The Movement Sensor with Sound Monitor detects your baby's slightest movements, even while she sleeps. While it may seem that your baby is perfectly still when she is sleeping, she really isn't. An alarm will sound alerting you to check your baby only if she becomes absolutely still for 20 seconds and no movement whatsoever is detected. The under-the-mattress Sensor Pad detects your baby's slightest movements. If your baby goes absolutely still for 20 seconds, the Sensor Pad sends a signal to the Nursery Unit which sounds an alarm alerting you to check your baby. If you are out of the baby's room, the portable Parents' Unit (Sound Monitor) will pick up the alarm as well as other nursery sounds. In addition, the Nursery Unit has an optional "tic" feature. If the Sensor Pad senses movement, the Nursery Unit will continuously tick. If it doesn't, the ticking will stop. Your crib must have a hard flat surface. If it does not, just place a piece of masonite or a board under the mattress and place the Sensor Pad on top of the masonite and under the mattress. You may order the masonite directly from us. Be sure to place the correct side of the Sensor Pad facing up. The B�©b�© Sounds�® Movement Sensor will give you the peace of mind you won't get from just a sound monitor or even a video monitor. And, this monitor is an excellent value. It is 2 monitors in 1 so you don't have to purchase a sound monitor.