Acoustical ceiling tiles are products that usually absorb reflection off hard surfaces and can be used either in a grid or direct glued to a solid ceiling deck. These are typically porous products that accept the sound/vibration - trap it and allow it to dissipate before leaving the products. These products carry a comparative NRC number that gives the percentage of absorption a product should have at key frequencies. There are acoustic ceiling tiles that are composite panels that absorb reverberation and block sound from coming in or going out of a space.