Weighting filter - AbsoluteAstronomy.com

A weighting filter is used to emphasise or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.

Audio applications

In each field of audio measurement, special units are used to indicate a weighted measurement as opposed to a basic physical measurement of energy level. For sound, the unit is the phon

Phon

The phon was proposed in DIN 45631 and ISO 532 B as a unit of perceived loudness level LN for pure tones by S. S. Stevens.-Definition:The purpose of the phon scale is to compensate for the effect of frequency on the perceived loudness of tones...

(1 kHz equivalent level).

Loudness measurements

In the measurement of loudness, for example, an A-weighting

A-weighting

A Weighting curve is a graph of a set of factors, that are used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly known example is frequency weighting in sound level measurement where a specific set of weighting curves known as A,...

filter is commonly used to emphasize frequencies around 3–6 kHz where the human ear is most sensitive, while attenuating very high and very low frequencies to which the ear is insensitive. The aim is to ensure that measured loudness corresponds well with subjectively perceived loudness.
A-weighting

A-weighting

is only really valid for relatively quiet sounds and for pure tones as it is based on the 40-phon Fletcher-Munson equal-loudness contour

Equal-loudness contour

An equal-loudness contour is a measure of sound pressure , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours...

. The B and C curves were intended for louder sounds (though they are less used) while the D curve is used in assessing loud aircraft noise (IEC 537).

Telecommunications

In the field of telecommunications, weighting filters are widely used in the measurement of electrical noise on telephone circuits, and in the assessment of noise as perceived through the acoustic response of different types of instrument (handset). Other noise-weighting curves have existed, e.g. DIN

Din

DIN or Din or din can have several meanings:* A din is a loud noise.* Dīn, an Arabic term meaning "religion" or "way of life".* Din is one of the ten aspects of the Ein Sof in Kabbalah ....

standards. The term psophometric weighting
Psophometric weighting
Psophometric weighting refers to any weighting curve used in the measurement of noise. In the field of audio engineering it has a more specific meaning, referring to noise weightings used especially in measuring noise on telecommunications circuits...

, though referring in principle to any weighting curve intended for noise measurement, is often used to refer to a particular weighting curve, used in telephony for narrow-bandwidth voiceband

Voiceband

In electronics, voiceband means the typical human hearing frequency range that is from 20 Hz to 20 kHz. In telephony, it means the frequency range normally transmitted by a telephone line, generally about 200–3600 Hz. Frequency-division multiplexing in telephony normally uses...

speech circuits.

Environmental noise measurement

A-weighted decibel

Decibel

The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...

s are abbreviated dB(A) or dBA. When acoustic (calibrated microphone) measurements are being referred to, then the units used will be dB

Decibel

SPL

-Sporting Leagues:*Scottish Premier League, or Clydesdale Bank Premier League, the highest professional football league in Scotland*Saudi Premier League, or Zain Saudi Premier League, the highest professional football league in Saudi Arabia...

(sound pressure level) referenced to 20 micropascals = 0 dB SPL. dBrn
DBrn
The symbol dBrn or dB is an abbreviation for decibels above reference noise.Weighted noise power in dB is referred to 1.0 picowatt. Thus, 0 dBrn = -90 dBm...

adjusted is a synonym for dBA.

The A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many sound level meters (see ITU-R 468

ITU-R 468 noise weighting

ITU-R 468 is a standard relating to noise measurement, widely used when measuring noise in audio systems. The standard defines a weighting filter curve, together with a quasi-peak rectifier having special characteristics as defined by specified tone-burst tests...

weighting for a further explanation).

A-weighting

A-weighting

is also in common use for assessing potential hearing damage

Hearing impairment

-Definition:Deafness is the inability for the ear to interpret certain or all frequencies of sound.-Environmental Situations:Deafness can be caused by environmental situations such as noise, trauma, or other ear defections...

caused by loud noise, though this seems to be based on the widespread availability of sound level meters incorporating A-Weighting rather than on any good experimental evidence to suggest that such use is valid. The distance of the measuring microphone from a sound source is often "forgotten", when SPL measurements are quoted, making the data useless. In the case of environmental or aircraft noise

Aircraft noise

Aircraft noise is noise pollution produced by any aircraft or its components, during various phases of a flight: on the ground while parked such as auxiliary power units, while taxiing, on run-up from propeller and jet exhaust, during take off, underneath and lateral to departure and arrival paths,...

, distance need not be quoted as it is the level at the point of measurement that is needed, but when measuring refrigerator

Refrigerator

A refrigerator is a common household appliance that consists of a thermally insulated compartment and a heat pump that transfers heat from the inside of the fridge to its external environment so that the inside of the fridge is cooled to a temperature below the ambient temperature of the room...

s and similar appliances the distance should be stated; where not stated it is usually one metre (1 m). An extra complication here is the effect of a reverberant room, and so noise measurement on appliances should state "at 1 m in an open field" or "at 1 m in anechoic chamber

Anechoic chamber

An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...

". Measurements made outdoors will approximate well to anechoic conditions.

A-weighted SPL measurements of noise level are increasingly to be found on sales literature for domestic appliances such as refrigerators and freezers, and computer fans. Although the threshold of hearing is typically around 0 dB SPL, this is in fact very quiet indeed, and appliances are more likely to have noise levels of 30 to 40 dB SPL.

Audio reproduction and broadcasting equipment

Human sensitivity to noise in the region of 6 kHz became particularly apparent in the late 1960s with the introduction of compact cassette

Compact Cassette

The Compact Cassette, often referred to as audio cassette, cassette tape, cassette, or simply tape, is a magnetic tape sound recording format. It was designed originally for dictation, but improvements in fidelity led the Compact Cassette to supplant the Stereo 8-track cartridge and reel-to-reel...

recorders and Dolby-B noise reduction

Dolby noise reduction system

Dolby NR is the name given to a series of noise reduction systems developed by Dolby Laboratories for use in analog magnetic tape recording. The first was Dolby A, a professional broadband noise reduction for recording studios in 1966, but the best-known is Dolby B , a sliding band system for the...

. A-weighted noise measurements were found to give misleading results because they did not give sufficient prominence to the 6 kHz region where the noise reduction was having greatest effect, and sometimes one piece of equipment would even measure worse than another and yet sound better, because of differing spectral content.

ITU-R 468 noise weighting

ITU-R 468 noise weighting

was therefore developed to more accurately reflect the subjective loudness of all types of noise, as opposed to tones. This curve, which came out of work done by the BBC

BBC

The British Broadcasting Corporation is a British public service broadcaster. Its headquarters is at Broadcasting House in the City of Westminster, London. It is the largest broadcaster in the world, with about 23,000 staff...

Research Department, and was standardised by the CCIR

CCIR

CCIR is a four-letter abbreviation that may stand for:*Campaign for Comprehensive Immigration Reform, a Washington, DC organization for immigrant rights...

and later adopted by many other standards bodies (IEC

International Electrotechnical Commission

The International Electrotechnical Commission is a non-profit, non-governmental international standards organization that prepares and publishes International Standards for all electrical, electronic and related technologies – collectively known as "electrotechnology"...

, BSI

BSI

BSI is a three letter acronym that can stand for:* Backside illumination, a type of digital image sensor* The Baker Street Irregulars, a fictional group featured in Sherlock Holmes stories* Banca della Svizzera Italiana* Bentley Systems Inc....

) and, , is maintained by the ITU. Noise measurements using this weighting typically also use a quasi-peak detector law rather than slow averaging. This also helps to quantify the audibility of bursty noise, ticks and pops that might go undetected with a slow rms measurement.

ITU-R 468 noise weighting with quasi-peak detection is widely used in Europe, especially in telecommunications, and in broadcasting particularly after it was adopted by the Dolby corporation who realised its superior validity for their purposes. Its advantages over A-weighting seem to be less well appreciated in the USA and in consumer electronics, where the use of A-weighting predominates—probably because A-weighting produces a 9 to 12 dB "better" specification, see specsmanship

Specsmanship

Specsmanship is the often inappropriate use of specifications or measurement results to establish putative superiority of one entity over another, generally when no such superiority exists...

. It is commonly used by broadcasters in Britain, Europe, and former countries of the British Empire such as Australia and South Africa.

Though the noise level of 16-bit audio systems (such as CD players) is commonly quoted (on the basis of calculations that take no account of subjective effect) as −96 dB relative to FS (full scale), the best 468-weighted results are in the region of −68 dB relative to Alignment Level (commonly defined as 18 dB below FS) i.e. −86 dB relative to FS.

The use of weighting curves is in no way to be regarded as 'cheating', provided that the proper curve is used. Nothing of relevance is being 'hidden', and even when, for example, hum is present at 50 or 100 Hz at a level above the quoted (weighted) noise floor this is of no importance because our ears are very insensitive to low frequencies at low levels, so it will not be heard. A-weighting is often used to compare and qualify ADC

Analog-to-digital converter

An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...

s, for instance, because it more accurately represents the way noise shaping

Noise shaping

Noise shaping is a technique typically used in digital audio, image, and video processing, usually in combination with dithering, as part of the process of quantization or bit-depth reduction of a digital signal...

hides dither

Dither

Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images...

noise in the ultrasonic range.

Other applications of weighting

In the measurement of gamma rays or other ionising radiation, a radiation monitor or dosimeter

Dosimeter

Dosimeters measure an individual's or an object'sexposure to something in the environment — particularly to a hazard inflicting cumulative impact over long periods of time, or over a lifetime...

will commonly use a filter to attenuate those energy levels or wavelengths that cause the least damage to the human body, while letting through those that do the most damage, so that any source of radiation may be measured in terms of its true danger rather than just its 'strength'. The sievert

Sievert

The sievert is the International System of Units SI derived unit of dose equivalent radiation. It attempts to quantitatively evaluate the biological effects of ionizing radiation as opposed to just the absorbed dose of radiation energy, which is measured in gray...

is a unit of weighted radiation dose for ionising radiation, which supersedes the older unit the REM

Röntgen equivalent man

Named after Wilhelm Röntgen , the roentgen equivalent in man or rem is a unit of radiation dose equivalent...

(roentgen equivalent man).

Weighting is also applied to the measurement of sunlight when assessing the risk of skin damage through sunburn

Sunburn

A sunburn is a burn to living tissue, such as skin, which is produced by overexposure to ultraviolet radiation, commonly from the sun's rays. Usual mild symptoms in humans and other animals include red or reddish skin that is hot to the touch, general fatigue, and mild dizziness. An excess of UV...

, or when assessing the sun protection factor of sun-screen

Sunscreen

Sunblock is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet radiation on the skin exposed to sunlight and thus helps protect against sunburn...

lotions. Light, UVA

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

and UVB in sunlight have different effects, with regard to both DNA damage and melanin

Melanin

Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

stimulation, depending on the wavelength of the radiation. Weighted measurements are often in SU (sun unit

Sun unit

The sun unit is a measure of sun intensity weighted for skin damage related to the UV index.-Basics of solar radiation:Solar radiation sustains life on earth and determines climate...

s) or DUV (Diffey weighted UV irradiance). The UV index

UV index

The ultraviolet index or UV Index is an international standard measurement of the strength of the ultraviolet radiation from the sun at a particular place on a particular day...

is also used.

Another use of weighting is in television, where the red, green and blue components of the signal are weighted according to their perceived brightness. This ensures compatibility with black and white receivers, and also benefits noise performance and allows separation into meaningful luminance and chrominance

Chrominance

Chrominance is the signal used in video systems to convey the color information of the picture, separately from the accompanying luma signal . Chrominance is usually represented as two color-difference components: U = B' − Y' and V = R' − Y'...

signals for transmission.

External links

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Audio applications

Loudness measurements

Telecommunications

Environmental noise measurement

Audio reproduction and broadcasting equipment

Other applications of weighting

See also

External links