[Ben Mugged]

Sonar is an active technology using induced sound to reflect off objects in the water. Unless they have some technology allowing them to be non-reflective to sound (not as easy as Radar) then their sub must either not be in the water or hiding against a larger reflective surface (like the bottom).

Being "silent" against listening technology is another subject entirely.

[Jacquerie]

Passive sonar is the preferred targeting method. Active sonar announces your bearing to the enemy before you have a bearing solution to them.

I suspect the Iranians obtained sound absorption or sound isolation technology from the Russians who obtained it from the treasonous Walker family of spies in the USN.