Directional microwave beam generator. Something like 5kw or so concentrated into a beam no more than a few inches thick. Pretty sure you'd be able to blow up somebody's head with that, even through glass or other materials that don't absorb microwaves well. Would be fairly quiet at the death end too. The wep itself would probably require some cooling fans though.
Could probably build one fairly easily with some knowledge of how magnetrons work or with an industrial one for some kind of huge industrial microwave.