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any signal that does not convey useful information. Electrical noise consists of electrical currents or voltages that interfere with the operation of electronic systems. Electrical noise limits the sensitivity of radio receiving systems and, when present at high enough levels, may cause false outputs from digital circuits. In radio receivers it is important that the noise produced by amplifiers, especially early-stage amplifiers, be kept as low as possible. The signal-to-noise (S/N) ratio is an important factor when evaluating much electronic equipment. Random noise originates when a current flows through a conductor that has resistance and is above absolute zero in temperature. It also arises in electron tubes and semiconductor devices, as well as from atmospheric disturbances and radiation from space (see staticstatic,
term formerly use to describe electrical noise in radio reception, especially noise that originates outside a transmitter and receiver, e.g., in the atmosphere or in man-made devices.
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). Nonrandom noise originates from the operation of other systems, e.g., automotive ignition systems, and from interfering signals. Noise also affects optical detection systems where light is treated by the particle, or quantum, theory. The output voltage of an optical detector is proportional to the intensity of the incident light. The noise can be from the detectors themselves, the electrical amplifiers that amplify the detector outputs, or thermal noise, which is caused by the vibration of atoms and molecules. Noise can also be inherent in the radiation being detected.


1. Fluctuations that occur in the output of any electronic or electrical device and, in a measuring instrument, ultimately limit its sensitivity. Noise can be caused by many different factors, for example by insufficient regulation of the voltage of a power supply, but the most important source is usually that produced by random agitation of electrons in a resistance, i.e. thermal noise. White noise is completely random and uncorrelated noise that is uniform in energy over equal intervals of frequency; up to a limiting frequency, thermal noise is white.

In electrical circuits a resistance at temperature T produces an exchangeable noise power in bandwidth B of

P = kTB
where k is the Boltzmann constant. If this noise power is generated in the first stages of the measuring instrument, it is amplified along with the signal; the signal/noise ratio is therefore reduced by amplification, although the signal is made stronger. The contribution to the noise power of any part of an electric circuit can be specified by its noise figure or noise factor, F, which is defined by
F = (P 1 + P 0 )/P 0
where P 1 is the extra noise power generated by the circuit and P 0 is the noise power present at the input terminals of the circuit due to the source resistance, i.e. the thermal noise. Thus if the circuit adds no extra noise, it has a noise figure of unity. F is usually expressed in decibels. Other forms of noise include shot noise, generated by the random arrival times of photons or electrons at a detector, and pink noise. See also sensitivity.

2. Fluctuations of a random nature in which the amplitude of the deflection from a given reference level cannot be predicted fom one moment to the next but whose statistical properties may be well-defined. Signals from radio sources are of this kind, and in a radio telescope the problem is to detect these random signals in the presence of noise generated in the measuring instruments. These intrinsic fluctuations of the signal are called self-noise, and those from the equipment or any other source are system noise. See also radio-source spectrum.



in communication lines, the action of external electromagnetic phenomena resulting in distortion of the transmitted information. Electrical processes in such lines that distort the information being transmitted are also referred to as noise.

Noise appears in different forms, depending on the type of transmitted information. For example, it appears as errors in the transmission of telegrams and data. In telephone lines it causes a crackling or rustling sound and can result in the poor intelligibility of speech and in the poor audibility of conversations transmitted through adjacent channels. In the transmission of messages or newspaper pages by facsimile systems it can mean insufficient sharpness of lines or the appearance of spurious lines. In remote control and remote signal’systems it can appear as distortions of commands. Noise effects depend on a great number of factors and usually are of a random character. For this reason problems of noise immunity in communication lines are solved by the methods of probability theory and mathematical statistics.

Noise can be divided into two types—additive and nonaddi-tive. Additive noise is summed linearly with the signal and is of three kinds, which differ in their statistical properties: fluctuation, harmonic, and impulse noise. The signal distortions contributed by each kind are determined by many factors, such as the ratio of signal power or amplitude to noise, the methods of transmission and reception, and the composition of the frequency spectra of the signal and the noise. Fluctuation noise is the most characteristic kind. Such noise includes thermal noise in electron tubes and semiconductor devices and can also result from, for example, the influence of adjacent channels in multichannel communication systems. Harmonic noise occurs relatively seldom in cable systems; its appearance indicates a defect in the cable. In overhead-line systems, however, harmonic noise is quite frequent; its main source is radiation from longwave radio broadcasting stations. Impulse noise does not substantially lower the quality of telephonic communication, but it is the principal cause of errors in the transmission of numerical and other forms of digital information. The sources of impulse noise include faulty electric contacts, switching in communication-line apparatus, lightning discharges, nearby radio stations, electrified railroads, and power transmission lines.

Nonadditive noise includes noise that causes spurious modulation of the signal. Such noise results from the nonlinear dependence of the characteristics of the communication channel on the signal parameters and on time and has a substantial effect particularly on the transmission of signals through very long communication channels.


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random vibrations of varying physical nature, which are distinguished by a complex temporal and spectral structure. In everyday life “noise” implies various undesirable acoustical disturbances during the apperception of speech or music, as well as any sounds that interfere with rest or work. Noise plays a significant role in many areas of science and technology, such as acoustics, radio engineering, radar technology, radio astronomy, information theory, computer technology, optics, and medicine.

Regardless of its physical nature, noise differs from periodic vibrations in the random change in the instantaneous values of the quantities characterizing a given process. Often, noise is a mixture of random and periodic vibrations. Depending on the temporal, spectral, and spatial structure of noise, various mathematical models are used to describe it. Averaged parameters determined on the basis of statistical laws that take into account the structure of noise at the source and the properties of the medium in which the noise is propagated are used for the quantitative evaluation of noise.

Noise is subdivided into statistically stationary and nonstationary noise. The theory and methods of measuring stationary noise, the classical model of which is white noise, are most highly developed. Stationary noise is characterized by the constancy of the average parameters: intensity (power), spectral distribution of intensity (spectral density), and the autocorrelation function (the time-averaged product of the instantaneous values of two noises shifted by the delay time). The noise observed in practice, which results from the action of many independent sources, such as the noise of a crowd of people, the sea, tools, or a vortex air flow or the noise at a radio-receiver output, is quasi-stationary. Noise that lasts for short time intervals (shorter than the averaging time in the measuring devices) is called nonstationary noise. Such noise includes, for example, the street noise of passing traffic, individual sounds in production processes, and infrequent impulse noise in radio engineering.

The investigation of noise pursues varied objectives: the study of noise sources in order to reduce their adverse effect on man and various systems; the search for methods and means of optimal reception, detection, and measurement of the parameters of various signals in the presence of noise; and the improvement of the accuracy of measurements in analog and digital data-processing devices. Sound-level meters, frequency analyzers, correlators, and other devices are used to measure the characteristics of noise.

Any vibrations in solids, liquids, and gases may be sources of audible and inaudible acoustical noise. In technology, the main sources of noise are various engines and machinery. An elevated noise level in machinery and mechanisms is often an indication of malfunction or design inefficiency. The precise fabrication and adjustment of parts and the dynamic balancing of all moving parts lead to a reduction in noise and, in general, to the reduced wear of parts and increased operating life and precision of operation.

Radio-electronic noise—random fluctuations of currents and voltages in electronic devices—results from the nonuniform emission of electrons in electronic vacuum devices (shot noise, the flicker effect), from the nonuniformity of the processes of generation and recombination of charge carriers (conduction electrons and holes) in semiconductor devices, from the thermal motion of charge carriers in conductors (thermal noise), and from the heat radiation of the earth, the earth’s atmosphere, the planets, the sun, stars, the interstellar medium, and other objects (cosmic noise). Noise limits the sensitivity of radio receivers.

In many cases, noise is used as a source of information. For example, in naval technology, submarines and surface vessels are detected and their positions fixed from the noise they generate when moving. In radio astronomy, the radio-frequency radiation of stars and other cosmic formations is investigated on the basis of noise in certain frequency ranges. Noiselike signals are used in the technology of radio and acoustical measurements, for example, in architectural acoustics. Some sounds in music are by their physical nature noise or have noiselike features. The voiced consonants encountered in speech are also noise in their properties.

The qualitative features of sensation during the perception of acoustical noise by the hearing organs and the organism as a whole depend on the intensity (seeLOUDNESS)and spectral composition of the noise. The adverse effect of noise on the human organism is manifested in specific damage to the hearing organs and nonspecific changes in other organs and systems of the body. The character, level, and frequency composition of noise are major factors, as are the length of exposure to noise and individual sensitivity. Prolonged exposure to intense noise may considerably disrupt the activity of the central nervous system, vascular tonus, and the functions of organs of the gastrointestinal tract and endocrine system, as well as gradually lead to hardness of hearing owing to neuritis of the vestibulocochlear nerve. An initial disruption of the perception of high frequencies (4,000–8,000 Hz) is characteristic of occupational hardness of hearing. The nonspecific action of noise may be manifested before any actual changes in hearing occur and is expressed as neurotic reactions, asthenia, and disruption of the functions of the autonomic nervous system. Coordination of movements is disrupted and labor productivity is reduced under the influence of noise. The term “noise disease” has appeared in the medical literature in connection with the unified etiology of the clinical disturbances.

A number of organizational, technical, and medical measures are implemented to prevent the adverse effects of acoustical noise on the human organism. The factors that produce noise are eliminated or weakened at the source. The propagation of noise from noise sources is prevented by insulation of noisy machinery assemblies against noise, installation of shock absorbers, and sound absorption, which attenuates noise by reducing reflections from surrounding design elements lined with sound-absorbing porous materials. Aerodynamic noise (for example, exhaust and noise in air ducts) is reduced by eliminating the causes of vortex formation, insulating air ducts against noise, and employing noise suppressors. It is important to alternate efficiently the work and rest periods of persons working under noisy conditions, to limit the length of their exposure to noise, and to monitor their health. The control of street noise is effected by replacing streetcars with trolleys and buses, restricting the use of horns, and other measures. Zones where noise levels reach 85 decibels are marked with warning signs, and people working in these zones are provided with individual sound-insulating earmuffs. Apart from the adverse effects of noise on man, some acoustical noise, such as the noise of the surf and forests, is known to have a healthful, relaxing effect.


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Sound which is unwanted, either because of its effect on humans, its effect on fatigue or malfunction of physical equipment, or its interference with the perception or detection of other sounds.
Unwanted electrical signal disturbances.
Interfering and unwanted currents or voltages in an electrical device or system.
Nonperiodic behavior of a system that results from the presence of random driving forces, such as thermal agitation, as opposed to chaotic behavior.
Random fluctuations of electronic signals appearing in a recorded spectrum.


Any sound which is unwanted because it is annoying, interferes with speech and hearing, or is intense enough to damage hearing.


In radio communications, the term implies any unwanted signal tending to obscure wanted signals. It is measured in decibels, or dB, at the audio output receiver. A decibel is a sound pressure of 20 micronewtons per square meter.


1. any undesired electrical disturbance in a circuit, degrading the useful information in a signal
2. undesired or irrelevant elements in a visual image
3. noises off Theatre sounds made offstage intended for the ears of the audience: used as a stage direction


Any part of a signal that is not the true or original signal but is introduced by the communication mechanism.

A common example would be an electrical signal travelling down a wire to which noise is added by inductive and capacitive coupling with other nearby signals (this kind of noise is known as "crosstalk").

A less obvious form of noise is quantisation noise, such as the error between the true colour of a point in a scene in the real world and its representation as a pixel in a digital image.


(1) Extraneous, unwanted signals that invade an electrical or optical system. In electronics, noise can come from strong electrical or magnetic signals in nearby lines, from poorly fitting electrical contacts and from power line spikes. In optics, noise comes from the stray reflections of light that emanate from the various components in the optical system. See signal-to-noise ratio and noise cancellation.

(2) Distortions in analog and digital video images that are caused by a variety of circumstances. The silver grains in the original film create granular noise. Electronic circuits create Gaussian noise. Drop outs and bit errors in digital tapes create impulse noise, and the digitization process creates quantization noise. See Gaussian noise, dynamic noise reduction and artifact.

(3) Distortions in a still image caused by variations in the pixels in the CCD or CMOS sensor. See digital camera.