Years ago there was a thread here that talked about how lower pitched whistles can be heard farther than higher pitched whistles. I'll save you from having to search for the thread. It was determined that lower pitches can be heard farther. The example given was the car with the customized sound system. You know the one. The one that thumps bass all around the neighborhood and nearly shatters everyones windows. You can hear this car's bass for a long time before you ever hear any of its mid range speakers or its tweeters.

So since I knew that lower pitches can be heard further I kept an eye out for something that can produce a low pitch for use in an emergency kit. And I think I may have found two.

The Air Blast Horn.
It's an air horn that doesn't use a compressed air can. You blow into it.

Here's a do-it-yourself version.

Not sure if this will replace the whistle in an emergency kit, but it's good to have options.