How To Calculate dB Loss

Decibels (dB) determine the relationship in signal strength between two sources. When the power of the first signal outweighs that of the second, a loss occurs; this can be desirable, as with the use of carpets to quiet a library, or it can be detrimental, as when a bad cable weakens electrical signals from an antenna on their way to your TV. Use the formula for finding the decibels as a ratio of the power of the signals to calculate the exact value of the loss. A scientific calculator with a log function helps to solve the equation.

Step 1

Measure the full-strength signal with an appropriate meter; to measure radio signals, for example, a radio signal power meter indicates the strength of radio waves at a particular location in units of milliwatts, microwatts or similar units. Write down the results, labeling them "full strength."

Step 2

Measure the attenuated signal with the same meter; this is the signal for which you expect a reduction in power. For example, an antenna picks up a radio signal; right at the antenna, the meter measures 20 miliwatts, but the long cable connected to the cable reduces the power to 5 miliwatts. In this instance, you measure the attenuated signal at the output end of the long cable. Write down the results, labeling them "attenuated."

Step 3

Divide the first signal's power by the second signal's power to find the ratio of the two signals. For instance, if signal A has a power of 20 mW and signal B has a power of 5 mW: 20/5 = 4.

Step 4

Take the log of the the ratio of the signals by pressing the log button on the scientific calculator. For instance: log 4 = 0.602.

Step 5

Multiply this answer by 10 to find the decibels. For the example: 0.602 x 10 = 6 decibels (dB).

Step 6

Determine if the decibel reading reflects a loss or gain of power by looking at signal A and signal B. Record a loss if signal A had a greater value than signal B, and a gain if signal B had the greater number. For the example, since the first signal (signal A) measured more than signal B, the result indicated a loss of 6 decibels (dB).