I purchased these along with a swann video surveillance system, kind of an after thought. They are very easy to install, very loud and work exactly as advertised. You simply hold them in you propsed installation place with the unit switched to on, and if you don't hear a siren they are close enough, then to test, just move one or the other away, and siren will sound, then clean surfaces, remove tape backing and stick on. front cover for batteries comes off easy and has on/off switch and test button. takes 4 watch style batteries. not sure how long they will last, but if it is not going off all the time I image a long time.
One word of caution, if you install on a door, check door opening clearance first, I installed on my molding and when i opened the door they rubbed and stopped me from opening the door, i had to remove and reposition like 1/8 inch so door would open.....REMOVING once installed is a pain in the rear, the doublesided tape is VERY STRONG, so be sure of placement before installation. Also, you only get one set of tape, so if you mess up, you have to go buy doublesided tape and cut to fit. (i did and it worked just fine)