Ultrasonic waves are high-pitched sound waves at frequencies too high for humans and most pets to hear. These ultrasonics are thought to be extremely uncomfortable for spiders and crawling insects, causing them to leave the room space. However, ultrasonics waves are like ordinary light waves - they will not penetrate solid objects such as walls and cupboards. Which means that ultrasonic devices can only be effective in the room you plug the device into, meaning that the spiders could hide in the walls, ceilings and in cupboards and behind curtains. Hence the second technology - electromagnetic waves.