Variable frequency technology - 2 usage modes for choosing:? -standard frequency,? -Variable frequency, the WWVVPET dog training device can emit the high ultrasonic sound of various frequencies, which can attract the dog's attention very well. Avoid being accustomed to listening and stopping responding to the ultrasound at a constant standard frequency. EASY TO USE AND EFFECTIVE - If the dog is barking or causing mischief, simply push and hold directly in its direction for 10 seconds or switch to a variable frequency ("?") if the dog is not responding . If the dog responds positively, praise and reward it to help strengthen its memory. SAFE FOR YOU AND YOUR DOGS - Unlike other electroshock dog training devices, the ultrasonic bark control device uses the latest ultrasonic technology with frequencies from 25 to 50 kHz. The sound does not affect humans (hearing limit of 20 kHz), but it can easily capture the dog's attention and stop their bad behavior. Consistent training and appropriate rewards are necessary to achieve the desired outcome. EFFECTIVE DOG CART CONTROL DEVICE - Ultrasonic bark control device provides a safe and effective solution to help owners control unwanted barking and stop some unwanted dog behaviors such as barking. excessive barking, digging, chasing, poor feeding or fighting, etc. Works well on many sizes of dogs (except dogs under 6 months and aggressive dogs).