What I don't understand is if they are just trying to locate sound source or looking for other waves, such as electromagnetic ones. When I walk under high voltage trellises I always hear my head humming.
You don't 'hear' EM waves. What you're hearing near high voltage lines is coronal discharges near the conductors; The field strength is high enough to ionize the air around the conductor causing it to move around, moving air = sound.
