Decibel (dB) is a unit of measurement expressing gain or loss. It is used to measure such things as sound, electrical or mechanical power, and voltage. In the telecommunications industry, the decibel is used to conveniently express the gain or loss in transmission systems, whether the medium is copper, optical fiber, or wireless.

The decibel is actually the relationship of some reference point and another point that is above or below the reference point. The base reference point is 0 dB, and subsequent measurements are relative to that reference point. There are a number of decibel notations, each indicating the context of the measurement, such as:

• dBi is the antenna gain in dB relative to an isotropic source.
• dBm is the power in dB relative to 1 milliwatt.
• dBW is the power in dB relative to 1 watt.
• dBmV is referenced to 1 millivolt. It is often used as a measure of signal levels (or noise) on a network.

The dB scale related to power is different from the dB scale related to voltage. In power measurements, the power level doubles every 3 dB, instead of every 6 dB as in voltage. Likewise, the dB scale related to audio output is different from the dB scales relating to voltage and power.

Since the range of audio intensities that the human ear can detect is so large, the scale frequently used to measure them is a scale based on multiples of 10. This type of scale is sometimes referred to as a “logarithmic scale.” The threshold of hearing is assigned a sound level of 0 dB. Asound that is 10 times more intense is assigned a sound level of 10 dB.

Asound that is 10 times more intense (10 × 10) is assigned a sound level of 20 dB. Asound that is 10 times more intense (10 × 10 × 10) is assigned a sound level of 30 dB. Asound that is 10 times more intense (10 × 10 × 10 × 10) is assigned a sound level of 40 dB.

There are a variety of test instruments available to handle virtually any measurement requirement, including analog impulse meters to measure quick bursts of sound. These devices typically have output jacks for connections to charting devices that plot continuous noise levels across a roll of paper.

Digital devices output measurements to light-emitting diode (LED) screens. Band filters allow selection of narrow frequency ranges to isolate specific noises for measurement. Optional calibrators are available for in-field adjustments.