Described is a system and method for detection using an ultrasonic wave. The system may include a first arrangement and a second arrangement. The first arrangement may include a transmitter and a receiver. The transmitter transmits an ultrasonic wave along a surface on which items are to be stored and the receiver receives the wave after it has passed over the surface. The second arrangement receives from the first arrangement data corresponding to properties of the received wave. The second arrangement compares the properties of the received wave to properties of the transmitted wave to generate current condition value. The second arrangement determines based on the current condition value and calibration data a current condition state corresponding to a degree of the surface currently occupied by the items. The calibration data includes at least one calibration value and a corresponding calibration condition state.