Noise is unwanted sound by the perceiver, because it is unpleasant, loud or interferes with hearing. This results in the subjective discretion between sound and noise, where any sound may be considered noise, depending on the perceiver. From a physics standpoint, noise is indistinguishable from sound, as both are vibrations through a medium, such as air or water. The difference arises when the brain receives and perceives a sound. By extension, in experimental sciences, "noise" refers to any random fluctuations of data that makes more difficult the perception of an expected signal.