Are there any rules anywhere relating to sound cover?
By this I mean, when perception is entirely dependent on hearing, how much background noise is needed to provide "cover"?
If not, does anyone have any house rules they wish to share (this thread should probably then move to advice)?
My take would be if the background noise is higher than the noise being detected then the noise being detected has the equivalent of total cover.
That would mean, though, for example, that invisible creatures should be able to wander around a combat without ever being heard.
What do you think?
Always assumed, all this stuff was taken into account with the invisibility stealth bonus (+20 when walking around, +40 when standing still) With +40 a clear indication that standing still and not making any noise grants even more bonuses.
Perception checks take into account all senses.
You could give an invisible creature or someone circumstantial bonus, which isn't really a house rule but something every dm can decide depending on situation, like an invisible creature on a battlefield walking around would be virtually impossible to spot/hear.
I'm not extremely knowledgeable about the rules but the perception skill does have modifiers for background noise with the unfavourable and terrible condition modifiers. Alongside those in most cases a creature or person experiencing such conditions would also likely be distracted making the DC even higher. So while the rules don't cover someone being unhearable due to loud noises(as far as I know) you're still looking at some considerable potential perception check DC increases alongside being invisible.
Using your combat example I'll take 6th level Duergar rogue sneaking in combat with his Invisibility SLA. Before invisibility he's likely got somewhere around a +14 if not better stealth check(6 ranks +3 class skill, +5 dex). With invisibility he's got a +34 while moving so even just using a 10 for the roll in this example he's got a 44 stealth check. On top of this the sounds of battle would likely offer a +2 to the perception DC, and being in combat it's also safe to rule that everyone is distracted for another +5 to the DC. So that's a DC 51 perception check to notice the Duergar rogue before distance is accounted for meaning that outside of special abilities, spells, or some very specific builds he's not getting noticed by a CR appropriate threat.
Even without a character built for stealth though you're still unlikely to be noticed in most situations. Take a 6th level wizard with no ranks in stealth in the same situation. You've got d20+20+dex mod for stealth and then another +7 before distance to the DC for bad conditions. Even someone pretty dedicate to perception at a similar level would have a difficult time noticing the wizard. For example a 6th level half-elf monk with Skill Focus Perception would have a +19(6R+3CS+5Wis+2Race+3Feat) or so perception check which is I believe a 10% chance to succeed with no positive dex on the wizard.
With all that I personally wouldn't houserule any further because it's already more than hard enough to notice an invisible opponent most of the time, and that's only when they're moving around.
Looking over this before posting and I realized I wrote more than I thought I would o.O
I don't think that the level of ambient noise is taken into account anywhere that I can think of.
I see this as being a different thing to "distraction". It's like the sound equivalent of cover.
If you're sitting in a quiet room, you can hear a pin drop. If you're in the middle of a heavy metal concert, you will never hear a pin drop. That't not because you're distracted, it's because the background noise in the convert "covers" the sound of the pin.
Ambient noise can make the difference between obvious and impossible. It's got to be worth more than a +2 circumstance bonus.
Has it not?
+2 circumstance bonus is by the rules, indeed. If it is not enough, consider giving the opposed perception checks a penalty due to the situation. Of course as the dm, if you want to include realism for ambient noises in your campaign feel free to do that and adjust the bonuses as you see fit.
Your rock concert example would be closer to terrible conditions which increases the DC by +5. I say this because one of the examples for this is a dragon's roar which while not completely comparable are both extremely loud. It is true normal people wouldn't hear a pin drop in the middle of a rock concert because it's likely a base DC 15 perception check to hear one and with a +10 to that you'd get a DC 25 check without distance which if you go by the idea that most people in real life are roughly equivalent to 1st and 2nd level characters it's easy to see that that's out of range of most normal human beings.
Something that has to be remembered though is that Pathfinder is fantasy and that you can have characters with a +15 perception who can routinely hear your entire conversation from 150ft away if there's nothing in the way.
In my opinion if you do feel that the difficulty has to be higher then I suggest maybe add two more tiers of conditions at +7 and +10 to the DC respectively to represent extremely bad conditions. I would not make it so that certain sounds make it impossible to hear others unless its by magic or the sound is literally deafening.
If you do end up going with your idea that sound can cover up things make it clear to your players with some form of guideline so that they can know. Also expect any stealth players to abuse the heck out of it when they can.