SRD:Sonic Blast (Epic Weapon Enhancement)
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On command, a sonic blast weapon emits a low thrumming hum (though this deals no damage to the wielder). On any hit, this becomes a thunderous roar that deals +3d6 points of bonus sonic damage to the creature struck. On a successful critical hit it instead deals +6d6 points of sonic damage (or +9d6 if the critical multiplier is x3, or +12d6 if the critical multiplier is x4). Bows, crossbows, and slings with this special ability bestow the bonus sonic damage upon their ammunition.