Does one add decibels?

Question

For example, if I have two computer fans each making 30 dB, how much noise do I hear? It's not 60, is it?

Answer 1

Decibels are an exponential scale, so 40 dB is twice as loud as 30 dB.

Answer 2

No, two 30 dB fans would equal 33 dB.

A 30 dB fan and a 10 dB fan would equal 30 dB. You wouldn't hear the much quieter fan.

================

Large increases in Intensity level of sound are not perceived as much "louder". In general a ten times increase in acoustic intensity is perceived as twice as loud. The decibel scale was invented to comply with this perceived measurement.

================

Decibels are a "relative measure", not an absolute measure. Each decibel measurement is related to a linear quantity.

A=10*log(a)

B=10*log(b)

To add the two signals you get 10*log(a+b) where you add the linear values.

================

In your original case a=b, and 30 dB = 10*log(a)

The sum of the two fans is:

10*log(a+b) or 10*log(2*a) = 10*log(a)+10*log(2) = 10*log(a)+3= 33 dB

You can calculate "a" if you want, but it is not necessary.

10*log(2)=3.010299957 but most people round it off to 3.

  

Answer 3

it's not: correct.

a=30 dB

b=30 dB

10 * log((10^(a / 10)) + (10^(b / 10))) =

10 * log((10^(30 / 10)) + (10^(30 / 10))) = 33.01 dB "noise"

hope this helps