Shepherd's whistle
They are also produced commercially. Simple whistles may cost less than 1 USD in bulk; expensive ones may be quite ornate. They are commonly made of plastic, aluminum, stainless steel, silver, brass, titanium, Corian, jade, buffalo horn and other materials.
Joseph Hudson (inventor)
Hudson entered a competition held by the Metropolitan police force in London in 1883 to design a better way of attracting people's attention. He won a contract to supply the police with their new devices, a small but loud 'whistle'. Prior to this time the police force had to rely on hand rattles and whistles were only thought of as musical instruments or toys. His whistle is still used by the force and many others worldwide.