The system we used in my campaigns has a Hearing stat (and Eyesight, Smell/Taste and Touch stats). To determine whether a character hears a noise the GM would assign a difficulty multiplier to a hearing check and the player would roll a d100 against it. For instance, a relatively easy hearing check might have a x 5 multiplier, so the player would multiply his character's Hearing stat by 5 and try to roll under that amount on a d100.
A roll which is under the required amount and divisible by 5 (for instance, 05, 10, 15, 20 etc) is a critical success. A roll under the required amount but not divisible by 5 is a marginal success. Same logic applies for rolls over the required number, interms of critical failures/marginal failures.
The base hearing stat might be reduced for wearing a helmet or other head covering that covered the ears. Many (actually most) military characters would have their hearing stat reduced during char gen due to long exposure to loud noises.
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 "It is better to be feared than loved" - Nicolo Machiavelli
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