Available in collar form or as discreet stand-alone units Best Rechargeable Bark Control Collars , these devices are activated by the sound of barking. They emit an irritating noise that only dogs can hear, every time they bark. The noise, while not harmful, is so abhorrent to dogs that they stop barking immediately. As soon as your open house is over, you can turn the device off and enjoy a pleasant conversation with your pooch.