# Introduction to dB

dB is the abbreviation of "decibel" in English, where deci means one-tenth, decibel is a tenth of a “Bel”. "Bel" is the abbreviation of "Bell", a unit named after the outstanding scientist Alexander Graham Bell. Bell obtained the patent for the invention of the telephone in 1876, and made a huge breakthrough in the application and development of the telephone. Bel is not a unit of the International System of Units, but influenced by the rules of the International System of Units, the first letter of the unit symbol represented by a person's name should be capitalized, so we see that B in dB should be capitalized. Since the unit "Bel" is relatively large and inconvenient to use, the more commonly used one-tenth bel, that is, decibels.

Three ways to define dB

1. The volume of the sound

In daily life, the notice board in the residential area indicates that the noise should be lower than 60 decibels, that is, it should be lower than 60dB. Here, dB (decibel) is defined as the logarithm of the ratio of the noise source power to the reference sound power multiplied by 10. Not a unit, but a numerical value used to describe the size of the sound.

2. Signal Strength

In the field of wireless communication, measuring the communication signal strength of a wireless base station in a location can also be expressed in dB. For example, the communication signal strength of No. 1 wireless base station in room 402 of a certain hotel is -90dBm, which is defined as the ratio of the useful signal strength of the room to all signals (including interference signals).

3. Gain

Gain generally refers to the degree of increase in current, voltage or power of components, circuits, equipment or systems, and defined in decibels (dB), that is, the unit of gain is generally decibels (dB), which is a relative value. Gain is often measured in logarithmic units in electronics, and is measured in bels.

• Gain = log10(P2/P1) bel

Where P1 and P2 are input and output power respectively.

Since the value of the gain is usually very large, it is generally expressed in decibels (dB, 1/10 of a decibel):

• Gain = 10×log10(P2/P1) dB

This is the relationship between the absolute value of the gain and the relative value in decibels. 