Just as humans living near trains or airports can become desensitized to the sound, dogs can become desensitized to the ultrasonic frequency after prolonged exposure to it. You can take steps to avoid this. Periodically turn the unit off, move it to a new or slightly different location. You can also leave the unit off for a day or two and then turning it back on. It may also help to turn the unit off during periods of unwanted barking.