PETNOVO - Stainless Steel UltraSonic Best Dog Training Whistle with Adjustable Frequencies Stainless Steel Whistle Adjustable pitch whistle simply tighten or loosen the screw to work out your dog's frequency. It is lightweight and may be worn on a key chain, or on a string or lanyard around neck. It is a safe and highly efficient product for pet education This whistle is audible to the human ear which is essential to gauge its efficiency