Although the Ultimate AT uses Electro-Vibrawave technology inside the walls of your home, the high-pitched ultrasonic sound that fills the inside of the room will not go through walls. This is why we recommend at least two to three units per floor for best results. Units can be placed in different areas of your home, depending on your building's floor plan and where the pests are a problem. By using multiple, strategically-placed units throughout your home or office, you effectively distribute the ultrasonic pest repelling sound throughout the structure. This optimizes your overall electronic pest control. Although the electromagnetic feature will travel throughout your home wiring with the use of one unit, it has been our experience that using more than one unit simultaneously presents the greatest results. One unit is better than no units, but multiple units will yield more powerful results than one unit alone.