I opted for the Auto model after seeing one at a friend's house. In background mode it is virtually silent, yet when the very sensitive particle detector calls for more filtration, there is no mistaking whether it is working. My woodstove released a puff of smoke this morning, which kicked the filter into action within about a minute (across the room); it ran on high for less than 5 minutes before dropping back to background. We are very pleased with the purchase!