The 535 whistle has a variable single frequency between 5200 and 12800Hz to enable the user to choose and set the exact frequency required. This is suitable for all breeds of dog for close/medium and distance work. Although very loud to the dog, this sound is not able to be heard by the human user hence the "silent" label. However the whistle is engineered to produce a relatively quiet whistle sound audible in the human range the sole purpose of this is to reassure the user that a sound in the required range is being produced. The reason for this is that if the whistle is incorrectly set up when selecting the frequency then it will not function properly without this facility the user will not realise the whistle is not working.