I mean, if a component emits 15db and another emits 20db, the noise won't be 15+20=35db? it may be just 20db (with the other 15db being "covered") ?
15dB + 20dB is not 35dB. 15dB + 20dB is 21dB. Why is that? Because dB is a logarithmic scale of relative values. 10dB = 10X, 20dB = 100X, 30dB = 1000X, and so on. To make matters more confusing, dB in terms of sound is a measure of intensity, not perceived loudness. 10X intensity only translates to 2X perceived loudness. Also, as NyteOwl notes, the sounds have to be perfectly in phase to be perfectly additive. How in phase the sounds are is of course completely dependent on listener position. This means that in real life, two noise sources are likely to always have a composite intensity at the listener position that is less than the combined value of each individual sound. Finally, human hearing is a very complicated system and you actually have the ability to "tune in" on sounds that are below the ambient noise or covered by the noise of other sounds. This tuning in process can happen subconsciously. So, to sum it all up: Yes, louder sounds and or ambient conditions can cover up some other sound. However, that even if the new sound is covered up for the purpose of measurement with s sound meter, that doesn't mean you won't hear it!