Sound level meter
A sound level meter (also called a phonometer ) is a measuring device for determining sound pressure levels . They show level values in decibels (dB), combined with the indication of the level; in particular, frequency and time weighting are displayed. Sound level meters are used where a quantification of noise is necessary, e.g. B. in the assessment of aircraft noise , machine noise, environmental noise and noise in the workplace.
Sound level meters are standardized in IEC 61672: 2013 (DIN EN 61672-1: 2014-07). It stipulates that at least one A-frequency filter must be present. Other frequency filters are also described.
Sound level meters basically consist of a measuring microphone with omnidirectional characteristics with a preamplifier, an evaluation unit and a display. They have housings with a cone shape towards the microphone, which reduces the reflection of the sound on the device. The measurement microphone converts the sound pressure into a corresponding analog electrical voltage . The preamplifier with range switch brings the voltage into the working range of the detector and the display . Here are frequency and time weighting applied. Modern sound level meters are equipped with a digital display, many older models have analog displays. The display shows the level of the time- and frequency-weighted effective value as an analog pointer deflection or digitally using numerical values or LED columns. In addition, sound level meters can be equipped with additional frequency band filters (octave or third octave band) and data memories.
Time evaluation
The time evaluation takes place in the event of a sound level that fluctuates over time in order to average the measured value. The time evaluations “fast” (F = fast, evaluation time 125 ms), “slow” (S = slow, evaluation time 1 s) and “impulse” (I = impulse, evaluation time 35 ms) are standardized. The I-time weighting is no longer taken into account in international standardization, as it correlates little with the actual impulse character of the sound events. An additional time evaluation arises with the cycle maximum method.
Frequency weighting
In most countries, the A-frequency weighting is used for noise measurements in the workplace . The A-frequency curve is based on the historical aurally correct volume ; Although it does not represent a generally valid representation of the spectral hearing sensitivity, it has almost universally established itself in standards and other regulations. This means that historical and current data can be compared with one another. The frequency weightings "B", "C" and "Z" (also "0" or without an index) are optional.
The A-weighting was originally only intended for quiet noises, for example in the 40 dB sound pressure level range, but it is now used in all level ranges. The C-frequency weighting is used when assessing low-frequency noises. The use of the B frequency weighting is nowhere prescribed. The D frequency weighting was designed for measuring aircraft noise at the time when aircraft were being measured without pressure compensation. Since air traffic with the Concorde was discontinued , this only affects military aircraft.
Exponentially averaging sound level meter
The standard sound level meter should be more accurately referred to as the exponentially averaging sound level meter because the AC signal from the microphone is converted to a DC signal by an RMS circuit , so there is an integration time constant now known as time weighting.
The output of the rms circuit has a linear voltage and is passed through a log converter to produce a linear display in decibels (dB).
An exponentially averaging sound level meter can only be used to a limited extent for hearing protection measurements because it only provides individual values for the current noise level. An integrating sound level meter is usually prescribed for this application. An integrating sound level meter integrates the frequency-weighted level measurement to reflect the sound exposure. The unit used for this is pressure squared times time: usually Pa 2 times s, but also Pa 2 times h. Because sound has historically always been expressed in decibels, exposure is usually expressed as the sound exposure level (SEL), which is the logarithmic conversion of sound exposure into decibels.
Measurands
L eq average level or equivalent continuous sound level
In many cases, a time average (continuous energy-equivalent averaging) is calculated for single-number specifications of the sound pressure level. This averaging level or equivalent continuous sound level L Aeq, T (with A- weighting, the averaging duration T is usually not specified ) is described in section 3.10 of IEC 61672-1: 2013. Since this is a time averaging, a time constant does not play a role - you can still find L AFeq in isolated cases .
This must be differentiated from the exposure level L AE, T , which is measured over a fixed time interval (e.g. L AE, 1h ) and is time-weighted.
Short- L eq
To indicate a level curve, mean values are formed over short time intervals (e.g. fast intervals of 0.125 s) and individually stored digitally. The history of this “short L eq ” can either be transferred to another device, or later retrieved from the device memory and converted into almost any conventional metric. The calculation can be performed both by special software and in a spreadsheet. In short, L eq has the advantage that older data can be recalculated for analysis should the legislation change. In some cases the “short L eq ” enables data to be converted from one metric to another. Currently, almost all permanently installed aircraft noise monitoring systems (which are actually just complex sound level meters) use “short L eq ” as their metric. A continuous data flow of the digital 1s "short L eq " values can be transmitted to a central display / processor system via telephone or internet. In short, L eq is a function that is part of most commercial integrated sound level meters. However, it is designated differently by different providers.
In short, L eq is a very valuable method of data storage. It was originally an idea of the French Laboratoire National d'Essais (ref 1) and has become the most common method of storing and displaying the real level over time in professional sound level meters. The alternative method is to generate the level over time by storing and displaying samples of the exposed sound level. Because this method has too many sound level meter artifacts, this data cannot easily be combined to form a single set of data.
Until 2003 there were separate standards for exponential and linearly integrating sound level meters: IEC 60651 and IEC 60804, which have since been withdrawn. Since then, the combined standard IEC 61672 describes both types of sound level meters. In order for the short L eq to remain valuable, the manufacturer must ensure that every single short L eq corresponds to IEC 61672.
L C, peak : peak sound pressure level
Many national laws require that the absolute peak value be measured in either “C” or “Z” frequency weighting to protect workers' hearing from sudden pressure spikes. The peak sound pressure level should not be confused with the maximum sound pressure level. The maximum sound pressure level is simply the highest reading that a sound level meter measures during a measurement, and it can actually be a few decibels below the peak sound pressure value. In the European Union the maximum permissible value of the peak sound pressure level is 140 dB (C), which corresponds to 200 Pa pressure. The maximum sound pressure level is shown (with a frequency weighting of 'A' and a time weighting of 'F' as L AF, max . The C-weighted peak sound pressure level is shown as L C, peak .
Personal noise dosimeter
A variant of the sound level meter is the noise dosimeter. This is officially known as the personal noise exposure meter and is subject to the international standard IEC 61252: 2003. Because the device can be worn on the body, the technical conditions are a bit simpler than for sound level meters. A personal noise dosimeter provides measurement data based on the sound exposure (usually in Pa 2 h). Older models, which give values in% dose, are no longer used in most countries. The problem with a% dose is that it relates to a political situation, and thus any scaling can be out of date when the regional law 100% value is adjusted. One of the devices currently in use is a miniature noise dosimeter, which many manufacturers refer to as a “doseBadge” or something similar. The device is so small that it resembles an X-ray dosimeter. These small devices have the advantage that they not only do not affect the sound field, but also do not disturb the wearer in their work and, because they do not require cables, they do not represent a source of danger.
Classification of sound level meters
Sound level meters are divided into two classes. These were referred to as types in older standards . The two classes differ in the requirements for the accuracy of the measurement. Class 1 devices measure a wider frequency spectrum with a lower tolerance than class 2 devices, which, however, cost less. The difference concerns both the sound level meter and the associated calibrator. Most national standards allow a class 2 sound level meter as a minimum requirement, and for many measurements there is no practical advantage in using a class 1 device. Class 1 sound level meters are sensibly used in the areas of research and law enforcement (e .g . Vehicle registration ) used. New in the IEC 61672 standard is a linear minimum range of 60 dB and a Z frequency weighting. The tolerances were restricted and the inclusion of permissible measurement inaccuracies described in the measurement section of the standard. This means that there is little likelihood that a sound level meter that was built according to the older standards 60651 and 60804 meets the requirements of IEC 61672.
Type approval
IEC 61672 Part 2 describes the type approval for the classification of sound level meters. The manufacturer must have the devices tested for compliance with the classes in a national laboratory. If the devices pass this test, type approval is granted. In Europe the best known institution is the Physikalisch-Technische Bundesanstalt (PTB) in Germany. If a manufacturer cannot show that at least one product from his company has received type approval, the quality of the products must be questioned. However, the associated costs are so high that a manufacturer will hardly be able to have their entire range checked. There are many cheap devices on the market for less than € 200 that are called sound level meters, but so far none of these devices has been able to prove that they actually meet the standard through type approval.
Sound level meters have to be calibrated regularly in order to guarantee an almost constant measurement sensitivity. This is particularly necessary due to changing atmospheric conditions such as temperature, humidity and air pressure. PTB also carries out type approvals for calibrators according to the IEC 60942: 2003 standard. In April 2008 the first commercial devices were approved in both Class 1 and Class 2 with the approval number PTB-1.61.4028829. These calibrators consist of a computer-controlled generator with additional sensors that compensate for humidity, temperature, battery voltage and static pressure. The generator output is fed to a transducer in a Hallb-inch cavity resonator into which the microphone of the sound level meter is inserted. The generated acoustic level is 94 dB (1 Pa) at a frequency of 1 kHz, at which all frequency weightings have the same sensitivity.
International standards
The following international standards define sound level meters, personal sound exposure dosimeters and their accessories. The national standards of most countries (with the exception of the USA) adopt these as far as possible. In terms of content, the national editions of standards are almost identical to the European ones if the national abbreviation is followed by the abbreviation EN in the title.
- IEC 61672: Electroacoustics - sound level meters , 2013 (English and French)
- IEC 61252: Electroacoustics - specifications for personal sound exposure meters , 1993 (English and French)
- IEC 60942: Electroacoustics - sound calibrators , 2003 (English and French)
These international standards were drawn up by the IEC Technical Committee 29: Electroacoustics in collaboration with the International Organization of Legal Metrology (OIML).
See also
- IEC 61260: Octave and fractional octave filters , 1996 (octave and third octave band filters)
- IEC 61094: Measurement microphones , 2000 (measurement microphones)
Until 2003 there were separate standards for exponential and linearly integrating sound level meters. From 2003, IEC 61672 describes both types.
Associations
- German Society for Acoustics (DEGA)
- The International Electrotechnical Commission (IEC)
- The International Institute for Noise Control
Some international manufacturers of approved sound level meters or acoustic devices have provided the IEC working group with their own engineers for cooperation for many years and were involved in the development of IEC 61672.
Each of these companies manufactures new sound level meters that declare compliance with IEC 61672 and has type approval for at least one device. There are many other suppliers who state that they manufacture sound level meters according to the IEC 61672 standard, but who did not participate in the development of the standard and do not yet have a type approval. None of these providers can be found in the following list, because without type approval it is not possible to distinguish commercial claims from reality.
The companies that have contributed to the IEC 61672 standard and have at least one type approval are listed in the following alpha-numerically with their duration of existence (5 years rounded).
- 01 dB-Metravib SA, France {40}
- Brüel and Kjær A / S, Denmark {65}
- Casella CEL Ltd, United Kingdom {210}
- Cesva SL, Spain {40}
- Cirrus Research Plc, United Kingdom {40}
- Norsonic AS, Norway {40}
- Pulsar Instruments Plc, United Kingdom {40}
- Quest Technologies Inc, USA {40}
- Rion Company Ltd, Japan {50}
- SINUS Messtechnik GmbH {20}
There are also some manufacturers who have received type approval, but who were not involved in the development of IEC 61672.
- NTi Audio AG, Liechtenstein {40}
- Svantek Sp. Z oo, Poland {20}
credentials
- A. Komorn & P. Luquet: Method de description objective d'un environnement acoustique . LNE report 1979.
- AD Wallis: From Mahogany to Computers Proceedings Euronoise, London. Plenary paper. Sept 1992.
- Leo L. Beranek: Acoustics Acoustical Society of America, 1993. ISBN 0-88318-494-X .
- RW Krug: Dosimeter standards, Europe & America, what difference does it make? Proc AIHCE 1993.
- Jürgen H. Maue: 0 decibels + 0 decibels = 3 decibels , Erich Schmidt Verlag GmbH & Co., Berlin 2009. ISBN 978-3-503-11488-7 .
See also
Web links
- Decrease in level of sound pressure and sound intensity with distance (PDF file; 76 kB)
Individual evidence
- ↑ http://www.laermorama.ch/m1_akustik/schallpegel_w.html
- ↑ Atmospheric influences on level calibrations. Retrieved March 27, 2020 .