What all the scientists agree is that dogs can hear higher frequency sounds (35-65+KHz) than humans which can hear only around 20KHz. There are lots of high frequency sounds in the world that dogs live in and many of the prey they enjoy hunting for emit these type of sounds such as small animals like rodents and bats.
Dogs have more than a dozen ear muscles ( actually thought to be around 18 in many textbooks) that help us not only understand how they are feeling but they have a function too. They use them like antennae and they can tilt their ears, go up or down and can hear sounds at four times the distance as us humans. It is like having radar on their head and our canine family members love to use them!
What is even more amazing about dogs is that they can hear with each ear independently. If you have ever thought 'I am teaching my dog a new trick and suddenly they run off after something' !!. They have either smelt it or more likely heard it! They can be listening to you with one ear and hear that small animal sound with the other ear. Smart dogs!! Dogs are flooded all the time with smells and sounds and these skills equip them perfectly for the search and rescue roles many perform and love doing. It all comes naturally to our dogs!
Finally, I am often asked about silent whistles and do they work. High-frequency sounds are more susceptible to degradation over distance , so these types of whistles might not be a good choice for recalling a dog at a distance. They also do not work well in highly populated wooded areas. But I think there is a more important consideration here when using products of a high frequency in that some dogs are frightened by very loud noises such as gunshots, fireworks and thunder. Carefully consideration if you are going to use this type of whistle as to the fact that they are emitting a very powerful sound that humans can't hear but dogs can .