The silent whistle that you blow to get your dog's attention isn't actually silent -- it simply plays a note that you can't hear. A dog's hearing works differently than a human's, and can detect sounds that our ears simply cannot. The tweet of a dog whistle is silent to you, but to your dog, it's anything but -- that's why it's so often used as a training tool.
Every whistle is made out of a tube -- the shorter the tube, the higher the pitch of the whistle. This is why dog whistles are short, and why they seem silent to humans. When you blow the whistle, it creates a sound so high-pitched that it doesn't register to human ears. The pitch is high enough that the average dog doesn't typically hear it unless it's being produced by a whistle, so when you blow the whistle, it catches his attention. In fact, blowing it too hard or too close to your dog can hurt his ears -- if you're interested in using a dog whistle, talk to your vet or a professional trainer for advice for responsible use first.
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