SPRENGER Dog Training Whistle-The human ear can hear only a quiet high pitched note,but a dog can pick up this sound at a distance of up to 400 yards (depending on direction and strength of the wind). The tone can be adjusted by turning the screw after removal of the protective cover. During training it is important the same pitch of note is always maintained.This can be achieved by use of the locking nut fitted on the adjustment screw.The plastic cover will protect the whistle from dust when carried in a pocket.