All dogs bark, but some don't know when to stop. If your dog's barking is becoming a nuisance, this may be the solution you're looking for. When the PetSafe® Indoor Bark Control is within range of a barking dog, an internal microphone picks up the sound. The unit automatically emits an ultrasonic tone that can be heard by dogs but is silent to most humans. Startled by the high-pitched tone, the dog should stop barking. In time it will associate its bark with this unpleasant noise. When the dog stops barking, the ultrasonic tone also stops. It is effective up to 8 metres. With this system, no collar is needed. There are two units included for multi-room coverage and each is freestanding or can attach to doors, windows, tables, etc. For indoor use only, this device is for dogs of any size, has a range up to 8 metres, and is water resistant. It uses 4 x 3 Volt lithium batteries (included) and includes a good/low battery indicator. The system includes 2 indoor bark control units; 2 self-adhesive, hook and loop mounting strips; 4 3-Volt Lithium CR2032 batteries; and an operating guide.