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The resulting sound power when adding equal sound power sources can be expressed as:
Lwt = 10 log(n N / No)
= 10 log(N / No) + 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)
No = 10-12 - reference sound power (W)n = number of sources
The adding of equal sound power sources can also be expressed graphically as

Note! Adding two identical sources will increase the total sound power level with 3 dB ( 10 log(2) ).
The sound power and sound power level is commonly used to specify the emitted noise or sound from technical equipment as fans, pump and other machines. The sound measured with microphones is the sound pressure.
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
| Number of Sources | Increase 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 |
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

| 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 |
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