I measure the ambient sound level at an outdoor location and find it to be 49 decibels. An additional source of noise is introduced to the location (say 20 children playing), and after re-measuring the sound level find no increase in the measured sound (ie. it's still 49 decibels).

What I would like to know is if it is therefore valid to conclude that because there was no measureable increase in sound levels, does it mean that the seoondary source of noise (20 children playing) must be at least 10 decibels lower than the original noise level??

I'm aware that an increase of 10 decibels represents a doubling of the audible sound level, and that doubling a noise source will produce an increase of 3 decibels in the sound level. However I don't know enough about acoustics to determine if the above scenario and resulting conclusion is valid or not.

Any help, or suggestions on how to find an answer would be greatly appreciated.