Frequency of debuff with Wall of Sound + Concussive Spell

[Zigg'dem]

Does the debuff from Concussive spell happen every time you take damage (as in a -2 every round you occupy the wall's space) you take damage from the Wall of Sound or just once at the initial damage (as in you can only take a total of -2 to the stats)?

[VRMH]

Every time you take damage, the "number of rounds equal to the actual spell level"-clock starts all over again. But the penalties do not stack.