so commonly the decibel (1/10th of a Bell) is used. The decibel (dB) is defined as:
one dB = 10 log^10 (Power Ratio) = 20 log^10 (Voltage Ratio)
The following table shows the relationship between the decibel and the Power and Voltage Ratios.

db Power Ratio Voltage Ratio
+40 10000 100
+20 100 10
+6 4 2
+3 2 1.4
0 1 1
-3 1/2 1/1.4
-6 1/4 1/2
-20 1/100 1/10
-40 1/10000 1/100

The dB scale is useful in compressing a wide range of amplitudes into a small set of numbers. The decibel scale is often used in sound and vibration measurements and in displaying frequency domain information.

Related Links: White Paper: Decibels

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Report Date: 08/08/2000
Last Updated: 09/05/2017
Document ID: 207E3JLJ

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