Refrigerator and controlling method thereof

A refrigerator, comprising:

• an inner case having a storage chamber;
• a cover plate provided at a rear portion of the inner case, wherein the storage chamber is formed at a front side of the cover plate;
• an evaporator that is provided in a space formed at a rear side of the cover plate, the evaporator including a refrigerant pipe and cooling fins;
• a sensor mounting area provided at the inner case;
• a reflective plate provided opposite the sensor mounting area such that the refrigerant pipe and the cooling fins are disposed between the sensor mounting area and the reflective plate;
• a heater that provides heat for defrosting the evaporator;
• a frost sensor that is configured to sense an amount of frost on the evaporator, the frost sensor including:
• a sound-source transmitter that generates a sound-source of a first volume to the evaporator, the sound-source transmitter being installed at the sensor mounting area;
• a sound-source receiver that receives a sound-source of a second volume formed after the sound-source transmitted by the sound-source transmitter reflects from or passes through the evaporator, the sound-source receiver being installed at the sensor mounting area;
• and
• a controller that controls the heater to generate heat, when the difference between the first volume and the second volume reaches a predetermined value.