Listening (DMG)

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One of the useful tricks that smart adventurers learn after a few trips into deadly dungeons is to pay attention and listen for strange noises. Noise is a valuable clue, alerting characters to possible danger and even occasionally giving them a definite picture of what dangers they face. After rashly bashing down a door only to discover a barracks full of unruly orcs, the player characters may find it more prudent to stop outside and listen before trying the same stunt again.
All characters have a percentage chance to hear noises, the percentage varying by race, as listed on Table 83. This ability is equal to that of a 1st-level thief (however, thieves can choose to increase this score). This is not the character's chance to hear someone talking to him or the tolling of the city watch's bell at night. This percentage should be used only when hearing is difficult or there are extraordinary circumstances involved.
 Table 83: Chance to Hear Noise by Race
Dwarf 	Elf 	Gnome 	Half-elf	Halfling	Human
15% (3)	20% (4)	25% (5)	15% (3) 	20% (4) 	15% (3)
The percentage chance is followed by a number in parentheses. The second number is the same chance on 1d20. You can either make a percentile check or roll 1d20, whichever is most convenient. In either case, a roll equal to or less than the number on the table means the character hears something.
Of course, the chance to hear noise given above represents more or less optimum conditionsóhelmet off, not moving, and all others remaining relatively still for one round while the character stands and tries to hear noises carried on the breeze or down a hallway. Under such conditions, the character will get a relatively clear idea of the nature of the noise—animal grunts, slithering, speech (including language and race), and perhaps even words.
Less than perfect conditions don't alter the chance to hear (which is low enough) but can affect the clarity. Some, like the muffling effect of doors or the echoing of stone passages, may still allow the character to hear a noise reasonably well, but may prevent precise identification.
In some situations, a character can hear muttering, growls, panting, or voices, but may be unable to identify the issuer of the sounds. The character would know there is something ahead, but wouldn't know what. In other situations, the chance to hear anything at all may be affected. Extreme cases can give you the excuse to provide misinformation. Guttural speech may sound like growls, the moaning wind could become a scream, etc.
In some cases a check is necessary even when the character is not attempting to discern some unknown noise. The character tries to hear the shouted words of a pirate captain over the raging storm. He can see the captain and can clearly tell the man is speaking. Indeed, the captain may even be speaking to him. However, a hearing check should be made to find out if the character can make out the captain's words over the fury of the storm. If the character were a little closer, the storm a little less, or the captain's lungs exceptionally strong, the character's chance of success would be increased.
In all cases, hearing a noise takes time. The amount of time spent listening to the captain is obviously the time it takes him to speak his peace. Standing and hearing noise in a corridor or at a door requires a round, with the entire party remaining still.
Furthermore, a character can make repeated checks in hopes of hearing more or gaining more information. However, once a character fails a check, he will not hear anything (even if he immediately makes a successful check on the next round) unless there is a substantial improvement in the conditions. The group will have to move closer, open the door, or take some other action to allow a new check.
If a check is successful, the character can keep listening to learn more. This requires continued checks, during which the player can attempt to discern specifies - number, race, nature of beast, direction, approaching or retreating, and perhaps even bits of conversation. The player states what he is trying to learn and a check is made.
Trying to overhear things this way is less than reliable. Thieves should not be allowed to use their hear-noise ability like super-sensitive microphones!