A:AnswerIt detects motion and you will be alerted by your smart phone not a siren box or speaker. There are sensitivity levels from very sensitive to not so much. Like you don't want to see a mouse run across the floor but you want to see a dog... etc.
A:AnswerDo not use the Dlink internet Wifi based sensor. Just buy a LED light bar with a motion sensor on it. They run on batteries and are easy to mount with two way tape.