Definition: deciBel

A unit of measurement of the loudness or strength of a signal. One deciBel is considered the smallest difference in sound level that the human ear can discern. Created in the early days of telephony as a way to measure cable and equipment performance and named after Alexander Graham Bell, deciBels (dBs) are a relative measurement derived from two signal levels: a reference input level and an observed output level. A deciBel is the logarithm of the ratio of the two levels. One Bel is when the output signal is 10x that of the input, and one deciBel is 1/10th of a Bel.

Sound Levels
A whisper is about 20 dB. A normal conversation is typically from 60 to 70 dB, and a noisy factory from 90 to 100 dB. Loud thunder is approximately 110 dB, and 120 dB borders on the threshold of pain. In 1883, the volcano on the Indonesian island of Krakatoa erupted. A hundred miles away, the sound level reached more than 170 dB, deafening everyone who survived. See dBm.

   INCREASE IN POWER LEVELS (WATTS)
   Formula is dB=10*log(P1/P2)

   DeciBels   Output Signal Strength
       3dB           2x
       6dB           4x
      10dB (1 Bel)  10x
      20dB         100x
      30dB       1,000x
      40db      10,000x


   ATTENUATION OF AMPLITUDE (VOLTS or AMPS)
   Formula is dB=20*log(A1/A2)

   DeciBels   Output Signal Strength
      -3dB        0.707x
      -6dB        0.5x
     -10dB        0.316x
     -20dB        0.1x
     -30dB        0.032x
     -40db        0.010x

Bels and Bells