Adding Decibels

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

Adding Equal Sound or Noise Power Sources

The resulting sound power when adding equal sound power sources can be expressed as:

Lwt =  10 log(n N / N0)

    = 10 log(N / N0) + 10 log(n) 

    = Lws + 10 log(n)         (1)

where

Lwt = the total sound power level (dB

Lws= sound power level from each single source (dB)

N = sound power (W)

N0 = 10-12 - reference sound power (W)

n = number of sources

Adding Equal Sound Power Calculator

N - sound power (W)

N0 - reference sound power (W)

n - number of sources

Sound Power Level (dB) :  

Lws - sound power level (dB)

n - number of sources

Sound Power Level (dB) :  

Adding of equal sound power sources can be expressed graphically

adding equal sound level sources

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

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.

Sound measured with microphones or sensors are sound pressure

Adding Equal Sound Pressure Levels

The resulting sound pressure level when adding equal sound pressure can be expressed as:

Lpt = Lps + 20 log(n)      (2)

where 

Lpt = total sound pressure level (dB)

Lps = sound pressure level from each single source (dB)

n = number of sources

Adding Equal Sound Pressure Levels Calculator

Lps - sound pressure level (dB)

n - number of sources

Sound Pressure Level (dB) :  

Number of SourcesIncrease in Sound Power Level
(dB)
Increase in Sound Pressure Level
(dB)
2 3 6
3 4.8 9.6
4 6 12
5 7 14
10 10 20
15 11.8 23.6
20 13 26

Adding Sound Power from Sources with different Sound Powers 

The sound power level from more than one source can be calculated as

Lwt =  10 log( (N1 + N2 ... + Nn) / No)    (3)

Adding two sources at different levels can be expressed graphically as

adding different sound levels diagram

Sound Power Level Difference between
two Sound Sources
(dB)
Added Decibel to the Highest Sound Power Level
(dB)
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

Related Topics

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

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