Not your father's whistle.
But ultra sonic unidirectional sound cannons. These things are truely amazing.
"Norris prods his assistant to locate the baby noise on a laptop, then aims the device at me. At first, the noise is dreadful -- just primally wrong -- but not unbearable. I repeatedly tell Norris to crank it up (trying to approximate battle-strength volume, without the nausea), until the noise isn't so much a noise as an assault on my nervous system. I nearly fall down and, for some reason, my eyes hurt. When I bravely ask how high they'd turned the dial, Norris laughs uproariously. ''That was nothing!'' he bellows. ''That was about 1 percent of what an enemy would get. One percent!'' Two hours later, I can still feel the ache in the back of my head."
It's been suggested that at a somewhat higher level, ultra sonic sound could liquify organs from a few miles away. We wouldn't want to set it that high, maybe just enough to cause a nose bleed and for them to soil their pants while writhing on the ground.