Decibel levels

[Kralizec]

Originally Posted by shake307

I was never a submariner. Only digitally since playing Seawolf SSN-21 in the 90s. I do know that submarines are always striving to be quiet. So, perusing the unit reference in Cold Waters, I see a Flight 1 LA Class is 115 decibels (if I recall correctly, I am at work and not playing). That is as loud as a rock concert or night club! These subs can't be that loud, can they? I get pumps are running and machinery is making noise, but damn, that seems really high. I was on an aircraft carrier and a destroyer and they weren't that loud, except during flight ops, not pier side though. Am I missing something?

You are comparing apples and oranges, and not just because of the different micropascal references!

At the rock concert: you are in the audience and 115dB is the measured level. I'm not saying that is incorrect, that is the actual pressure that an SPL meter shows you that you hold in your hand (wear earplugs please)
But the 115dB sound pressure in the case of the LA Class is her Source level. The difference between the two numbers is the distance: source level is defined at 1m distance, and sound level drops 6dB every time the distance doubles. This is called Transmission loss.
So let's turn this into an apples vs apples comparison (or if you prefer, oranges vs oranges):
At the rock concert, let's say you are 8 meters from the loudspeaker. That means the distance from 1m doubled three times and each time the sound got 6dB weaker. We can calculate from this that the source level of the loudspeaker is 115+18=133dB
Turn this into water decibels by adding 26dB and we get 159dB(re 1μPa). Now that is a LOT louder than the 6900 ton nuclear attack submarine! Big thing designed to be quiet vs small thing designed to be loud. Isn't engineering the best?

Or compare them the other way: the propulsion and the machinery are at the stern of the LA class, and the spherical array at the bow. The distance between them is about 100m. Depending on the position along the hull where you measure, the noise level will be different: 6dB less every time the distance doubles, or generally speaking, 20×log(R). That is 40dB transmission loss just from distance, baffles come after that. So now we can compare: 115dB, minus 40, minus 26. That is 49dB(re 20μPa), which is a lot less loud than what you hear at a rock concert. It's about as loud as a comfortable conversation about submarines between friends who don't disagree which sub is best.
If you were a diver and an LA Class came up behind you, I genuinely wonder how soon you'd hear her. I don't have the faintest idea what the threshold of hearing is when one is SCUBA diving.