Adding Decibels

The logarithmic decibel scale is convenient calculating sound power levels for two or more sound sources

The decibel (dB) is a logarithmic unit used to express the ratio of two signal values - like power, sound power or pressure, voltage, intensity etc. - where one value is a reference value. 

Adding Equal Signal Levels

The total signal level in decibel from equal signal sources can be calculated as

Lt =  10 log (n S / Sref)

    = 10 log (S / Sref) + 10 log (n) 

    = Ls + 10 log (n)                                                         (1)


Lt = total signal level (dB)

S = signal (signal unit)

Sref = signal reference (signal unit)

n = number of sources

Ls = signal level from each single source (dB)

The signal units depends on the nature of the signal - W for power, Pa for pressure and so on.

Note! -  adding sound pressure levels.

Example - Total Sound Power from Two Identical Fans

For sound power it is common to use 10-12 W as the reference sound power. Total sound power from two identical fans each generating 1 W in noise power can be calculated as

Lt =  10 log (2 (1 W) / (1 10-12 W))

              = 123 dB

Sound power and sound power level are often used to specify the noise or sound emitted from technical equipment like fans, pumps or other machines. The "sound" measured with microphones or sensors (meters) are sound pressure.

Adding Equal Signals Units Calculator

S - signal (signal unit)

Sref - signal reference (signal unit)

n - number of signal sources

Adding Equal Signal Levels (decibels) Calculator

Ls - signal level from single source (decibel, dB)

n - number of signal sources

Adding equal signal sources can be expressed graphically

Decibel - adding equal sound level sources

Note! Adding two identical sources (doubling the signal) will increase the total signal level with 3 dB (10 log(2)).

Number of
Increase in
Sound Power Level
2 3
3 4.8
4 6
5 7
10 10
15 11.8
20 13

Adding Signals from Sources with different Strengths

The total signal level from sources with different strengths can be calculated as

Lt =  10 log ((S1 + S2 ... + Sn) / Sref)                                                (2)

Example - Total Sound Power from Two different Fans

The total noise power from two fans - one with sound power 1 W and the other with sound power 0.5 W - can be calculated as

Lt =  10 log (((1 W) + (0.5 W)) / (1 10-12 W))

      = 122 dB

Adding two signal sources with different levels can be expressed graphically in decibels as

Decibel - adding different sound levels diagram

Signal Level Difference
two Sources
to Add to the
Highest Signal Level
0 3
1 2.5
2 2
3 2
4 1.5
5 1
6 1
7 1
8 0.5
9 0.5
10 0.5
> 10 0

Example - Adding Sound Power in Decibels

The sound power from one of the fans in the example above can be calculated as

Ls1 = 10 log((1 W) / (1 10-12 W))

   = 120 dB  

The sound power from the other fan can be calculated as

Ls2 = 10 log((0.5 W) / (1 10-12 W))

   = 117 dB  

The difference in decibel is

Ls1 - Ls2

= (120 dB) - (117 dB)

= 3 dB

From the table or diagram above a difference of 3 dB requires that 2 dB must be added to the highest sound pressure source as

Lt = (120 dB) + (2 dB)

     = 122 dB

Related Topics

  • Acoustics - Room acoustics and acoustic properties - decibel A, B and C - Noise Rating (NR) curves, sound transmission, sound pressure, sound intensity and sound attenuation
  • Noise and Attenuation - Noise is usually defined as unwanted sound - noise, noise generation, silencers and attenuation in HVAC systems
  • Measurements & Instrumentation - Measurement and instrumentation principles

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