Dogs hear higher frequencies better than people do. That's why dog whistles seem to be silent. Dogs hear the high frequency sounds that we can't. Dog trainers use dog whistles to get a dog's attention. Hand signals are often combined with dog whistles. Smaller dogs are more sensitive to higher frequencies compared to larger dogs. Younger dogs generally have more acute hearing than older ones. Dog Whistle for Android generates whistle frequencies from 12000Hz to 44000Hz, among which you can find one that your dog will response to, but humans are not able to hear. You may not be able to hear some of the high frequency whistles, but you furious friends can.