For a decent idea of how much the PK3 isolates, put on some ipod earbuds... or any earbud for that matter

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The lack of isolation does have a big advantage for me though. I tried running with the PK3 today and found I liked it more than running with an IEM. Running with IEMs isn't very good for me. The sounds of my breathing, heartbeat, and feet pounding on the track are extremely loud. The PK3 didn't suffer from any of this. Of course, I had to poke my fingers at the PK3 every now and then to keep them in place, but I had to do that when running with IEMs anyway.