Audio noise measurement
Encyclopedia
Audio noise measurement is carried out when assessing the quality of audio equipment, such as is used in recording studios, broadcast studios, and in the home (Hi-Fi).
Noise
in general refers to unwanted sound
, often loud, but in audio systems it is the low-level hiss or buzz that intrudes on quiet passages that is of most interest. All recordings will contain some background noise that was picked up by microphones, such as the rumble of air conditioning, or the shuffling of an audience, but in addition to this every piece of equipment which the recorded signal subsequently passes through will add a certain amount of electronic noise, which ideally should be so low as to contribute insignificantly to what is heard.
to the signals passing through them, generally described as hum, buzz or hiss. All buildings have low-level magnetic and electrostatic fields in and around them emanating from mains supply wiring, and these can induce hum into signal paths, typically 50 Hz or 60 Hz (depending on the country's electrical supply standard) and lower harmonics. Shielded cables help to prevent this, and on professional equipment where longer interconnections are common, balanced signal connections (most often with XLR
or TRS
connectors) are usually employed. Hiss is the result of random signals, often arising from the random motion of electrons in transistors and other electronic components, or the random distribution of oxide particles on analog magnetic tape. It is predominantly heard at high frequencies, sounding like steam or compressed air.
Attempts to measure noise in audio equipment as RMS voltage, using a simple level meter or voltmeter, do not produce useful results; a special noise-measuring instrument is required. This is because noise contains energy spread over a wide range of frequencies and levels, and different sources of noise have different spectral content. For measurements to allow fair comparison of different systems they must be made using a measuring instrument that responds in a way that corresponds to how we hear sounds. From this, three requirements follow. Firstly, it is important that frequencies above or below those that can be heard by even the best ears are filtered out and ignored by bandwidth limiting (usually 22 Hz to 22 kHz). Secondly, the measuring instrument should give varying emphasis to different frequency components of the noise in the same way that our ears do, a process referred to as ‘weighting’. Thirdly, the rectifier or detector that is used to convert the varying alternating noise signal into a steady positive representation of level should take time to respond fully to brief peaks to the same extent that our ears do; it should have the correct ‘dynamics’.
The proper measurement of noise therefore requires the use of a specified method, with defined measurement bandwidth and weighting curve, and rectifier dynamics. The two main methods defined by current standards are A-weighting and ITU-R 468(formerly known as CCIR weighting).
uses a weighting curve based on ‘equal-loudness contours’ that describe our hearing sensitivity to pure tones, but it turns out that the assumption that such contours would be valid for noise components was wrong. While the A-weighting curve peaks by about 2dB around 2 kHz, it turns out that our sensitivity to noise peaks by some 12dB at 6 kHz. Another weakness of A-weighting is that it is usually combined with an rms (root mean square) rectifier, which measures mean power, with no attempt made to account for proper hearing dynamics.
When measurements started to be used in reviews of consumer equipment in the late 1960s it became apparent that they did not always correlate with what was heard. In particular, the introduction of Dolby B noise-reduction on cassette recorders was found to make them sound a full 10dB less noisy, yet they did not measure 10dB better. Various new methods were then devised, including one which used a harsher weighting filter and a quasi-peak rectifier, defined as part of the German DIN45 500 ‘Hi Fi’ standard. This standard, no longer in use, attempted to lay down minimum performance requirements in all areas for ‘High Fidelity’ reproduction.
The introduction of FM radio, which also generates predominantly high-frequency hiss, also showed up the unsatisfactory nature of A-weighting, and the BBC Research Department
undertook a research project to determine which of several weighting filter and rectifier characteristics gave results that were most in line with the judgment of panel of listeners, using a wide variety of different types of noise. BBC Research Department Report EL-17 formed the basis of what became known as CCIR recommendation 468, which specified both a new weighting curve and a quasi-peak
rectifier. This became the standard of choice for broadcasters worldwide, and it was also adopted by Dolby, for measurements on its noise-reduction systems which were rapidly becoming the standard in cinema sound, as well as in recording studios and the home.
Though they represent what we truly hear, ITU-R 468 noise weighting
gives figures that are typically some 11dB worse than A-weighted, a fact that brought resistance from marketing departments reluctant to put worse specifications on their equipment than the public had been used to. Dolby tried to get round this by introducing a version of their own called CCIR-Dolby which incorporated a 6dB shift into the result (and a cheaper average reading rectifier), but this only confused matters, and was very much disapproved of by the CCIR.
With the demise of the CCIR, the 468 standard is now maintained as ITU-R 468, by the International Telecommunications Union, and forms part of many national and international standards, in particular by the IEC (International Electrotechnical commission), and the BSI (British Standards Institute). It is the only way to measure noise, that allows fair comparisons; and yet the flawed A-weighting has made a comeback in the consumer field recently, for the simple reason that it gives the lower figures that are considered more impressive by marketing departments.
’, both of which have multiple definitions, sometimes treated as synonyms. The exact meaning must be specified along with the measurement.
Since the early 1990s various writers such as Julian Dunn have suggested that dynamic range be measured in the presence of a low-level test signal. Thus, any spurious signals caused by the test signal or distortion will not degrade the signal-to-noise ratio. This also addresses concerns about muting circuits.
In 2000 the AES released AES Information Document 6id-2000 which defined dynamic range as "20 times the logarithm of the ratio of the full-scale signal to the r.m.s. noise floor in the presence of signal, expressed in dB FS" with the following note:
Noise
Noise
In common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
in general refers to unwanted sound
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
, often loud, but in audio systems it is the low-level hiss or buzz that intrudes on quiet passages that is of most interest. All recordings will contain some background noise that was picked up by microphones, such as the rumble of air conditioning, or the shuffling of an audience, but in addition to this every piece of equipment which the recorded signal subsequently passes through will add a certain amount of electronic noise, which ideally should be so low as to contribute insignificantly to what is heard.
Origins of noise - the need for Weighting
Microphones, amplifiers and recording systems all add some electronic noiseElectronic noise
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
to the signals passing through them, generally described as hum, buzz or hiss. All buildings have low-level magnetic and electrostatic fields in and around them emanating from mains supply wiring, and these can induce hum into signal paths, typically 50 Hz or 60 Hz (depending on the country's electrical supply standard) and lower harmonics. Shielded cables help to prevent this, and on professional equipment where longer interconnections are common, balanced signal connections (most often with XLR
XLR connector
The XLR connector is a style of electrical connector, primarily found on professional audio, video, and stage lighting equipment. The connectors are circular in design and have between 3 and 7 pins...
or TRS
TRS connector
A TRS connector is a common family of connector typically used for analog signals including audio. It is cylindrical in shape, typically with three contacts, although sometimes with two or four . It is also called an audio jack, phone jack, phone plug, and jack plug...
connectors) are usually employed. Hiss is the result of random signals, often arising from the random motion of electrons in transistors and other electronic components, or the random distribution of oxide particles on analog magnetic tape. It is predominantly heard at high frequencies, sounding like steam or compressed air.
Attempts to measure noise in audio equipment as RMS voltage, using a simple level meter or voltmeter, do not produce useful results; a special noise-measuring instrument is required. This is because noise contains energy spread over a wide range of frequencies and levels, and different sources of noise have different spectral content. For measurements to allow fair comparison of different systems they must be made using a measuring instrument that responds in a way that corresponds to how we hear sounds. From this, three requirements follow. Firstly, it is important that frequencies above or below those that can be heard by even the best ears are filtered out and ignored by bandwidth limiting (usually 22 Hz to 22 kHz). Secondly, the measuring instrument should give varying emphasis to different frequency components of the noise in the same way that our ears do, a process referred to as ‘weighting’. Thirdly, the rectifier or detector that is used to convert the varying alternating noise signal into a steady positive representation of level should take time to respond fully to brief peaks to the same extent that our ears do; it should have the correct ‘dynamics’.
The proper measurement of noise therefore requires the use of a specified method, with defined measurement bandwidth and weighting curve, and rectifier dynamics. The two main methods defined by current standards are A-weighting and ITU-R 468(formerly known as CCIR weighting).
A-weighting
A-weightingA-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,...
uses a weighting curve based on ‘equal-loudness contours’ that describe our hearing sensitivity to pure tones, but it turns out that the assumption that such contours would be valid for noise components was wrong. While the A-weighting curve peaks by about 2dB around 2 kHz, it turns out that our sensitivity to noise peaks by some 12dB at 6 kHz. Another weakness of A-weighting is that it is usually combined with an rms (root mean square) rectifier, which measures mean power, with no attempt made to account for proper hearing dynamics.
ITU-R 468 weighting
The introduction of FM radio, which also generates predominantly high-frequency hiss, also showed up the unsatisfactory nature of A-weighting, and the BBC Research Department
BBC Research Department
-Function:It has responsibility for researching and developing advanced and emerging media technologies for the benefit of the corporation, and wider UK and European media industries, and is also the technical design authority for a number of major technical infrastructure transformation projects...
undertook a research project to determine which of several weighting filter and rectifier characteristics gave results that were most in line with the judgment of panel of listeners, using a wide variety of different types of noise. BBC Research Department Report EL-17 formed the basis of what became known as CCIR recommendation 468, which specified both a new weighting curve and a quasi-peak
Quasi-peak
Quasi-peak means 'not quite peak', or 'aiming towards peak but not actually peak'. The term is commonly used when referring to electronic detectors or rectifiers. Despite the above definition, the term quasi-peak should not be interpreted as vague in any way...
rectifier. This became the standard of choice for broadcasters worldwide, and it was also adopted by Dolby, for measurements on its noise-reduction systems which were rapidly becoming the standard in cinema sound, as well as in recording studios and the home.
Though they represent what we truly hear, ITU-R 468 noise weighting
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...
gives figures that are typically some 11dB worse than A-weighted, a fact that brought resistance from marketing departments reluctant to put worse specifications on their equipment than the public had been used to. Dolby tried to get round this by introducing a version of their own called CCIR-Dolby which incorporated a 6dB shift into the result (and a cheaper average reading rectifier), but this only confused matters, and was very much disapproved of by the CCIR.
With the demise of the CCIR, the 468 standard is now maintained as ITU-R 468, by the International Telecommunications Union, and forms part of many national and international standards, in particular by the IEC (International Electrotechnical commission), and the BSI (British Standards Institute). It is the only way to measure noise, that allows fair comparisons; and yet the flawed A-weighting has made a comeback in the consumer field recently, for the simple reason that it gives the lower figures that are considered more impressive by marketing departments.
Signal to noise ratio and Dynamic range
Audio equipment specifications tend to include the terms ‘signal to noise ratio’ and ‘dynamic rangeDynamic range
Dynamic range, abbreviated DR or DNR, is the ratio between the largest and smallest possible values of a changeable quantity, such as in sound and light. It is measured as a ratio, or as a base-10 or base-2 logarithmic value.-Dynamic range and human perception:The human senses of sight and...
’, both of which have multiple definitions, sometimes treated as synonyms. The exact meaning must be specified along with the measurement.
Analog
Dynamic range used to mean the difference between maximum level and noise level, with maximum level defined as a clipping signal with a specified THD+N. The term has become corrupted by a tendency to refer to the dynamic range of CD players as meaning the noise level on a blank recording with no dither, (in other words, just the analog noise content at the output). This is not particularly useful; especially since many CD players incorporate automatic muting in the absence of signal.Since the early 1990s various writers such as Julian Dunn have suggested that dynamic range be measured in the presence of a low-level test signal. Thus, any spurious signals caused by the test signal or distortion will not degrade the signal-to-noise ratio. This also addresses concerns about muting circuits.
Digital
In 1999, Dr. Steven Harris & Clif Sanchez Cirrus Logic published a white paper titled "Personal Computer Audio Quality Measurements" stating:In 2000 the AES released AES Information Document 6id-2000 which defined dynamic range as "20 times the logarithm of the ratio of the full-scale signal to the r.m.s. noise floor in the presence of signal, expressed in dB FS" with the following note:
See also
- Audio quality measurementAudio quality measurementAudio quality measurement seeks to quantify the various forms of corruption present in an audio system or device. The results of such measurement are used to maintain standards in broadcasting, to compile specifications, and to compare pieces of equipment....
- Distortion measurement
- NoiseNoiseIn common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
- Sound level meterSound level meterSound level meters measure sound pressure level and are commonly used in noise pollution studies for the quantification of almost any noise, but especially for industrial, environmental and aircraft noise. However, the reading given by a sound level meter does not correlate well to...
- ITU-R 468 noise weightingITU-R 468 noise weightingITU-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...
- Noise measurementNoise measurementNoise measurement is carried out in various fields.In acoustics, it can be for the purpose of measuring environmental noise, or part of a test procedure using white noise, or some other specialised form of test signal....
- Headroom
- Weighting filterWeighting filterA weighting filter is used to emphasise or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.- Audio applications :...
- Equal-loudness contourEqual-loudness contourAn 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...
- Fletcher-Munson curves