The whistle is about two inches long and bullet shaped. While manufacturers contend deer can hear the whistle up to a quarter mile away, no published research verifies the device's effectiveness. Georgia's Game and Fish Department, for example, found that in hundreds of observations from vehicles equipped with deer whistles, deer didn't respond. Whistles on vehicles going 25-30 mph produced no ultrasonic sound, although some ultrasonic and lower frequencies were produced when the whistles were blown by mouth. According to wildlife biologists at the University of Georgia, neither deer nor humans can hear ultrasonic sound.
Whistles blown by mouth near captive deer produce no response.
Moreover, deer would only be able to hear the whistles if there were a straight shot between the deer and the whistle. If curves, trees, or other obstacles came between the deer and the whistle, the device would be ineffective. Finally, Washington State University researcher Leonard Askham felt the evidence tended to favor a conclusion that deer whistles do not work. He taped the barking of his neighbors' dogs, rigged an amplified speaker to the front of his truck and then broadcast the tape as he cruised down highways. But he abandoned the scheme, amid concern that the barking was not only scaring deer, but awakening residents of southern Arkansas.