The caster would whisper a cacophonous sound that only the target could hear. It caused damage to the target and if they couldn't resist the spell they had to move away from the caster. If the target did resist the spell, then they only took half the damage and didn't have to retreat. If the spell was cast with a higher spell level, then the spell caused more damage to the target.
A target that couldn't hear would not be effected by the spell.
Only a vocal component was needed for the casting of this spell.