1. Any undesired sound. It is measured on a decibel scale ranging from the threshold of hearing (0 dB) to the threshold of pain (130 dB). Between these limits a whisper registers about 20 dB, heavy urban traffic about 90 dB, and a heavy hammer on steel plate about 110 dB. A high noise level (industrial or from overamplified music, for example) can cause permanent hearing impairment.
2. Any unwanted disturbance within a useful frequency band in a communication channel.
Noise. Decibel scale.
A descriptive name for random error or variation in observations which is not explained by the model.
A term used by analysts of time series to describe random fluctuations that may obscure the true signal. A sequence of errors, in successive observations, that consists of independent random values from a normal distribution with zero mean is termed white noise.
Any signal that occurs in an electronic or communication system and is considered extraneous to the desired signal being propagated. Noise can be introduced, for example, by external disturbances and may be deleterious in a given system since it can produce spurious signals, i.e. errors.
The noise immunity is a measure of the magnitude of external disturbances that a digital circuit can tolerate without producing errors. Logic values are represented electronically by two different voltage levels. Any noise introduced into logic circuitry by external disturbances is added (or subtracted) from the real digital logic signal. The noise margin is the maximum noise voltage that can be added or subtracted from the logic signal before a threshold voltage for a logic state is passed. See also Gaussian noise, impulse noise, white noise.
The electrical energy in a transmission line which does not form part of the signal transmitted. There are a number of sources of noise; for example, inductive effects between wires in close proximity, electromagnetic radiation from other devices, or the random movements of electrons in a physical transmission line.
Unwanted electrical signals that occur within electrical devices, circuits, or systems and result in a spurious signal at the output. The source of the noise can be man-made, the following being examples.
• Mains hum, caused by electromagnetic induction from power cabling, transformers, etc.: this is a periodic signal (at the mains frequency), but is an unwanted signal.
• Spark interference, such as from a car ignition system, fluorescent light starter, or commutating motor: this is high-frequency noise seen as a burst of high-voltage spikes.
• Microphony: acoustic interference with sensitive apparatus can introduce unwanted electrical signals, particularly with vacuum-tube apparatus.
• Radiofrequency interference: this may be on the desired receiving channel, on an adjacent channel to the desired channel, or may be an out-of-band or spurious response of the receiver; it is an inherent problem with heterodyne systems and requires careful design for spurious response rejection (see cochannel rejection; spurious rejection; electromagnetic compatibility).
• Quantization noise, resulting from the inaccurate representation of analogue signals in a digital system of limited resolution.
These man-made sources of noise can generally be minimized by careful component choice, circuit and system design, and appropriate filtering.
The second category of noise sources is naturally occurring noise due to the particle-like nature of electrical conduction, and includes the following.
• Thermal noise (or Johnson noise), due to the random motion of charge carriers in a conductor or semiconductor, giving rise to a fluctuating voltage: this voltage increases with temperature as the carriers move more rapidly.
• Shot noise, due to charge carriers crossing a potential barrier, such as a p-n junction or metal-semiconductor contact.
• Flicker noise, due to the random trapping and release of charge carriers in many electrical and naturally occurring systems: it has an inverse frequency response and is often called l/f (one over f) noise.
• Radiation noise, caused by cosmic radiation, and similar short-wavelength radiation such as gamma rays or X-rays: this is impulse noise and can result in errors in digital systems.
• Contact noise, occurring in discontinuous conductors such as carbon resistors, which are made from compressed particles: this is an excess noise source over the normal thermal noise.
A signal that conveys no useful information (e.g. background sound that makes it difficult to hear a conversation). If the useful signal comprises data that are being recorded, random (white) noise can be reduced by summing the recorded signals; incoherent noise is effectively damped out and the coherent signal is enhanced, thus improving the signal-to-noise ratio.
1. The opposite of the information content of a signal. In an econometric model, noise refers to the random component of the data-generating process.
2. Environmental noise is the noise from transport and industrial activities. In the European Union the assessment and monitoring of environmental noise is regulated by a set of directives that serve as a basis for the development of local measures.
- ultrafast laser spectroscopy
- ultrafiltration
- ultraforming process
- ultra high-definition
- ultra high frequency
- ultrahigh frequency
- ultra high-pressure processing
- ultra high temperature
- Ultra-Long Duration Balloon
- ultra-luminous infrared galaxy
- ultramafic
- ultrametric
- ultramicroscope
- ultramicrotome
- ultramylonite
- ultra orthoflow process
- ultraplinian eruption
- ultrasonic cleaning
- boundary representation
- boundary-scan testing
- boundary slip
- boundary stratotype
- boundary value problem
- boundary-value problem
- boundary wave