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In its stat block, a bat is shown to have a passive perception of 11; the standard 10 plus wisdom modifier. However, the bats ability Echolocation states that it "can't use its blindsight while deafened", which heavily implies that the bat relies on hearing for detection. This, married with its other ability Keen Hearing, which says that a bat has "advantage on Wisdom (Perception) checks that rely on hearing", and the knowledge that advantage on perception checks also translates to a +5 to passive perception makes me think that barring special cases (such as perception checks that rely on detecting a certain smell), a bat should be treated as having a passive perception of 16.

Is this correct, or is there a reason a bats passive perception would stay at 11?

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    \$\begingroup\$ Hello and welcome to RPG.SE. Please take the tour when you have a chance. Good question. \$\endgroup\$
    – Mark Wells
    Mar 2 at 22:59
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The bat usually will have advantage on passive perception checks.

This is an interesting rules interaction you’ve found, but we can prove it.

The Bat’s senses state:

Senses Blindsight 60 ft.

Next, we observe the Bat’s echolocation ability:

Echolocation. The bat can't use its blindsight while deafened.

The rules for deafened state:

A deafened creature can't hear.

This indicates that the Bat’s blindsight functions entirely on hearing. Since the Bat’s blindsight is entirely non-functioning when the Bat can’t hear, this means that any perception check involving the Bat’s blindsight is a perception check that relies on hearing.

Finally, the Bat’s Keen Hearing ability states:

Keen Hearing. The bat has advantage on Wisdom (Perception) checks that rely on hearing.

And so we conclude:

The Bat’s passive perception is 16 when using its Blindsight to “see”, since the rules for passive perception state:

If the character has advantage on the check, add 5.

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    \$\begingroup\$ I might be reading it wrong, but in the example for the bat's blindsight doesn't the lack of the parenthetical note tell us that it is not naturally blind? An example for this note might be found in the Flying Sword statblock, where it says "blindsight 60 ft. (blind beyond this radius)". \$\endgroup\$ Mar 3 at 1:58
  • \$\begingroup\$ @KogarashiKaito Maybe I’m reading it wrong, I’ll look into it. \$\endgroup\$ Mar 3 at 2:10
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    \$\begingroup\$ @KogarashiKaito You were right, but that bit wasn’t actually important to the argument. \$\endgroup\$ Mar 3 at 2:18
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It has advantage if relying on hearing

It does not necessarily have 16 because it might also have disadvantage for all sorts of reasons.

Further, it is by no means clear that if it is using its (hearing based) Blindsight for the Perception check that it is "rely[ing] on hearing". To my mind, relying on hearing means listening for noises made by the thing you are trying to perceive; not listening for the echos of sounds you made yourself - after all, you already know where you are.

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  • \$\begingroup\$ I'm not sure I understand the last sentence. When bats listen to the echoes of sounds they made, they aren't trying to find themselves: they're trying to find things around themselves. The fact that they made the sound originally doesn't change that. Similarly, just because you are carrying a torch doesn't mean the only thing you are trying to see is yourself. \$\endgroup\$ Mar 3 at 15:34
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No, because not all things are constant

You listed out the important stats:

  • Passive Perception is 11 (10 + Wisdom mod)
  • Blindsight relies on Echolocation
  • Echolocation can't be use while its deafened
  • Keen Hearing gives advantage when check using hearing

So when not deafened, it can use blindsight which gives advantage. But that's not always the case.

What if the bat is flying inside a silence spell? It still has a Passive Perception, it just can't use Echolocation and thus no advantage. So stating that it has a PP of 11 is accurate.

Also, other factors might not deafen the bat, but might nullify Keen Hearing. As a DM I would say that things like a very noisy chamber, loud music, spells that cause loud noise (such as Thunderwave) might effect it's ability for keen hearing and cancel out the advantage. Back to PP of 11.

Also, not all things perceived are audible. You cannot "hear" a secret door, a trapped chest, or a flickering light. Since these don't use hearing at all, there is no advantage. Once again, PP of 11.

So while it may be common to have everything align to have PP of 16, it is more correct to say they have a base PP of 11, but often get advantage to make it 16.

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    \$\begingroup\$ This isn't the best answer available, but why was it downvoted without comment? \$\endgroup\$
    – Dave
    Mar 3 at 22:04
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    \$\begingroup\$ @Dave, I try to not take it personally. :) \$\endgroup\$
    – MivaScott
    Mar 3 at 23:22
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    \$\begingroup\$ As you should, but a comment is still common courtesy when the reason for the downvote isn't abundantly clear. \$\endgroup\$
    – Dave
    Mar 3 at 23:27

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