The decibel is a unit used in acoustics to describe the magnitude of sound levels. These levels can either describe how loud something is (eg 85 dB due to a passing bus), or they can describe the ability of a product or system to reduce sound. For example, a 35 dB suspended ceiling will reduce a sound level of 75 dB in one room down to 40 dB in an adjacent room. The bigger the number, the greater is the sound energy level or sound difference involved.