Deutsche Version 
Incoherent or noncoherent means the signals of the overdubbed channels are irrelative like a violin and a trumpet, that means having no correlative relationship. Sometimes we say uncorrelated when we mean incoherent. Given two incoherent sources their combined effect is the sum of their acoustic power. 
Fill in as many sound level boxes as necessary (max 10) and then click the calculate bar, to get the calculated sum. Provided, that each sound source has its own random phasing. 
A program to combine as much as thirty (30) noise sources
Conversion of sound pressure level to sound pressure and sound intensity
The ten octave bands of our hearing range
The formula for the sum level of sound pressures of n incoherent radiating sources is
The reference sound pressure p_{0} is 20 µPa = 0.00002 Pa = 2 × 10^{−5} Pa (RMS) ≡ 0 dB.
From the formula of the sound pressure level we find
This inserted in the formula for the sound pressure level to calculate the sum level shows
L_{Σ} = Total level and L_{1}, L_{2}, ... L_{n} = sound pressure level of the separate sources in dBSPL.
Incoherent means: lacking cohesion, connection, or harmony. It is not coherent.
0  1  2  3  4  5  6  7  8  9  10 
3.01  2.54  2.12  1.76  1.46  1.19  0.97  0.79  0.64  0.51  0.41 
Level difference between the two sound sources 
Adding of equal loud incoherent sound sources

Formulas: Δ L = 10 × log n or n = 10^{(ΔL/10)}
Δ L = level difference; n = number of equal loud sound sources.
n = 2 equally loud incoherent sound sources result in a higher level of 10 × log_{10} 2 = +3.01 dB compared to the case that only one source is available. n = 3 equally loud incoherent sound sources result in a higher level of 10 × log_{10} 3 = +4.77 dB compared to the case that only one source is available. n = 4 equally loud incoherent sound sources result in a higher level of 10 × log_{10} 4 = +6.02 dB compared to the case that only one source is available. n =10 equally loud incoherent sound sources result in a higher level of 10 × log_{10} 10 = +10.00 dB compared to the case that only one source is available. 
Adding (combining) levels of equal loud sound sources
To use the calculator, simply enter a value. The calculator works in both directions of the ↔ sign. 
The total level in dB is the level of one sound source plus the increase of level in dB.
Example: The measurable noise of a motorcycle is at a certain distance
60 dB (A). How big is the total level of 4 motorcycles with the same volume? Solution: 60 dB (A) + 10 log 4 = 60 + 6 = 66 dB (A). If you are doing noise measurements of motorcycles you should at least consider the "honesty" of the dBAreadings without low frequencies. 
You can easily add up coherent and incoherent sound level and sound pressure values. It is often desired to add the psychoacoustic perceived loudness or volume. See: 
How many decibels (dB) level change is double, half, or four times as loud?
How many dB to appear twice as loud (two times)? Here are all the different ratios. Ratio means "how many times" or "how much" ... Doubling of loudness. 
Level change 
Volume Loudness 
Voltage Sound pressure 
Acoustic Power Sound Intensity 
+40 dB  16  100  10000 
+30 dB  8  31.6  1000 
+20 dB  4  10  100 
+10 dB  2.0 = double  3.16 = √10  10 
+6 dB  1.52 times  2.0 = double  4.0 
+3 dB  1.23 times  1.414 times = √2  2.0 = double 
    ±0 dB          1.0             1.0             1.0      
−3 dB  0.816 times  0.707 times  0.5 = half 
−6 dB  0.660 times  0.5 = half  0.25 
−10 dB  0.5 = half  0.316  0.1 
−20 dB  0.25  0.100  0.01 
−30 dB  0.125  0.0316  0.001 
−40 dB  0.0625  0.0100  0.0001 
Log. size  Psycho size  Field size  Energy size 
dB change  Loudness multipl.  Amplitude multiplier  Power multiplier 
Ratio  Change in Sound Loudness Level 
Change in Sound 
Change in Sound Power Level 
20  +43.22 dB  +26.02 dB  +13.01 dB 
15  +39.07 dB  +23.52 dB  +11.76 dB 
10  +33.22 dB  +20 dB  +10 dB 
5  +23.22 dB  +13.98 dB  +6.99 dB 
4  +20 dB  +12.04 dB  +6.02 dB 
3  +15.58 dB  +9.54 dB  +4.77 dB 
2  +10 dB  +6.02 dB  +3.01 dB 
     1           ±0 dB          ±0 dB         ±0 dB     
1/2 = 0.5  −10 dB  −6.02 dB  −3.01 dB 
1/3 = 0.3333  −15.58 dB  −9.54 dB  −4.77 dB 
1/4 = 0.25  −20 dB  −12.04 dB  −6.02 dB 
1/5 = 0.2  −23.22 dB  −13.98 dB  −6.99 dB 
1/10 = 0.1  −33.22 dB  −20 dB  −10 dB 
1/15 = 0.0667  −39.07 dB  −23.52 dB  −11.76 dB 
1/20 = 0.05  −43.22 dB  −26.02 dB  −13.01 dB 
Noise
Noise is annoying, harassing and unwanted sound. It is not a physical phenomenon, but
only mental processes change a sound to noise. There are a number of definitions of noise. Important ones are: 1  the acoustic ratio that characterize the noise and by measurable physical sizes, such as the amplitude or the sound pressure level, frequency, and the time behavior of the sound, can be described. 2  the situational ratio, i.e. location, time and situation in which the person is situate during the occurrence of the noise, and the relation to the activities, intentions and the current being of the person who is exposed to the noise. 3  the personal ratio of the person who is exposed to the noise, with their acquired cognitive and emotional implications for the sound source. The fact that noise is not only dependant on physically measurable sizes, but "of more", makes the derivation of methods and calculation methods for the objective description to a problem and explains the problems of noise control, which are often found between the measured noise values and the perceived harassment. Kurt Tucholsky wrote aptly: "Our own dog does not make noise, it only barks. 
Castrated sound level values in weighted dBA are added the same way like sound level values in unweighted dB. 
Pro audio equipment often lists an Aweighted noise spec – not because it correlates well with our hearing – but because it can "hide" nasty hum components that make for bad noise specs. Words to bright minds: Always wonder what a manufacturer is hiding when they use Aweighting. *) 
*) http://www.google.com/search?q=Always+wonder+what+a+manufacturer+Rane&filter=0
Formulas for working with sound
1 pascal (Pa) = 1 newton/m^{2} = 10 dyne/cm^{2} = 10 microbar ≡ 94 dB SPL (Sound Pressure Level) Sound Pressure Level (SPL) Sound pressure level L_{p} = 20 × lg (p / p_{0}) in decibels (dB), where p is the measured pressure as sound field size and p_{0} is the reference pressure in the same system of units. p_{0} = 20 micropascals or micronewtons/m^{2} = 0.00002 Pa = 0.0002 microbar or dyne/cm^{2}. This reference sound pressureas a sound field size corresponds to a sound wave in free air with an acoustic pressure of p_{0} = 0.00002 Pa (N/m²). Sound Intensity Level (SIL) or Acoustic Intensity Level Sound intensity level L_{I} = 10 × lg (I / I_{0}) in decibels (dB), where I is the measured intensity as sound energy size and I_{0} is the reference sound intensity in the same system of units. I_{0} = 10^{−12} watt per m^{2}. This reference sound intensity as a sound energy size corresponds to a sound wave in free air with an acoustic intensity (energy) of I_{0} = 10^{−12} watt/m^{2}. 
What does sound level mean?
Sound engineers and "ear people" think by the short word "sound level" simply of "sound pressure level" as sound field quantity. Acousticians and "noise fighters" mean by the short word "sound level" probably "sound intensity level" as sound energy quantity. Equating sound pressure with sound intensity must cause problems.I ~ p^{2}. 
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