Noise is sound that is not wanted 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, like air or water. The difference arises from how 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. 
Specifically, acoustic noise is any sound in the acoustic domain, either deliberate (music, speech, etc.) or unintended. In contrast, noise in electronics may not be audible to the human ear and often requires instruments for detection. 
In audio engineering it can also refer to the unwanted residual electronic noise signal that gives rise to acoustic noise heard as hiss. This signal noise is commonly measured using A-weighting or ITU-R 468 weighting.
Sound is measured based on the amplitude and frequency of a sound wave. Noise is most commonly discussed in terms of decibels (dB), the measure of loudness, or intensity of a sound; this measurement describes the amplitude of a sound wave. On the other hand, pitch describes the frequency of a sound and is measured in hertz (Hz). 
Recording and reproduction
In audio, recording, and broadcast systems, audio noise refers to the residual low level sound (four major types: hiss, rumble, crackle, and hum) that is heard in quiet periods of program. This variation from the expected pure sound or silence can be caused by the audio recording equipment, the instrument, or ambient noise in the recording room. 
Environmental noise is the accumulation of all noise present in a specified environment. The principal sources of environmental noise are surface motor vehicles, aircraft, trains and industrial sources.  These noise sources expose millions of people to noise pollution that creates not only annoyance, but also significant health consequences such as elevated incidence of hearing loss and cardiovascular disease. There are a variety of mitigation strategies and controls available to reduce sound levels including source intensity reduction, land use planning strategies, noise barriers and sound baffles, time of day use regimens, vehicle operational controls and architectural acoustics design measures.
Regulation of noise
Certain geographic areas or specific occupations may be at a higher risk of being exposed to constantly high levels of noise; in order to prevent negative health outcomes, regulations may be set. Noise regulation includes statutes or guidelines relating to sound transmission established by national, state or provincial and municipal levels of government. Environmental noise is governed by laws and standards which set maximum recommended levels of noise for specific land uses, such as residential areas, areas of outstanding natural beauty, or schools. These standards usually specify measurement using a weighting filter, most often A-weighting.
In 1972, the Noise Control Act was passed to promote a healthy living environment for all Americans, where noise does not pose a threat to human health. This policy's main objectives were: (1) establish coordination of research in the area of noise control, (2) establish federal standards on noise emission for commercial products, and (3) promote public awareness about noise emission and reduction.
The Quiet Communities Act of 1978 promotes noise control programs at the state and local level and developed a research program on noise control. Both laws authorized the Environmental Protection Agency to study the effects of noise and evaluate regulations regarding noise control.
Noise in the workplace
In the US, the National Institute of Occupational Safety and Health (NIOSH) provides recommendation on noise exposure in the workplace.  In 1972 (revised in 1998), NIOSH published a document outlining recommended standards relating to the occupational exposure to noise, with the purpose of reducing the risk of developing permanent hearing loss related to exposure at work.  This publication set the recommended exposure limit (REL) of noise in an occupation setting to 85 dBA for 8 hours. However, in 1973 the Occupational Safety and Health Administration (OSHA) maintained the requirement of an 8-hour average of 90 dBA. The following year, OSHA required employers to provide a hearing conservation program to workers exposed to 85 dBA average 8-hour workdays. 
The European Environment Agency regulates noise control and surveillance within the European Union.  The Environmental Noise Directive was set to determine levels of noise exposure, increase public access to information regarding environmental noise, and reduce environmental noise.  Additionally, in the European Union, underwater noise is a pollutant according to the Marine Strategy Framework Directive (MSFD). The MSFD requires EU Member States to achieve or maintain Good Environmental Status, meaning that the "introduction of energy, including underwater noise, is at levels that do not adversely affect the marine environment".
Health effects from noise
Exposure to noise is associated with several negative health outcomes. Depending on duration and level of exposure, noise may promote hearing loss, high blood pressure, ischemic heart disease, sleep disturbances, birth defects, and even decreased school performance. 
Noise exposure has increasingly been identified as a public health issue, especially in an occupational setting, as demonstrated with the creation of NIOSH's Noise and Hearing Loss Prevention program.  Noise has also proven to be an occupational hazard, as it is the most common work-related illness.
While noise-induced hearing loss is permanent, it is also very preventable.  Particularly in the workplace, regulations may exist depicting maximum allowed levels of noise. This can be especially important for professionals working in atmospheres with consistent exposure to noise, such as musicians, music teachers and sound engineers.  Examples of measures taken to prevent noise-induced hearing loss in the workplace include engineering noise control, the Buy-Quiet initiative, creation of the Safe-in-Sound Award, and noise surveillance. 
Roland Barthes distinguishes between physiological noise, which is merely heard, and psychological noise, which is actively listened to. Physiological noise is felt subconsciously as the vibrations of the noise (sound) waves physically interact with the body while psychological noise is perceived as our conscious awareness shifts its attention to that noise.
Luigi Russolo, one of the first composers of noise music, wrote the essay The Art of Noises. He argues that any kind of noise could be used as music, as audiences become more familiar with noises caused by technological advancements; noise has become so prominent that pure sound no longer exists.