decibel

decibel (dĕs`əbĕl', –bəl), abbr. dB, unit used to measure the loudness of sound. It is one tenth of a bel (named for A. G. Bell), but the larger unit is rarely used. The decibel is a measure of sound intensity as a function of power ratio, with the difference in decibels between two sounds being given by dB=10 log₁₀(P₁/P₂), where P₁ and P₂ are the power levels of the two sounds. The faintest audible sound, corresponding to a sound pressure of about 0.0002 dyne per sq cm, is arbitrarily assigned a value of 0 dB. The loudest sounds that can be tolerated by the human ear are about 120 dB. The level of normal conversation is about 50 to 60 dB. The decibel is also used to measure certain other quantities, such as power loss in telephone lines.

---

decibel (dB)

Unit for measuring the relative intensities of sounds or the relative amounts of acoustic or electric power. Because it requires about a tenfold increase in power for a sound to register twice as loud to the human ear, a logarithmic scale is useful for comparing sound intensity. Thus, the threshold of human hearing (absolute silence) is assigned the value of 0 dB and each increase of 10 dB corresponds to a tenfold increase in intensity and a doubling in loudness. The “threshold of pain” for intensity varies from 120 to 130 dB among different individuals. A related unit is the bel = 10 dB.

---

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.

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. 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

Quite a lot was named after Alexander Graham Bell. Throughout the 20th century, the Bell name was ubiquitous. It will live on with the deciBel.