KEKU Dog Training Whistle - Two Pack - Stops Barking - Dog Training Aide - Adjustable to Lock in Ultrasonic Frequency A dog whistle - two pack - so you'll always have one when needed. This offer - exclusively through Amazon - includes a second whistle as a bonus versus one being offered on most sites. The whistle is the same but you get two for the price of one. This whistle, like all silent whistles, is not completely silent. You will hear it when you use it. All whistles produce a secondary audible sound that is caused by the air that goes across the whistle. Blowing lightly on the whistle reduces the audible sound, and actually increases the ultrasonic sound that your dog hears. Used by many for stopping barking or unwanted behaviors. Can be used to train your dog to come, sit, or other commands. Simple and easy to train your dog with. Durable design - no breakable parts. 100% satisfaction guarantee with shipping by Amazon. Adjustable frequency. Easily find the optimal sound for your pet.