Noise pollution known as environmental noise or sound pollution, is the propagation of noise with harmful impact on the activity of human or animal life. The source of outdoor noise worldwide is caused by machines and propagation systems. Poor urban planning may give rise to noise pollution, side-by-side industrial and residential buildings can result in noise pollution in the residential areas; some of the main sources of noise in residential areas include loud music, transportation noise, lawn care maintenance, nearby construction, or young people yelling. Noise pollution associated with household electricity generators is an emerging environmental degradation in many developing nations; the average noise level of 97.60 dB obtained exceeded the WHO value of 50 dB allowed for residential areas. Research suggests that noise pollution is the highest in low-income and racial minority neighborhoods. Documented problems associated with urban environment noise go back as far as ancient Rome. High noise levels can contribute to cardiovascular effects in humans and an increased incidence of coronary artery disease.
In animals, noise can increase the risk of death by altering predator or prey detection and avoidance, interfere with reproduction and navigation, contribute to permanent hearing loss. While the elderly may have cardiac problems due to noise, according to the World Health Organization, children are vulnerable to noise, the effects that noise has on children may be permanent. Noise poses a serious threat to a child’s physical and psychological health, may negatively interfere with a child's learning and behavior. Noise pollution affects both behavior. Unwanted sound can damage physiological health. Noise pollution can cause hypertension, high stress levels, hearing loss, sleep disturbances, other harmful effects. Sound becomes unwanted when it either interferes with normal activities such as sleep or conversation, or disrupts or diminishes one's quality of life. Noise-induced hearing loss can be caused by prolonged exposure to noise levels above 85 A-weighted, decibels. A comparison of Maaban tribesmen, who were insignificantly exposed to transportation or industrial noise, to a typical U.
S. population showed that chronic exposure to moderately high levels of environmental noise contributes to hearing loss. Noise exposure in the workplace can contribute to noise-induced hearing loss and other health issues. Occupational hearing loss is one of the most common work-related illnesses in the U. S. and worldwide. Less addressed is. Indeed, tolerance for noise is independent of decibel levels. However, Murray Schafer's soundscape research was groundbreaking in this regard. In his eponymous work, he makes compelling arguments about how humans relate to noise on a subjective level, how such subjectivity is conditioned by culture, he notes that sound is an expression of power, as such, material culture tend to have louder engines not only for safety reasons, but for expressions of power by dominating the soundscape with a particular sound. Other key research in this area can be seen in Fong's comparative analysis of soundscape differences between Bangkok and Los Angeles, California, US. Based on Schafer's research, Fong's study showed how soundscapes differ based on the level of urban development in the area.
He found. Fong's important findings tie not only soundscape appreciation to our subjective views of sound, but demonstrates how different sounds of the soundscape are indicative of class differences in urban environments. Noise pollution can have negative affects on children on the autistic spectrum; those with Autism Spectrum Disorder can have hyperacusis, an abnormal sensitivity to sound. People with ASD that experience hyperacusis may have unpleasant emotions, such as fear and anxiety, sensations in noisy environments with loud sounds; this can cause individuals with ASD to avoid environments with noise pollution which can cause isolation and negatively impact their quality of life. Sudden explosive noises typical of high-performance car exhausts and car alarms are types of noise pollution that can affect individuals with ASD. Noise can have a detrimental effect on animals, increasing the risk of death by changing the delicate balance in predator or prey detection and avoidance, interfering the use of the sounds in communication in relation to reproduction and in navigation.
These effects may alter more interactions within a community through indirect effects. Acoustic overexposure can lead to permanent loss of hearing. European robins living in urban environments are more to sing at night in places with high levels of noise pollution during the day, suggesting that they sing at night because it is quieter, their message can propagate through the environment more clearly; the same study showed that daytime noise was a stronger predictor of nocturnal singing than night-time light pollution, to which the phenomenon is attributed. Anthropogenic noise reduced the species richness of birds found in Neoptropical urban parks. Zebra finches become less faithful to their partners; this could alter a population's evolutionary trajectory by selecting traits, sapping resources devoted to other activities and thus leading to profound genetic and evolutionary consequences. Underwater noise pollution due to human activities is prevalent in the sea. Cargo ships generate high levels of noise due to diesel engines.
This noise pollutio
General Services Administration
The General Services Administration, an independent agency of the United States government, was established in 1949 to help manage and support the basic functioning of federal agencies. GSA supplies products and communications for U. S. government offices, provides transportation and office space to federal employees, develops government-wide cost-minimizing policies and other management tasks. GSA employs about 12,000 federal workers and has an annual operating budget of $20.9 billion. GSA oversees $66 billion of procurement annually, it contributes to the management of about $500 billion in U. S. federal property, divided chiefly among 8,700 owned and leased buildings and a 215,000 vehicle motor pool. Among the real estate assets managed by GSA are the Ronald Reagan Building and International Trade Center in Washington, D. C. – the largest U. S. federal building after the Pentagon – and the Hart-Dole-Inouye Federal Center. GSA's business lines include the Federal Acquisition Service and the Public Buildings Service, as well as several Staff Offices including the Office of Government-wide Policy, the Office of Small Business Utilization, the Office of Mission Assurance.
As part of FAS, GSA's Technology Transformation Services helps federal agencies improve delivery of information and services to the public. Key initiatives include FedRAMP, Cloud.gov, the USAGov platform, Data.gov, Performance.gov, Challenge.gov. GSA is a member of the Procurement G6, an informal group leading the use of framework agreements and e-procurement instruments in public procurement. In 1947 President Harry Truman asked former President Herbert Hoover to lead what became known as the Hoover Commission to make recommendations to reorganize the operations of the federal government. One of the recommendations of the commission was the establishment of an "Office of the General Services." This proposed office would combine the responsibilities of the following organizations: U. S. Treasury Department's Bureau of Federal Supply U. S. Treasury Department's Office of Contract Settlement National Archives Establishment All functions of the Federal Works Agency, including the Public Buildings Administration and the Public Roads Administration War Assets AdministrationGSA became an independent agency on July 1, 1949, after the passage of the Federal Property and Administrative Services Act.
General Jess Larson, Administrator of the War Assets Administration, was named GSA's first Administrator. The first job awaiting Administrator Larson and the newly formed GSA was a complete renovation of the White House; the structure had fallen into such a state of disrepair by 1949 that one inspector of the time said the historic structure was standing "purely from habit." Larson explained the nature of the total renovation in depth by saying, "In order to make the White House structurally sound, it was necessary to dismantle, I mean dismantle, everything from the White House except the four walls, which were constructed of stone. Everything, except the four walls without a roof, was stripped down, that's where the work started." GSA worked with President Truman and First Lady Bess Truman to ensure that the new agency's first major project would be a success. GSA completed the renovation in 1952. In 1986 GSA headquarters, U. S. General Services Administration Building, located at Eighteenth and F Streets, NW, was listed on the National Register of Historic Places, at the time serving as Interior Department offices.
In 1960 GSA created the Federal Telecommunications System, a government-wide intercity telephone system. In 1962 the Ad Hoc Committee on Federal Office Space created a new building program to address obsolete office buildings in Washington, D. C. resulting in the construction of many of the offices that now line Independence Avenue. In 1970 the Nixon administration created the Consumer Product Information Coordinating Center, now part of USAGov. In 1974 the Federal Buildings Fund was initiated, allowing GSA to issue rent bills to federal agencies. In 1972 GSA established the Automated Data and Telecommunications Service, which became the Office of Information Resources Management. In 1973 GSA created the Office of Federal Management Policy. GSA's Office of Acquisition Policy centralized procurement policy in 1978. GSA was responsible for emergency preparedness and stockpiling strategic materials to be used in wartime until these functions were transferred to the newly-created Federal Emergency Management Agency in 1979.
In 1984 GSA introduced the federal government to the use of charge cards, known as the GMA SmartPay system. The National Archives and Records Administration was spun off into an independent agency in 1985; the same year, GSA began to provide governmentwide policy oversight and guidance for federal real property management as a result of an Executive Order signed by President Ronald Reagan. In 2003 the Federal Protective Service was moved to the Department of Homeland Security. In 2005 GSA reorganized to merge the Federal Supply Service and Federal Technology Service business lines into the Federal Acquisition Service. On April 3, 2009, President Barack Obama nominated Martha N. Johnson to serve as GSA Administrator. After a nine-month delay, the United States Senate confirmed her nomination on February 4, 2010. On April 2, 2012, Johnson resigned in the wake of a management-deficiency report that detailed improper payments for a 2010 "Western Regions" training conference put on by the Public Buildings Service in Las Vegas.
In July 1991 GSA contractors began the excavation of what is now the Ted Weiss Federal Building in New York City. The planning for that buildin
In electronics, noise is an unwanted disturbance in an electrical signal. Noise generated by electronic devices varies as it is produced by several different effects. In communication systems, noise is an error or undesired random disturbance of a useful information signal; the noise is a summation of unwanted or disturbing energy from natural and sometimes man-made sources. Noise is, however distinguished from interference, for example in the signal-to-noise ratio, signal-to-interference ratio and signal-to-noise plus interference ratio measures. Noise is typically distinguished from distortion, an unwanted systematic alteration of the signal waveform by the communication equipment, for example in signal-to-noise and distortion ratio and total harmonic distortion plus noise measures. While noise is unwanted, it can serve a useful purpose in some applications, such as random number generation or dither. Different types of noise are generated by different processes. Thermal noise is unavoidable at non-zero temperature, while other types depend on device type or manufacturing quality and semiconductor defects, such as conductance fluctuations, including 1/f noise.
Johnson–Nyquist noise is unavoidable, generated by the random thermal motion of charge carriers, inside an electrical conductor, which happens regardless of any applied voltage. Thermal noise is white, meaning that its power spectral density is nearly equal throughout the frequency spectrum; the amplitude of the signal has nearly a Gaussian probability density function. A communication system affected by thermal noise is modeled as an additive white Gaussian noise channel. Shot noise in electronic devices results from unavoidable random statistical fluctuations of the electric current when the charge carriers traverse a gap. If electrons flow across a barrier they have discrete arrival times; those discrete arrivals exhibit shot noise. The barrier in a diode is used. Shot noise is similar to the noise created by rain falling on a tin roof; the flow of rain may be constant, but the individual raindrops arrive discretely. The root-mean-square value of the shot noise current in is given by the Schottky formula.
I n = 2 I q Δ B where I is the DC current, q is the charge of an electron, ΔB is the bandwidth in hertz. The Schottky formula assumes independent arrivals. Vacuum tubes exhibit shot noise because the electrons randomly leave the cathode and arrive at the anode. A tube may not exhibit the full shot noise effect: the presence of a space charge tends to smooth out the arrival times. Conductors and resistors do not exhibit shot noise because the electrons thermalize and move diffusively within the material. Shot noise has been demonstrated in mesoscopic resistors when the size of the resistive element becomes shorter than the electron–phonon scattering length. Flicker noise known as 1/f noise, is a signal or process with a frequency spectrum that falls off into the higher frequencies, with a pink spectrum, it occurs in all electronic devices and results from a variety of effects. Burst noise consists of sudden step-like transitions between two or more discrete voltage or current levels, as high as several hundred microvolts, at random and unpredictable times.
Each shift in offset voltage or current lasts for several milliseconds to seconds. It is known a popcorn noise for the popping or crackling sounds it produces in audio circuits. If the time taken by the electrons to travel from emitter to collector in a transistor becomes comparable to the period of the signal being amplified, that is, at frequencies above VHF and beyond, the transit-time effect takes place and noise input impedance of the transistor decreases. From the frequency at which this effect becomes significant, it increases with frequency and dominates other sources of noise. While noise may be generated in the electronic circuit itself, additional noise energy can be coupled into a circuit from the external environment, by inductive coupling or capacitive coupling, or through the antenna of a radio receiver. Intermodulation noise Caused. Crosstalk Phenomenon in which a signal transmitted in one circuit or channel of a transmission systems creates undesired interference onto a signal in another channel.
Interference Modification or disruption of a signal travelling along a mediumAtmospheric noise This noise is called static noise and it is the natural source of disturbance caused by lightning discharge in thunderstorm and the natural disturbances occurring in nature. Industrial noise Sources such as automobiles, ignition electric motors and switching gear, High voltage wires and fluorescent lamps cause industrial noise; these noises are produced by the discharge present in all these operations. Solar noise Noise that originates from the Sun is called solar noise. Under normal conditions there is constant radiation from the Sun due to its high temperature. Electrical disturbances such as corona discharges, as well as sunspots can produce additional noise; the intensity of solar noise varies over time in a solar cycle. Cosmic noise Distant stars generate. While these stars are too far away to individually affect
Computer simulation is the reproduction of the behavior of a system using a computer to simulate the outcomes of a mathematical model associated with said system. Since they allow to check the reliability of chosen mathematical models, computer simulations have become a useful tool for the mathematical modeling of many natural systems in physics, climatology, chemistry and manufacturing, human systems in economics, social science, health care and engineering. Simulation of a system is represented as the running of the system's model, it can be used to explore and gain new insights into new technology and to estimate the performance of systems too complex for analytical solutions. Computer simulations are realized by running computer programs that can be either small, running instantly on small devices, or large-scale programs that run for hours or days on network-based groups of computers; the scale of events being simulated by computer simulations has far exceeded anything possible using traditional paper-and-pencil mathematical modeling.
Over 10 years ago, a desert-battle simulation of one force invading another involved the modeling of 66,239 tanks and other vehicles on simulated terrain around Kuwait, using multiple supercomputers in the DoD High Performance Computer Modernization Program. Other examples include a 1-billion-atom model of material deformation; because of the computational cost of simulation, computer experiments are used to perform inference such as uncertainty quantification. A computer model is the algorithms and equations used to capture the behavior of the system being modeled. By contrast, computer simulation is the actual running of the program that contains these equations or algorithms. Simulation, therefore, is the process of running a model, thus one would not "build a simulation". Computer simulation developed hand-in-hand with the rapid growth of the computer, following its first large-scale deployment during the Manhattan Project in World War II to model the process of nuclear detonation, it was a simulation of 12 hard spheres using a Monte Carlo algorithm.
Computer simulation is used as an adjunct to, or substitute for, modeling systems for which simple closed form analytic solutions are not possible. There are many types of computer simulations; the external data requirements of simulations and models vary widely. For some, the input might be just a few numbers, while others might require terabytes of information. Input sources vary widely: Sensors and other physical devices connected to the model. Lastly, the time at which data is available varies: "invariant" data is built into the model code, either because the value is invariant or because the designers consider the value to be invariant for all cases of interest; because of this variety, because diverse simulation systems have many common elements, there are a large number of specialized simulation languages. The best-known may be Simula. There are now many others. Systems that accept data from external sources must be careful in knowing what they are receiving. While it is easy for computers to read in values from text or binary files, what is much harder is knowing what the accuracy of the values are.
They are expressed as "error bars", a minimum and maximum deviation from the value range within which the true value lie. Because digital computer mathematics is not perfect and truncation errors multiply this error, so it is useful to perform an "error analysis" to confirm that values output by the simulation will still be usefully accurate. Small errors in the original data can accumulate into substantial error in the simulation. While all computer analysis is subject to the "GIGO" restriction, this is true of digital simulation. Indeed, observation of this inherent, cumulative error in digital systems was the main catalyst for the development of chaos theory. Computer models can be classified according to several independent pairs of attributes, including: Stochastic or deterministic – see external links below for examples of stochastic vs. deterministic simulations Steady-state or dynamic Continuous or discrete Dynamic system simulation, e.g. electric systems, hydraulic systems or multi-body mechanical s
Health effects from noise
Noise health effects are the physical and psychological health consequences of regular exposure to consistent elevated sound levels. Elevated workplace or environmental noise can cause hearing impairment, hypertension, ischemic heart disease and sleep disturbance. Changes in the immune system and birth defects have been attributed to noise exposure. Although age-related health effects occur with age, in many countries the cumulative impact of noise is sufficient to impair the hearing of a large fraction of the population over the course of a lifetime. Noise exposure has been known to induce tinnitus, hypertension and other cardiovascular adverse effects. Chronic noise exposure has been associated with sleep disturbances and increased incidence of diabetes. Adverse cardiovascular effects occur from chronic exposure to noise due to the sympathetic nervous system's inability to habituate; the sympathetic nervous system maintains lighter stages of sleep when the body is exposed to noise, which does not allow blood pressure to follow the normal rise and fall cycle of an undisturbed circadian rhythm.
Stress from time spent around elevated noise levels has been linked with increased workplace accident rates and aggression and other anti-social behaviors. The most significant sources are vehicles, prolonged exposure to loud music, industrial noise. There are an 10,000 deaths per year as a result of noise in the European Union. Noise-induced hearing loss is a permanent shift in pure-tone thresholds, resulting in sensorineural hearing loss; the severity of a threshold shift is dependent on severity of noise exposure. Noise-induced threshold shifts are seen as a notch on an audiogram from 3000–6000 Hz, but most at 4000 Hz. Exposure to loud noises, either in a single traumatic experience or over time, can damage the auditory system and result in hearing loss and sometimes tinnitus as well. Traumatic noise exposure can happen at work, at play, and/or by accident Noise induced hearing loss is sometimes unilateral and causes patients to lose hearing around the frequency of the triggering sound trauma.
Tinnitus is an auditory disorder characterized by the perception of a sound in the ear in the absence of an external sound source. There are two types of tinnitus: objective. Subjective can only be heard "in the head" by the person affected. Objective tinnitus can be heard from those around the affected person. Though the pathophysiology of tinnitus isn't known, noise exposure can be a contributing factor. Noise-induced tinnitus can be temporary or permanent depending on the type and amount of noise a person was exposed to. Noise has been associated with important cardiovascular health problems hypertension. Noise levels of 50 dB at night may increase the risk of myocardial infarction by chronically elevating cortisol production. Roadway noise levels are sufficient to constrict arterial blood flow and lead to elevated blood pressure. Vasoconstriction can result through medical stress reactions. Causal relationships have been discovered between noise and psychological effects such as annoyance, psychiatric disorders, effects on psychosocial well-being.
Exposure to intense levels of noise can cause violent reactions. Noise has been shown to be a factor that attributed to violent reactions; the psychological impacts of noise include an addiction to loud music. This was researched in a study where non-professional musicians were found to have loudness addictions more than non-musician control subjects. Psychological health effects from noise include anxiety. Individuals who have hearing loss, including noise induced hearing loss, may have their symptoms alleviated with the use of hearing aids. Individuals who do not seek treatment for their loss are 50% more to have depression than their aided peers; these psychological effects can lead to detriments in physical care in the form of reduced self-care, work-tolerance, increased isolation. Auditory stimuli can serve as psychological triggers for individuals with post traumatic stress disorder. Research commissioned by Rockwool, a multi-national insulation manufacturer headquartered in Denmark, reveals that in the UK one third of victims of domestic disturbances claim loud parties have left them unable to sleep or made them stressed in the last two years.
Around one in eleven of those affected by domestic disturbances claims it has left them continually disturbed and stressed. More than 1.8 million people claim noisy neighbours have made their life a misery and they cannot enjoy their own homes. The impact of noise on health is a significant problem across the UK given that more than 17.5 million Britons have been disturbed by the inhabitants of neighbouring properties in the last two year. For one in ten Britons this is a regular occurrence; the extent of the problem of noise pollution for public health is reinforced by figures collated by Rockwool from local authority responses to a Freedom of Information Act request. This research reveals in the period April 2008 - 2009 UK councils received 315,838 complaints about noise pollution from private residences; this resulted in environmental health officers across the UK serving 8,069 noise abatement notices, or citations under the terms of the Anti-Social Behaviour Act. Westminster City Council has received more complaints per head of population than any other district in the UK with 9,814 grievances about noise, which equates to 42.32 c
A-weighting is the most used of a family of curves defined in the International standard IEC 61672:2003 and various national standards relating to the measurement of sound pressure level. A-weighting is applied to instrument-measured sound levels in an effort to account for the relative loudness perceived by the human ear, as the ear is less sensitive to low audio frequencies, it is employed by arithmetically adding a table of values, listed by octave or third-octave bands, to the measured sound pressure levels in dB. The resulting octave band measurements are added to provide a single A-weighted value describing the sound. Other weighting sets of values – B, C, D and now Z – are discussed below; the curves were defined for use at different average sound levels, but A-weighting, though intended only for the measurement of low-level sounds, is now used for the measurement of environmental noise and industrial noise, as well as when assessing potential hearing damage and other noise health effects at all sound levels.
It is used when measuring low-level noise in audio equipment in the United States. In Britain and many other parts of the world and audio engineers more use the ITU-R 468 noise weighting, developed in the 1960s based on research by the BBC and other organizations; this research showed that our ears respond differently to random noise, the equal-loudness curves on which the A, B and C weightings were based are only valid for pure single tones. A-weighting began with work by Fletcher and Munson which resulted in their publication, in 1933, of a set of equal-loudness contours. Three years these curves were used in the first American standard for sound level meters; this ANSI standard revised as ANSI S1.4-1981, incorporated B-weighting as well as the A-weighting curve, recognising the unsuitability of the latter for anything other than low-level measurements. But B-weighting has since fallen into disuse. Work, first by Zwicker and by Schomer, attempted to overcome the difficulty posed by different levels, work by the BBC resulted in the CCIR-468 weighting maintained as ITU-R 468 noise weighting, which gives more representative readings on noise as opposed to pure tones.
A-weighting is valid to represent the sensitivity of the human ear as a function of the frequency of pure tones, but only for quiet levels of sound. In effect, the A-weighting is based on the 40-phon Fletcher–Munson curves which represented an early determination of the equal-loudness contour for human hearing. However, because decades of field experience have shown a good correlation between the A scale and occupational deafness in the frequency range of human speech, this scale is employed in many jurisdictions to evaluate the risks of occupational deafness and other auditory problems related to signals or speech intelligibility in noisy environnements; because of perceived discrepancies between early and more recent determinations, the International Organization for Standardization revised its standard curves as defined in ISO 226, in response to the recommendations of a study coordinated by the Research Institute of Electrical Communication, Tohoku University, Japan. The study produced new curves by combining the results of several studies, by researchers in Japan, Denmark, UK, USA.
This has resulted in the recent acceptance of a new set of curves standardized as ISO 226:2003. The report comments on the large differences, the fact that the original Fletcher–Munson contours are in better agreement with recent results than the Robinson-Dadson, which appear to differ by as much as 10–15 dB in the low-frequency region, for reasons that are not explained. Fortuitously, the 40-phon Fletcher–Munson curve is close to the modern ISO 226:2003 standard, it will be noted that A-weighting would be a better match to the loudness curve if it fell much more steeply above 10 kHz, it is that this compromise came about because steep filters were difficult to construct in the early days of electronics. Nowadays, no such limitation need exist. If A-weighting is used without further band-limiting it is possible to obtain different readings on different instruments when ultrasonic, or near ultrasonic noise is present. Accurate measurements therefore require a 20 kHz low-pass filter to be combined with the A-weighting curve in modern instruments.
This is defined in IEC 61012 as AU weighting and while desirable, is fitted to commercial sound level meters. A-frequency-weighting is mandated by the international standard IEC 61672 to be fitted to all sound level meters; the old B- and D-frequency-weightings have fallen into disuse, but many sound level meters provide for C frequency-weighting and its fitting is mandated — at least for testing purposes — to precision sound level meters. D-frequency-weighting was designed for use when measuring high level aircraft noise in accordance with the IEC 537 measurement standard; the large peak in the D-weighting curve is not a feature of the equal-loudness contours, but reflects the fact that humans hear random noise differently from pure tones, an effect, pronounced around 6 kHz. This is because individual neurons from different regions of the cochlea in the inner ear respond to narrow ba
Sound level meter
A sound level meter is used for acoustic measurements. It is a hand-held instrument with a microphone; the diaphragm of the microphone responds to changes in air pressure caused by sound waves. That is; this movement of the diaphragm, i.e. the sound pressure deviation, is converted into an electrical signal. A microphone is distinguishable by the voltage value produced when a known, constant sound pressure is applied; this is known as the microphone sensitivity. The instrument needs to know the sensitivity of the particular microphone being used. Using this information, the instrument is able to convert the electrical signal back to a sound pressure, display the resulting sound pressure level. Sound level meters are used in noise pollution studies for the quantification of different kinds of noise for industrial, environmental and aircraft noise; the current international standard that specifies sound level meter functionality and performances is the IEC 61672-1:2013. However, the reading from a sound level meter does not correlate well to human-perceived loudness, better measured by a loudness meter.
Specific loudness is a compressive nonlinearity that depends on level and frequency, which can be calculated in a number of different ways. The IEC 61672-1 specifies "three kinds of sound measuring instruments", they are the "conventional" sound level meter, the integrating-averaging sound level meter, the integrating sound level meter. The standard sound level meter can be called an exponentially averaging sound level meter as the AC signal from the microphone is converted to DC by a root-mean-square circuit and thus it must have a time constant of integration. Three of these time-weightings have been internationally standardized,'S' called Slow,'F' called Fast and'I' called Impulse, their names were changed in the 1980s to be the same in any language. I-time-weighting is no longer in the body of the standard because it has little real correlation with the impulsive character of noise events; the output of the RMS circuit is linear in voltage and is passed through a logarithmic circuit to give a readout linear in decibels.
This is 20 times the base 10 logarithm of the ratio of a given root-mean-square sound pressure to the reference sound pressure. Root-mean-square sound pressure being obtained with a standard frequency weighting and standard time weighting; the reference pressure is set by International agreement to be 20 micropascals for airborne sound. It follows that the decibel is, in a sense, not a unit, it is a dimensionless ratio. An exponentially averaging sound level meter, which gives a snapshot of the current noise level, is of limited use for hearing damage risk measurements. An integrating meter integrates—or in other words'sums'—the frequency-weighted noise to give sound exposure and the metric used is pressure squared times time Pa²·s, but Pa²·h is used. However, because the unit of sound was described in decibels, the exposure is most described in terms of sound exposure level, the logarithmic conversion of sound exposure into decibels. Note: in acoustics all'levels' are in decibels A common variant of the sound level meter is a noise dosemeter.
However, this is now formally known as a personal sound exposure meter and has its own International standard IEC 61252:1993. A noise dosimeter or noise dosemeter is a specialized sound level meter intended to measure the noise exposure of a person integrated over a period of time; this is intended to be a body-worn instrument and thus has a relaxed technical requirement, as a body-worn instrument—because of the presence of the body—has a poorer overall acoustic performance. A PSEM gives a read-out based on sound exposure Pa²·h, the older'classic' dosimeters giving the metric of'percentage dose' are no longer used in most countries; the problem with "%dose" is that it relates to the political situation and thus any device can become obsolete if the "100%" value is changed by local laws. Traditionally, noise dosemeters were large devices with a microphone mounted near the ear and having a cable going to the instrument body, itself belt worn; these devices had several issues the reliability of the cable and the disturbance to the user's normal work mode, caused by the presence of the cable.
In 1997 following a UK research grant an EU patent was issued for the first of a range of devices that were so small that they resembled a radiation badge and no cable was needed as the whole unit could be fitted near the ear. Today these devices measure not only simple noise dose, but some have four separate dosemeters, each with many of the functions of a full-sized sound level meter, including in the latest models full octave band analysis. IEC standards divide sound level meters into two "classes". Sound level meters of the two classes have the same functionality, but different tolerances for error. Class 1 instruments have a wider frequency range and a tighter tolerance than a lower cost, Class 2 unit; this applies to both the sound level meter itself as well as the associated calibrator. Most national standards permit the use of "at least a Cl