This spell causes a blast of light and sound which can cause creatures to become stunned, perhaps also blinded and deafened.

You choose a point within range and each creature within a 10ft radius must make a Constitution saving throw. On a failed save, the creature is affected according to its Hit Dice.

2 HD or less - The creature is blinded, deafened, and stunned for 1d4 rounds, then blinded and deafened for 1 round, and then deafened for 1 final round. 

3 or 4 HD - The creature is blinded, deafened and stunned for 1d4 rounds, then blinded and deafened for 1 round.

5 or more HD - The creature is blinded, deafened and stunned for 1 round.

Sightless creatures with no ability to hear are not affected by flash bang. A silence spell removes the deafened effect and a darkness spell removes the blindness. If flash bang is cast into a area that has both darkness and silence in effect then it is countered.