The Acme Silent Whistle 535 was pioneered by the Acme Whistle Company of Birmingham, England in 1935, where it is still manufactured to this day. This whistle has an adjustable pitch that is loud to dogs, but is barely audible to humans. Effective up to 2 miles with ultrasonic frequencies ranging from 5400-12800 Hertz. Constructed of stainless steel, it has a 4 in. chain with a cover at the end to protect the mouthpiece. Perfect for wild bird hunting in big country or to have a distinctive whistle at a field trial or hunt test.