It activates whenever it detects barking nearby.

Stop Dog Barking: Best Anti-Barking Device Reviews | Hounds 101
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Even though the Dog Silencer™ can "hear" barking up to 75-ft. away, the corrective ultrasound emitted from the unit can actually travel up to 300 feet in a clear, straight path. If the dog is 100 feet away, for example, his barking probably won't activate the unit automatically. However, pressing the remote control will activate the Dog Silencer™ manually and the dog will still be able to hear the corrective sound.

If for some reason Rover is outside the Dog Silencer's 75 foot sound sensor range, you can manually trigger the device with the simple press of a button. The remote control can be used from inside your home, car, or even in your pocket! As an additional option, you can turn the Dog Silencer™ microphone sensitivity all the way down and solely activate your Pro using the remote control. Not only does this special convenience allow you to train dogs more than 75 feet away, but you can customize your own dog's training to include staying off furniture, away from the trash and anything else you want to teach your dog to do. The key chain remote can be used up to 75 feet away from the main unit.
The only ultrasonic bark control device equipped with remote control. This is the perfect trainer if you have a problem with a barking dog!
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The Super Bark Free Pro Series is an innovative and advanced dog training device designed to stop a dog's annoyance or boredom barking habits. The new Super Bark Free Pro Series features a specially designed sound selector that lets you choose ultrasonic frequencies, which can only be heard by dogs, or lower sound tones audible to human ears. Both sounds have been shown effective in training dogs to stop barking. The Audio Activation Sensor can detect a dog's barking up to a 60 ft. radius from the unit, and with the sliding sensitivity control and audible sound output selector you can custom adjust the internal pick-up microphone to the sound of dog barking. Super Bark Free Pro Series takes four "D" size batteries (not included) for remote operation and can alternately use the included A/C adaptor with a 50 ft. low lead cable, so you aren't dependant on just battery power. Your Dog Silencer package includes a tiny keychain remote control - the only bark control device in the industry to include such a luxury
Photo provided by FlickrNew For 2 dogs remote control dog training collar bark stop collar electric shock device treatment ring stop pet dog barking collar
Photo provided by FlickrDog Silencer™ detects barking up to 75 feet away and emits ultrasonic sound to curb annoying barking. The Dog Silencer™ is weatherproof and one of the only anti-barking devices on the market to include a remote control.
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Indoor and Outdoor devices to stop dog barking – with or without a collar. Powerful ultrasonic sound, automatic bark detection and even remote control options. Our free-standing units are great for your own dog, and even better for the neighbor's dog!They don't seem to work worth a damn, but remote sound-emitting anti-barking devices are, essentially, noise-blasting gizmos, fueled either by batteries or by AC power. The devices contain built-in sensors that respond to a nearby barking dog by releasing a piercing, high-pitched sound that some canines find intolerable. Your Dog Silencer Pro package includes a tiny keychain remote control - the only bark control device in the industry to include such a luxury. If for some reason Rover is outside the Dog Silencer Pro's 75 foot sound sensor range, you can manually trigger the device with the simple press of a button. The remote control can be used from inside your home, car, or even in your pocket! Your Bark Control Pro also comes with an exclusive keychain remote control. If for some reason the barking dog is farther than the unit’s 50 foot sound sensor range, you can manually trigger the device with the simple press of a button. The remote control can be used from inside your home, car, or even in your pocket!