When I was a kid, one of my friends had a silent dog whistle. It was a small device made out of brass with holes at both ends and an additional hole in the side. When he blew into the whistle, we could hear nothing. He claimed, however, that dogs could hear the sound of the whistle. I don’t remember any practical application of the device. Several of us were allowed to blow through it and none of us could hear anything except a small amount of air going through the tube.

Now that I am older and have access to the Internet and a search engine, I got a bit more specific information about high frequency dog whistles. It is true that the range of sounds that a dog can hear is different from that of humans. The maximum upper sound frequency range of human hearing is about 20,000 Hz for children. By the time a person is around 30 years of age, hearing declines to about 16,000 Hz. The top end of a dog’s hearing is about 45,000 Hz, with some variation depending on the breed of the dog. Most dogs can hear sounds that are so high-pitched that humans cannot hear them. In general, smaller breeds of dogs can hear higher pitches than dogs that are larger.

Human speech generally ranges from 250 to 8000 Hz. Humans have evolved to be most sensitive to the sounds of other humans speaking.

An article in Psychology Today put it into a practical example: “If we wanted to extend the right side of a piano keyboard to the limits of human hearing, we would have to add 28 keys. If we extended the keyboard to match a dog’s hearing, we would have to add 52 keys . . . The last 24 of these would produce sounds that were so high-pitched that humans cannot hear them.”

In addition to being able to detect odors that humans cannot, dogs can hear sounds that we cannot. Scientists believe that these special adaptations evolved to improve the hunting ability of dogs. Being able to hear high-frequency noises made by the small prey animals they hunted enabled dogs to capture more food than those who could not hear those sounds. In mammals, there is a physiological correlation between the size of the skull and the frequency of sounds that can be heard. The smaller the skull, the higher the frequency range of hearing. Thus small dogs can detect sounds that are beyond the hearing range of larger dogs. Smaller mammals, such as mice can detect sounds that are even higher.

I was rummaging around the Internet reading articles about what dogs can hear that humans cannot in the middle of the night because my sister’s dog, who is staying at our home for a few weeks, will bark an alert when I have no idea what is causing the dog to react. There is no sound that I can hear, and no visual clue that I can see that will explain why the dog has barked. I was accusing the dog of seeing ghosts, but it is more logical that the dog is hearing sounds that I cannot hear.

The dog can hear the approach of a large truck before I am aware that there is one in the neighborhood. The information about dogs being able to hear high pitched sounds, however, doesn’t seem to correspond to the approach of a truck, because we associate lower pitched sounds with a truck. A diesel motor has a lower pitch than the motor of a car. My theory is that what the dog is hearing is not the sound of the motor, but rather the sound of the brakes. A large truck has air brakes and air escapes the system when the brakes are applied. We can hear the whoosh of escaping air when we are close to a large truck, but a smaller escape of air when brakes are tapped may emit a higher pitch that carries for a longer distance - a sound that we cannot hear, but a dog can.

Since lower frequencies travel farther that higher frequencies through a medium such as air or water, a dog whistle must emit a sound that is not only in the frequency range above 20,000 Hz, it also must be loud for that sound to be heard from a distance. The use of a dog whistle to call a dog or to make a dog stop barking requires the skill of a specialized dog trainer. A system of rewards has to be instituted that gives the dog incentive to respond to the sound. The use of a “silent” dog whistle requires the same discipline, consistency, and practice as do other dog training signals such as voice or hand commands.

All of that adds up to the result that a dog, no matter how beloved, who is in our home as a visitor for a short amount of time, will respond to sounds that we cannot hear. It is unlikely that I will learn enough about training dogs to correct the dog’s barking during the span of his visit and even if I were able to train him temporarily he likely would forget the training in the long periods of time when he is not in my home.

Another field of speculation about which I was unable to find much solid information in my basic Internet search is about the dreams of dogs. I’m pretty sure that this dog dreams when he sleeps. He will occasionally grunt, whine, or even bark when it appears that he is sleeping. Since he snores, I’m pretty sure that these sounds are emitted when he is asleep. My theory is that a dog dreams in a manner that is similar to humans. In his dreams there are memories of sounds that are perceived as being real even when there are no external sounds to stimulate the memory.

The information I have isn’t very helpful when it comes to the fact that the dog will occasionally wake me when I want to be sleeping. I’ll probably continue to say that the dog hears ghosts and know that his visit is short and he’ll soon be back in my sister’s care. Who knows? Maybe she believes in ghosts.