Police helicopter Captain reported a NMAC event when a medical helicopter on a collision course did not respond to multiple calls to deviate heading. The Police helicopter took evasive action to avoid a collision.

Narrative

I was PIC of Aircraft X orbiting a police scene at 1000 ft. AGL approximately 9 miles SW of ZZZ outside of the Bravo [airspace]. I observed on my Foreflight Ipad Aircraft Y; a Bell 407 helicopter operated by Company A approaching me from the Southeast at a distance of 3 miles and at 1100 ft. AGL. I immediately announced my position on the local helicopter frequency (XXX.XX). Aircraft Y did not deviate his flight path and continued straight at my position. I again announced my position when he was 2 miles to my South/Southeast with no response. When he was approximately 1 mile from me; I announced again with no response. At this time; I took evasive action and quickly descended 300 feet as Aircraft Y flew directly over the top of me. He never deviated his course; never announced his position; nor did he respond to my numerous calls on the local frequency. As he approached ZZZ1 Class D; I switched over to that frequency to listen. Aircraft Y called up the ATC of ZZZ2 and asked for permission to transition through the Delta [airspace].