are a-weighting, c-weighting, and z-weighting. a-weighting mirrors the range of hearing, with frequencies of 20 hz to 20,000 hz. this gives more weight to higher frequencies and less weight to lower frequencies. c-weighting has been used to measure peak sound pressure or impulse noise, similar to loud short-lived noises from machinery in occupational settings. z-weighting, also known as zero-weighting, represents noise levels without any frequency weights. understanding sound pressure levels is key to assessing measurements of noise pollution. several metrics describing noise exposure include: energy average equivalent level of the a-weighted sound, laeq: this measures the average sound energy over a given period for constant or continuous noise, such as road traffic. laeq can be further broken up into different types of noise based on time of day; however, cutoffs for evening and nighttime hours may differ between countries, with the united states, belgium, and new zealand noting evening hours from 19:00-22:00 or 7:00pm–10:00pm and nighttime hours from 22:00-7:00 or 10:00pm–7:00am and most european countries noting evening hours from 19:00-23:00 or 7:00pm–11:00pm and nighttime hours from 23:00-7:00 or 11:00pm–7:00am). laeq terms include: day-night average level, dnl or ldn: this measurement assesses the cumulative exposure to sound for a 24-hour period (leq over 24 hrs) of the year, with a 10 db(a) penalty or weight added to nighttime noise measurements given the increased sensitivity to noise at night. this is calculated from the following equation (united states, belgium, new zealand): l d n = 10 ⋅ log 10 ⁡ 1 24 ( 15 ⋅ 10 l d a y 10 + 9 ⋅ 10 l n i g h t + 10 10 ) =10\cdot \log _\left(15\cdot 10^+9\cdot 10^+10}\right)} day-evening-night average level, denl or lden: this measurement, commonly used in european countries, assesses the 24 hour average in a year (similar to dnl); however, this measurement separates evening (4 hours, 19:00-23:00 or 7:00pm–11:00pm) from night hours (8 hours, 23:00-7:00 or 11:00pm–7:00am) and adds a 5 db penalty to evening and 10 db penalty to nighttime hours. this is calculated from the following equation (most of europe): l d e n = 10 ⋅ log 10 ⁡ 1 24 ( 12 ⋅ 10 l d a y 10 + 4 ⋅ 10 l e v e n i n g + 5 10 + 8 ⋅ 10 l n i g h t + 10 10 ) =10\cdot \log _\left(12\cdot 10^+4\cdot 10^+5}+8\cdot 10^+10}\right)} daytime level, laeqd or lday: this measurement assesses daytime noise, usually from 7:00-19:00 (7am-7pm), yet may vary by country. nighttime level, laeqn or lnight: this measurement assesses nighttime noise, depending on country cutoff hours discussed above. maximum level, lamax: this measurement represents the maximal noise level when examining point sources or single events of noise; however, this value does not factor in duration of the event. sound exposure level of a-weighted sound, sel: this measurement represents the total energy for a particular event. sel is used to describe discrete events in terms of a-weighted sound. the difference between sel and lamax is that sel is derived using multiple time points of a particular event in calculating sound levels rather than the peak value. percentile-derived measurements (l10, l50, l90, etc.): noise may be described in terms of its statistical distribution over a set time, in which investigators may obtain values, or cut-points, at any percentile level. the l90 is the sound level that exceeds 90% of the time period; this is commonly referred to as background noise. researchers with the us national park service found that human activity doubles the background-noise levels in 63 percent of protected spaces like national parks, and increases them tenfold in 21 percent. in the latter places, “if you could have heard something 100 feet away, now you can only hear it 10 feet away,” instrumentation a sound level meter is one of the main tools for measuring sounds in the environment and the workplace. sound level meters sound can be measured in the air using a sound level meter, a device consisting of a microphone, an amplifier, and a time meter. sound level meters can measure noise at different frequencies (usually a- and c-weighted levels). there are two settings for response time constants, fast (time constant = 0.125 seconds, similar to human hearing) or slow (1 second, used for calculating averages over widely varying sound levels). sound level meters meet the required standards set by the international electrotechnical commission (iec) and in the united states, the american national standards institute as type 0, 1, or 2 instruments. type 0 devices are not required to meet the same criteria expected of types 1 and 2 since scientists use these as laboratory reference standards. type 1 (precision) instruments are to study the precision of capturing sound measurements, while type 2 instruments are for general field use. type 1 devices acceptable by the standards have a margin of error of ±1.5 db, while type 2 instruments meet a margin of error of ±2.3 db. dosimeters sound can also be measured using a noise dosimeter, a device similar to a sound level meter. individuals have used dosimeters to measure personal exposure levels in occupational settings given their smaller, more portable size. unlike many sound level meters, a dosimeter microphone attaches to the worker and monitors levels throughout a work shift. additionally, dosimeters can calculate the percent dose or time-weighted average (twa). smartphone applications measuring the noise level from a leaf blower, using the niosh sound level meter app in recent years, scientists and audio engineers have been developing smartphone apps to conduct sound measurements, similar to the standalone sound level meters and dosimeters. in 2014, the national institute for occupational safety and health (niosh) within the centers for disease control and prevention (cdc) published a study examining the efficacy of 192 sound measurement apps on apple and android smartphones. the authors found that only 10 apps, all of which were on the app store, met all acceptability criteria. of these 10 apps, only 4 apps met accuracy criteria within 2 db(a) from the reference standard. as a result of this study, they created the niosh sound level meter app to increase accessibility and decrease costs of monitoring noise using crowdsourcing data with a tested and highly accurate application. the app is compliant with ansi s1.4 and iec 61672 requirements. the app calculates the following measures: total run time, instantaneous sound level, a-weighted equivalent sound level (laeq), maximum level (lamax), c-weighted peak sound level, time-weighted average (twa), dose, and projected dose. dose and projected dose are based on sound level and duration of noise exposure in relation to the niosh recommended exposure limit of 85 db(a) for an eight-hour work shift. using the phone's internal microphone (or an attached external microphone), the niosh sound level meter measures instantaneous sound levels in realtime and converts sound into electrical energy to calculate measurements in a-, c-, or z-weighted decibels. app users are able to generate, save, and e-mail measurement reports. the niosh sound level meter is currently only available on apple ios devices. noise control the citylink sound tube in flemington, melbourne, australia, is designed to reduce roadway noise without detracting from the area's aesthetics. a man inserting an earplug in his ear to reduce the noise exposure the hierarchy of controls concept is often used to reduce noise in the environment or the workplace. engineering noise controls can be used to reduce noise propagation and protect individuals from overexposure. when noise controls are not feasible or adequate, individuals can also take steps to protect themselves from the harmful effects of noise pollution. if people must be around loud sounds, they can protect their ears with hearing protection (e.g., ear plugs or ear muffs). buy quiet programs and initiatives have arisen in an effort to combat occupational noise exposures. these programs promote the purchase of quieter tools and equipment and encourage manufacturers to design quieter equipment. noise from roadways and other urban factors can be mitigated by urban planning and better design of roads. roadway noise can be reduced by the use of noise barriers, limitation of vehicle speeds, alteration of roadway surface texture, limitation of heavy vehicles, use of traffic controls that smooth vehicle flow to reduce braking and acceleration, and tire design. an important factor in applying these strategies is a computer model for roadway noise, that is capable of addressing local topography, meteorology, traffic operations, and hypothetical mitigation. costs of building-in mitigation can be modest, provided these solutions are sought in the planning stage of a roadway project. aircraft noise can be reduced by using quieter jet engines. altering flight paths and time of day runway has benefited residents near airports. legal status and regulation country-specific regulations up until the 1970s governments tended to view noise as a "nuisance" rather than an environmental problem. many conflicts over noise pollution are handled by negotiation between the emitter and the receiver. escalation procedures vary by country, and may include action in conjunction with local authorities, in particular the police. egypt in 2007, the egyptian national research center found that the average noise level in central cairo was 90 decibels and that the noise never fell below 70 decibels. noise limits set by law in 1994 are not enforced. in 2018, the world hearing index declared cairo to be the world's second-noisiest city. india noise pollution is a major problem in india. the government of india has rules and regulations against firecrackers and loudspeakers, but enforcement is extremely lax. awaaz foundation is a non-governmental organization in india working to control noise pollution from various sources through advocacy, public interest litigation, awareness, and educational campaigns since 2003. despite increased enforcement and stringency of laws now being practiced in urban areas, rural areas are still affected. the supreme court of india had banned playing of music on loudspeakers after 10pm. in 2015, the national green tribunal directed authorities in delhi to ensure strict adherence to guidelines on noise pollution, saying noise is more than just a nuisance as it can produce serious psychological stress. however, implementation of the law remains poor. sweden how noise emissions should be reduced, without the industry being hit too hard, is a major problem in environmental care in sweden today. the swedish work environment authority has set an input value of 80 db for maximum sound exposure for eight hours. in workplaces where there is a need to be able to converse comfortably the background noise level should not exceed 40 db. the government of sweden has taken soundproofing and acoustic absorbing actions, such as noise barriers and active noise control. united kingdom figures compiled by rockwool, the mineral wool insulation manufacturer, based on responses from local authorities to a freedom of information act (foi) request reveal 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 behavior (scotland) act. in the last 12 months, 524 confiscations of equipment have been authorized involving the removal of powerful speakers, stereos and televisions. 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 complaints per thousand residents. eight of the top 10 councils ranked by complaints per 1,000 residents are located in london. united states the noise control act of 1972 established a u.s. national policy to promote an environment for all americans free from noise that jeopardizes their health and welfare. in the past, environmental protection agency coordinated all federal noise control activities through its office of noise abatement and control. the epa phased out the office's funding in 1982 as part of a shift in federal noise control policy to transfer the primary responsibility of regulating noise to state and local governments. however, the noise control act of 1972 and the quiet communities act of 1978 were never rescinded by congress and remain in effect today, although essentially unfunded. the national institute for occupational safety and health (niosh) at the centers for disease control and prevention (cdc) researches noise exposure in occupational settings and recommends a recommended exposure limit (rel) for an 8-hour time-weighted average (twa) or work shift