I usually carry a horn on my plane. One of the can-horns that boaters and sport fans use. I've done some tests and wish to share the following observations:
1. My wife, even when standing in a parking lot and listening closely, cannot hear it from the ground 1000 feet below as I circle, blasting it out the window on my Cherokee while trying to "fly casual".
2. The guy walking across the apron at the local airport, however, can, and the reaction of being honked at by a taxiing aircraft is amusing. The facial expression is what you'd expect from, say, a dog that has encountered a barking cat.
3. When my 5 year old sets it off inside the cabin, it is very audible. Also, doing a zero-g pushover to try and get him to stop just makes him press the button harder as he floats up out of his seat.
Finally,
4. While not every air horn is built with manufacturing defects, the one that is will fail at about 11PM at night on a cross country flight over rocky terrain at 7500 feet, and the failure will be loud and sudden in the form of an explosive BLAM!
I now carry auxiliary pants as part of my Minimum Equipment List.
Next experiment, a train horn from eBay driven by a portable air tank in my back seat.
