This is a listing for one dog whistle. How about using it for training your dog? The color is black, and the size is approximately 7.5cm. When you blow the whistle, ultrasonic waves that are inaudible to humans are emitted. Removing the black cap reveals an adjustment screw. Turning this screw allows you to change the frequency, and the volume of the sound audible to humans also changes accordingly. However, it's not possible to adjust it to a frequency that is completely inaudible to humans. You will still hear the sound of the whistle blowing. By the way, as far as I know, other dog whistles are similar, and if you raise the frequency to the point where humans can't hear anything, it won't travel as far or its effectiveness will be greatly reduced by obstacles, so it will overlap to some extent with the human hearing range. You can train your dog by establishing signals based on the number and length of the whistles. Made of metal, it is resistant to scratches. The product will be sealed in a plastic bag and then shipped in a brown envelope. I also sell other dog whistles and pet supplies. Please search using the following hashtag: #harumi_pet