Holy Bluetooth, Batman...!
I have the latest Interphone unit on my helmet...the 5, I think. It's deficient in many respects, and the battery is on its way out after a year. However, I recently learned the BT power is substantial.
I live in a very urban area by most standards; no skyscrapers, but a continual sprawl of 2/3 story building, both residential and commercial.
I recently left for work after shuffling around luggage and gear. I connected with my iTunes on my iPhone as usual and went on my way. When I almost a couple kilometers from my home, the music suddenly cut out. I tried to troubleshoot this, but the unit was acting as if it were not connected to the iPhone. I pulled over and went to get the phone out of my usual pocket to see what the issue was...and was shocked to find the phone was not there. I drove back home, and it was still at my house, next to the bike parking area.
I was blown away, the headpiece received the iPhone music signal for well over 1.5 kilometers...how can that be?