What grandso what to know, what are dog whistle are for?

Answers:
The whistle is a "silent whistle". It is used to give commands to dogs over distance just as any other whistle is used. It is called a "silent whistle" because it operates on a frequency above that of the average person's hearing. Most people will hear a faint sound. In other words it is very LOUD to dogs but very QUIET to people.
It is not a "silencer" and does nothing to make a dog stop barking nor will it repel an agressive dog.

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People use them to get a dogs attention but in most cases if you properly train or socialize a dog it will come to the sound of your voice.

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I think your asking what dog whistle are used for, Right?
They are used for training a dog. The dog is trained to respond to the way the owner or trainer blow's the whistle. 2 short whistle blows may mean go right. 1 long whistle blow many mean stop. See! Hope this answers your grandsons question.

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I have no clue what you mean in the first part of the question, do you mean my grandson wants to know what a dogs whistle is for?
Dog whistle
From Wikipedia, the free encyclopedia
A dog whistle (also known as silent whistle or Galton's whistle) is a type of whistle used in the training of dogs and cats. It was invented by Francis Galton. The frequency range of a dog whistle is largely out of the range of human hearing. Typically, a dog whistle is within the range of 16000 Hz to 22000 Hz with only the frequencies below 20000 Hz audible to the human ear. Some dog whistles have adjustable sliders for active control of the frequency produced.
Depending on the way the whistle is used, a trainer may simply gather a dog's attention or inflict pain for the purpose of behavior modification.
The name dog whistle is often used for both lung-powered whistles as well as electronic devices that emit ultrasonic sound via piezoelectric emitters. The electronic variety are sometimes coupled with bark detection circuits in an effort to curb barking behavior.
These kind of whistles are also used to determine the hearing range of people or for physics demonstrations.
Some political observers have used the phrase "dog-whistle politics" to refer to the use of words and phrases that one's political allies will recognize and appreciate but that may not mean anything to other listeners.

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Dog whistles are used as training aids for dogs. I think that is what you are asking!

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calling dogs -- the frequency is too high for human hearing,
but a dog does, and usually don't delay in responding.