A weighting filter is used to emphasize 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 The phon is a logarithmic unit of loudness level for tones and complex sounds. Loudness is measured in sones, a linear unit. Human sensitivity to sound is variable across different frequencies; therefore, although two different tones may pres ...

(1 kHz equivalent level).

Sound

Sound has three basic components, the

wavelength In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats. In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...

frequency Frequency is the number of occurrences of a repeating event per unit of time. Frequency is an important parameter used in science and engineering to specify the rate of oscillatory and vibratory phenomena, such as mechanical vibrations, audio ...

, and

speed In kinematics, the speed (commonly referred to as ''v'') of an object is the magnitude of the change of its position over time or the magnitude of the change of its position per unit of time; it is thus a non-negative scalar quantity. Intro ...

. In sound measurement, we measure the loudness of the sound in

decibel The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a Power, root-power, and field quantities, power or root-power quantity on a logarithmic scale. Two signals whos ...

s (dB). Decibels are logarithmic with 0 dB as the reference. There are also a range of frequencies that sounds can have. Frequency is the number of times a sine wave repeats itself in a second. Normal

auditory system The auditory system is the sensory system for the sense of hearing. It includes both the ear, sensory organs (the ears) and the auditory parts of the sensory system. System overview The outer ear funnels sound vibrations to the eardrum, incre ...

s can usually hear between 20 and 20,000 Hz. When we measure sound, the measurement instrument takes the incoming auditory signal and analyzes it for these different features. Weighting filters in these instruments then filter out certain frequencies and decibel levels depending on the filter. A weighted filters are most similar to natural human hearing. This allows the sound level meter to determine what decibel level the incoming sound would likely be for a normal hearing human's auditory system.

Loudness measurements

In the measurement of loudness, for example, an

A-weighting A-weighting is a form of frequency weighting and the most commonly 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-weighti ...

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 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 An equal-loudness contour is a measure of sound pressure level, 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 an ...

. 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). B curves filter out more medium loudness levels when compared to an A curves. This curve is rarely ever used in the assessment or monitoring of noise levels anymore. C curves differ from both A and B in the fact that they filter less of the lower and higher frequencies. The filter is a much flatter shape and is used in sound measurement in especially loud and noisy environments. A weighted curves follow a 40 phon curve while C weighted follows a 100 phon curve. The three curves differ not in their measurement of exposure levels, but in the frequencies measured. A weighted curves allow more frequencies equal to or less than 500 Hz through, which is most representative of the human ear.

Loudness measurements with weighting filters

There are a variety of reasons for measuring sound. This includes following regulations to protect worker's hearing, following noise ordinances, in

telecommunications Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

, and many more. At the basis of sound measurement is the idea of breaking down an incoming signal based on its different properties. Every incoming sinusoidal wave of sound has a frequency and amplitude. Using this information, a sound level can be deduced from the root-sums-of-squares of the amplitudes of all the incoming auditory information. Whether using a

sound level meter A sound level meter (also called sound pressure level meter (SPL)) is used for acoustics, acoustic measurements. It is commonly a hand-held instrument with a microphone. The best type of microphone for sound level meters is the condenser microp ...

or a

noise dosimeter A noise dosimeter (American English) or noise dosemeter (British English) is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time. It is often used to comply with Health ...

, the processing is somewhat similar. With a calibrated sound level meter, the incoming sounds are going to be picked up by the microphone and then measured by the internal electronic circuits. The sound measurement that the device outputs can be filtered through an A, B, or C weighting curve. The curve used will have slight effects on the resulting decibel level.

Telecommunications

In the field of

, 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 or Din or din may refer to: People and language * Din (name), people with the name * Dīn, an Arabic word with three general senses: judgment, custom, and religion from which the name originates * Dinka language (ISO 639 code: din), spoken ...

standards. The term ''

psophometric weighting Psophometric weighting (from : psóphos "noise") 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 ...

'', 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 A voice frequency (VF) or voice band is the range of audio frequencies used for the transmission of speech. Frequency band In telephony, the usable voice frequency band ranges from approximately 300 to 3400 Hz. It is for this reason th ...

speech circuits.

Environmental noise measurement

A-weighted

s are abbreviated dB(A) or dBA. When acoustic (

calibrated In measurement technology and metrology, calibration is the comparison of measurement values delivered by a device under test with those of a calibration standard of known accuracy. Such a standard could be another measurement device of known ...

microphone) measurements are being referred to, then the units used will be dB SPL (

sound pressure level Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...

) referenced to 20 micropascals = 0 dB SPL. The A-weighting curve has been widely adopted for environmental noise measurement, and is standard in many sound level meters (see ITU-R 468 weighting for a further explanation). A-weighting is also in common use for assessing potential hearing damage 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 pollution refers to noise produced by aircraft in flight that has been associated with several negative stress-mediated health effects, from sleep disorders to cardiovascular disorders. Governments have enacted extensive control ...

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

refrigerator A refrigerator, commonly shortened to fridge, is a commercial and home appliance consisting of a thermal insulation, thermally insulated compartment and a heat pump (mechanical, electronic or chemical) that transfers heat from its inside to ...

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 An anechoic chamber (''an-echoic'' meaning "non-reflective" or "without echoes") is a room designed to stop reflection (physics), reflections or Echo (phenomenon), echoes of either sound or electromagnetic waves. They are also often isolate ...

". 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 The Compact Cassette, also commonly called a cassette tape, audio cassette, or simply tape or cassette, is an analog magnetic tape recording format for audio recording and playback. Invented by Lou Ottens and his team at the Dutch company ...

recorders and Dolby-B noise reduction. 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 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 The British Broadcasting Corporation (BBC) is a British public service broadcaster headquartered at Broadcasting House in London, England. Originally established in 1922 as the British Broadcasting Company, it evolved into its current sta ...

Research Department, and was standardised by the CCIR and later adopted by many other standards bodies ( IEC, BSI/) 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 US 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. 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 ADCs, for instance, because it more accurately represents the way

noise shaping Noise shaping is a technique typically used in digital audio, Image processing, image, and video processing, usually in combination with dithering, as part of the process of Quantization (signal processing), quantization or Audio bit depth, bit-dep ...

hides

dither Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images. Dither is routinely used in processing of both digital audio and video data, and is ofte ...

noise in the ultrasonic range.

Other applications of weighting

In the measurement of

gamma rays A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

or other

ionising radiation Ionizing (ionising) radiation, including nuclear radiation, consists of subatomic particles or electromagnetic waves that have enough energy per individual photon or particle to ionize atoms or molecules by detaching electrons from them. Some par ...

, a radiation monitor or

dosimeter A radiation dosimeter is a device that measures the equivalent dose, dose uptake of external ionizing radiation. It is worn by the person being monitored when used as a personal dosimeter, and is a record of the radiation dose received. Modern el ...

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 The sievert (symbol: SvPlease note there are two non-SI units that use the same Sv abbreviation: the sverdrup and svedberg.) is a derived unit in the International System of Units (SI) intended to represent the stochastic health risk of ionizin ...

is a unit of weighted radiation dose for

, which supersedes the older unit the REM ( roentgen equivalent man). Weighting is also applied to the measurement of sunlight when assessing the risk of skin damage through

sunburn Sunburn is a form of radiation burn that affects living tissue, such as skin, that results from an overexposure to ultraviolet (UV) radiation, usually from the Sun. Common symptoms in humans and other animals include red or reddish skin tha ...

, since different wavelengths have different biological effects. Common examples are the SPF of sunscreen, and the UV index. 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 Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls wit ...

and

chrominance Chrominance (''chroma'' or ''C'' for short) is the signal used in video systems to convey the color information of the picture (see YUV color model), separately from the accompanying Luma (video), luma signal (or Y' for short). Chrominance is usu ...

signals for transmission.

Notes

References

{{Reflist, refs= {{Cite book , title=OSHA Technical Manual (OTM) , chapter=Section III: Chapter 5 - Noise , publisher=Occupational Safety and Health Administration , website=www.osha.gov , url=https://www.osha.gov/dts/osta/otm/new_noise/ , access-date=2020-11-25 {{Cite web , title=Understanding Sound - Natural Sounds , publisher=U.S. National Park Service , website=www.nps.gov , url=https://www.nps.gov/subjects/sound/understandingsound.htm , access-date=2020-11-25 {{Cite web , title=A, B, and C Contour Filters for Sound Measurement , website=hyperphysics.phy-astr.gsu.edu , url=http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/acont.html , access-date=2020-10-12 {{Cite web , title=2. How is sound measured? , website=ec.europa.eu , url=https://ec.europa.eu/health/scientific_committees/opinions_layman/en/hearing-loss-personal-music-player-mp3/l-3/2-sound-measurement-decibel.htm , access-date=2020-11-26 {{Cite web, author-first=Chris, author-last=Woodford, author-link=Chris Woodford (author) , title=How decibel sound level meters work , website=Explain that Stuff , date=2009-01-28 , url=http://www.explainthatstuff.com/soundlevelmeters.html , access-date=2020-11-25 {{anchor, CITEREFFS1037Federal Standard 1037, Glossary of Telecommunication Terms, entry dBa: https://www.its.bldrdoc.gov/fs-1037/dir-010/_1471.htm

External links

Noise measurement briefing

AES pro audio reference definition of "weighting filters"

Audio engineering Acoustics Sound Radiation Light