Sound level meter

An integrating-averaging Cirrus Optimus which complies with IEC 61672-1:2002

A sound level meter is used for acoustic (sound that travels through air) measurements. It is commonly a hand-held instrument with a microphone. The diaphragm of the microphone responds to changes in air pressure caused by sound waves. That is why the instrument is sometimes referred to as a Sound Pressure Level (SPL) Meter. This movement of the diaphragm, i.e. the sound pressure deviation (pascal Pa), is converted into an electrical signal (volts V).

A microphone is distinguishable by the voltage value produced when a known, constant sound pressure is applied. This is known as the microphone sensitivity. The instrument needs to know the sensitivity of the particular microphone being used. Using this information, the instrument is able to accurately convert the electrical signal back to a sound pressure, and display the resulting sound pressure level (decibels dB).

Sound level meters are commonly used in noise pollution studies for the quantification of different kinds of noise, especially for industrial, environmental and aircraft noise. However, the reading from a sound level meter does not correlate well to human-perceived loudness, which is better measured by a loudness meter. The current international standard that specifies sound level meter functionality and performances from is the IEC 61672-1:2013.

Classification

Types

The IEC 61672-1 specifies "three kinds of sound measuring instruments".[1] They are the "conventional" sound level meter, the integrating-averaging sound level meter, and the integrating sound level meter.

The standard sound level meter can be called an exponentially averaging sound level meter as the AC signal from the microphone is converted to DC by a root-mean-square (RMS) circuit and thus it must have a time-constant of integration; today referred to as the time-weighting. Three of these time-weightings have been internationally standardised, 'S' (1 s) originally called Slow, 'F' (125 ms) originally called Fast and 'I' (35 ms) originally called Impulse. Their names were changed in the 1980s to be the same in any language. I-time-weighting is no longer in the body of the standard because it has little real correlation with the impulsive character of noise events.

The output of the RMS circuit is linear in voltage and is passed through a logarithmic circuit to give a readout linear in decibels (dB). This is 20 times the base 10 logarithm of the ratio of a given root-mean-square sound pressure to the reference sound pressure. Root-mean-square sound pressure being obtained with a standard frequency weighting and standard time weighting. The reference pressure is set by International agreement to be 20 micropascals for airborne sound. It follows that the decibel is in a sense not a unit, it is simply a dimensionless ratio—in this case the ratio of two pressures.

An exponentially averaging sound level meter, which gives a snapshot of the current noise level, is of limited use for hearing damage risk measurements; an integrating or integrating-averaging meter is usually mandated. An integrating meter simply integrates—or in other words 'sums'—the frequency-weighted noise to give sound exposure and the metric used is pressure squared times time, often Pa²·s, but Pa²·h is also used. However, because sound was historically described in decibels, the exposure is most often described in terms of sound exposure level (SEL), the logarithmic conversion of sound exposure into decibels.

Note: in acoustics all 'levels' are in decibels.

Personal noise dosimeter

A common variant of the sound level meter is a noise dosemeter (dosimeter in American English). However, this is now formally known as a personal sound exposure meter (PSEM) and has its own International standard IEC 61252:1993.

A noise dosimeter (American) or noise dosemeter (British) is a specialized sound level meter intended specifically to measure the noise exposure of a person integrated over a period of time; usually to comply with Health and Safety regulations such as the Occupational Safety and Health (OSHA) 29 CFR 1910.95 Occupational Noise Exposure Standard[2] or EU Directive 2003/10/EC.

This is normally intended to be a body-worn instrument and thus has a relaxed technical requirement, as a body-worn instrument—because of the presence of the body—has a poorer overall acoustic performance. A PSEM gives a read-out based on sound exposure, usually Pa²·h, and the older 'classic' dosimeters giving the metric of 'percentage dose' are no longer used in most countries. The problem with "%dose" is that it relates to the political situation and thus any device can become obsolete if the "100%" value is changed by local laws. Today, one of the most common devices in use is a miniature PSEM called by many manufacturers a 'dosebadge', or some similar name, as it is so small and light that it somewhat resembles a radiation badge. These tiny devices have the three advantages that not only do they not affect the sound field, but they are so small that they do not interfere with the worker in any way and his work pattern does not change; as well, having no microphone cable, they should have a lower risk of failure, by the cable 'catching on machinery'.

Classes

IEC standards divide sound level meters into two "classes". Sound level meters of the two classes have the same functionality, but different tolerances for error. Class 1 instruments have a wider frequency range and a tighter tolerance than a lower cost, Class 2 unit. This applies to both the sound level meter itself as well as the associated calibrator. Most national standards permit the use of "at least a Class 2 instrument". For many measurements, there is little practical point in using a Class 1 unit; these are best employed for research and law enforcement.

Similarly, the American National Standards Institute (ANSI) specifies sound level meters as three different Types 0, 1 and 2. These are described, as follows, in the Occupational Safety and Health OSHA Technical Manual TED01-00-015, Chapter 5, OSHA Noise and Hearing Conservation, Appendix III:A,[3] "These ANSI standards set performance and accuracy tolerances according to three levels of precision: Types 0, 1, and 2. Type 0 is used in laboratories, Type 1 is used for precision measurements in the field, and Type 2 is used for general-purpose measurements. For compliance purposes, readings with an ANSI Type 2 sound level meter and dosimeter are considered to have an accuracy of ±2 dBA, while a Type 1 instrument has an accuracy of ±1 dBA. A Type 2 meter is the minimum requirement by OSHA for noise measurements, and is usually sufficient for general purpose noise surveys. The Type 1 meter is preferred for the design of cost-effective noise controls. For unusual measurement situations, refer to the manufacturer's instructions and appropriate ANSI standards for guidance in interpreting instrument accuracy."

Measurements

Frequency weighting

Main article: A-weighting
A, C and Z frequency weightings for sound

Certified sound level meters offer noise measurements with A, C and Z frequency weighting. So what is the difference?[4]

If a sound is produced with equal sound pressure across the whole frequency spectrum, it could be represented in the graph alongside by the Z-Weighting line. What humans are physically capable of hearing is represented by the A-Weighting curve. Acoustic sound contains more lower and higher frequencies than humans perceive. The C-Weighting curve represents what humans hear when the sound is turned up; we become more sensitive to the lower frequencies. The A and C weightings are thus most meaningful for describing the frequency response of the human ear toward real world sounds.

The IEC 61672-1:2013 mandates the inclusion of an A-frequency-weighting filter in all sound level meters, and also describes C and Z (zero) frequency weightings. The older B and D frequency-weightings are now obsolete and are no longer described in the standard.

In almost all countries, the use of A-frequency-weighting is mandated to be used for the protection of workers against noise-induced hearing loss. The A-frequency curve was based on the historical equal-loudness contours and while arguably A-frequency-weighting is no longer the ideal frequency weighting on purely scientific grounds, it is nonetheless the legally required standard for almost all such measurements and has the huge practical advantage that old data can be compared with new measurements. It is for these reasons that A-frequency-weighting is the only weighting mandated by the international standard, the frequency weightings 'C' and 'Z' being optional fitments.

Originally, the A-frequency-weighting was only meant for quiet sounds in the region of 40 dB sound pressure level (SPL), but is now mandated for all levels. C-frequency-weighting however is still used in the measurement of the peak value of a noise in some legislation, but B-frequency-weighting - a half way house between 'A' and 'C' has almost no practical use. D-frequency-weighting was designed for use in measuring aircraft noise, when non-bypass jets were being measured and after the demise of Concord, these are all military types. For all civil aircraft noise measurements A-frequency-weighting is used as is mandated by the ISO and ICAO standards.

LAT or Leq: Equivalent continuous sound level

Sound exposure level—in decibels—is not much used in industrial noise measurement. Instead, the time-averaged value is used. This is the time average sound level or as it is usually called the 'equivalent continuous sound level' has the formal symbol LAT as described in paragraph 3,9 "Definitions" of IEC 61672-1 where many correct formal symbols and their common abbreviations are given. These mainly follow the formal ISO acoustic definitions. However, for mainly historical reasons, LAT is commonly referred to as Leq.

Formally, LAT is 10 times the base 10 logarithm of the ratio of a root-mean-square A-weighted sound pressure during a stated time interval to the reference sound pressure and there is no time constant involved. To measure LAT an integrating-averaging meter is needed; this in concept takes the sound exposure, divides it by time and then takes the logarithm of the result.

Short Leq

An important variant of overall LAT is "short Leq" where very short Leq values are taken in succession, say at 1/8 second intervals, each being stored in a digital memory. These data elements can either be transmitted to another unit or be recovered from the memory and re-constituted into almost any conventional metric long after the data has been acquired. This can be done using either dedicated programs or standard spreadsheets. Short Leq has the advantage that as regulations change, old data can be re-processed to check if a new regulation is met. It also permits data to be converted from one metric to another in some cases. Today almost all fixed airport noise monitoring systems, which are in concept just complex sound level meters, use short Leq as their metric, as a steady stream of the digital one second Leq values can be transmitted via telephone lines or the Internet to a central display and processing unit. Short Leq is a feature of most commercial integrating sound level meters—although some manufacturers give it many different names.

Short Leq is a very valuable method for acoustic data storage; initially, a concept of the French Government's Laboratoire National d'Essais (ref 1), it has now become the most common method of storing and displaying a true time history of the noise in professional commercial sound level meters. The alternative method which is to generate a time history by storing and displaying samples of exponential sound level has too many artifacts of the sound level meter to be as valuable and such sampled data cannot be readily combined to form an overall set of data.

Until 2003 there were separate standards for exponential and linear integrating sound level meters, (IEC 60651 and IEC 60804—both now withdrawn), but since then the combined standard IEC 61672 has described both types of meter. For short Leq to be valuable the manufacturer must ensure that each separate Leq element fully complies with IEC 61672.

LCpk: peak sound pressure level

Most national regulations also call for the absolute peak value to be measured to protect workers hearing against sudden large pressure peaks, using either 'C' or 'Z' frequency weighting. 'Peak sound pressure level' should not be confused with 'MAX sound pressure level'. 'Max sound pressure level' is simply the highest RMS reading a conventional sound level meter gives over a stated period for a given time-weighting (S, F, or I) and can be many decibel less than the peak value. In the European Union the maximum permitted value of the peak sound level is 140 dB(C) and this equates to 200 Pa pressure. The symbol for the A-frequency and S-time weighted maximum sound level is LASmax. For the C-frequency weighted peak it is LCpk or LC,peak.

Standardization

Sound level meters

Octave filters

Personal noise dosimeters

Measurement microphones

Room acoustics

Equipment safety

IEC61010-1 Ed. 2.0 (2001–02)

International standards

The following International standards define sound level meters, PSEM and associated devices. Most countries' national standards follow these very closely, the exception being the USA. In many cases the equivalent European standard, agreed by the EU, is designated for example EN 61672 and the UK national standard then becomes BS. EN 61672.

These International Standards were prepared by IEC technical committee 29:Electroacoustics, in cooperation with the International Organization of Legal Metrology (OIML).

Until 2003 there were separate standards for exponential and linear integrating sound level meters, but since then IEC 61672 has described both types. The classic exponential meter was originally described in IEC 123 for 'industrial' meters followed by IEC 179 for 'precision' meters. Both of these were replaced by IEC 651, later renamed IEC 60651, while the linear integrating meters were initially described by IEC 804, later renamed IEC 60804. Both IEC 60651 and 60804 included four accuracy classes, called "types". In IEC 61672 these were reduced to just two accuracy classes 1 and 2. New in the standard IEC 61672 is a minimum 60 dB linear span requirement and Z-frequency-weighting, with a general tightening of limit tolerances, as well as the inclusion of measurement uncertainty in the testing regime. This makes it unlikely that a sound level meter designed to the older 60651 and 60804 standards will meet the requirements of IEC 61672 : 2013. These 'withdrawn' standards should no longer be used, especially for any official purchasing requirements, as they have significantly poorer accuracy requirements than IEC 61672.

Military standards

Organizations

Pattern approval

One of the more difficult decisions in selecting a sound level meter is "How do you know if it complies with its claimed standard?" This is a difficult question and IEC 61672 part 2[7] tries to answer this by the concept of "pattern approval". A manufacturer has to supply instruments to a national laboratory which tests one of them and if it meets its claims issue a formal Pattern Approval certificate.[8] In Europe the most common—and the most rigorous—approval is often considered to be that from the PTB in Germany (Physikalisch-Technische Bundesanstalt). If a manufacturer cannot show at least one model in his range that has such approval, it is reasonable to be wary, but the cost of this approval militates against any manufacturer having all his range approved. Inexpensive sound level meters (under $200) are unlikely to have a Pattern Approval and may produce incorrect measurement results.

Even the most accurate approved sound level meter must be regularly checked for sensitivity—what most people loosely call 'calibration'. To assist in this sensitivity checking, PTB also Pattern Approves sound calibrators to IEC 60942:2003 and in April 2008, the first commercial units were formally approved both at Class 1 and Class 2 level with the approval number PTB-1.61.4028829.

These units consist of a computer controlled generator with additional sensors to correct for humidity, temperature, battery voltage and static pressure. The output of the generator is fed to a transducer in a half-inch cavity into which the sound level meter microphone is inserted. The acoustic level generated is 94 dB which is 1 pascal and is at a frequency of 1 kHz where all the frequency weightings have the same sensitivity.

ANSI/IEC: the Atlantic divide

Sound level meters are also divided into two types in "the Atlantic divide". Sound level meters meeting the USA American National Standards Institute (ANSI) specifications [9] cannot usually meet the corresponding International Electrotechnical Commission (IEC) specifications [10] at the same time, as the ANSI standard describes instruments that are calibrated to a randomly incident wave, i.e. a diffuse sound field, while internationally meters are calibrated to a free field wave, that is sound coming from a single direction. Further, USA dosimeters have an exchange rate of level against time where every 5 dB increase in level halves the permitted exposure time; whereas in the rest of the world a 3 dB increase in level halves the permitted exposure time. The 3 dB doubling method is called the "equal energy" rule and there is no possible way of converting data taken under one rule to be used under the other. Despite these differences, many developing countries try to specify both USA and international specifications in one instrument in their national regulations. Because of this, many commercial PSEM have dual channels with 3 and 5 dB doubling, with some even having 4 dB for the U.S. Air Force.

Other applications

Noise monitoring stations

Noise monitoring station at Muir Woods National Monument in California[11]

Some applications require the ability to monitor noise continuously on permanent or semi-permanent basis. Some manufacturers offer permanent and semi-permanent noise monitoring stations for this purpose. Such monitoring stations are typically based on a sound level meter at the heart and some added capabilities such as remote communication, GPS, and weather stations. These can often also be powered using solar power. Applications for such monitoring stations include airport noise, construction noise, mining noise, traffic noise, rail noise, community noise, wind farm noise, industrial noise, etc.

Modern monitoring stations can also offer remote communication capabilities using cellular modems, WiFi networks or direct LAN wires. Such devices allow for real time alerts and notifications via email and text messages upon exceeding a certain dB level. Systems can also remotely email reports on a daily, weekly or monthly basis. Real time data publication is often also desired, which can be achieved by pushing data to a website.

Building acoustics

Some advanced sound level meters can also include Reverberation Time (RT60) measurement capabilities. Measurements can be done using the Integrated Impulse Response or Interrupted Noise Methods. It is important that such sound level meters comply with latest ISO 3382-2 and ASTM E2235-04 measurement standards. An additional often desired feature is an onboard signal-generator that provides pink or white noise source for optional amplifiers and omni-directional speakers. Such measurements are also required for calculation of wall/partition sound insulation.

Reverberation time

Some advanced sound level meters include reverberation time (RT60) measurement capabilities.

See also

General:

References

  1. ↑ https://webstore.iec.ch/publication/5708
  2. ↑ "OSHA 29 CFR 1910.95 Occupational Noise Exposure Standard". Occupational Health and Safety Administration. 2011-03-03. Retrieved 2012-09-10.
  3. ↑ "OSHA Noise and Hearing Conservation, Appendix III:A". Occupational Health and Safety Administration. 1996-03-07. Retrieved 2013-04-09.
  4. ↑ http://www.nti-audio.com/en/support/faq/frequency-weightings-for-sound-level-measurements.aspx
  5. ↑ "Department of Defense Design Criteria, Noise Limits". Department of Defense. 12 February 1997. Retrieved 2012-09-18.
  6. ↑ "Test Operations Procedure:Sound Level Measurements". US Department of Defense. 1 January 2011. Retrieved 2012-09-18.
  7. ↑ http://webstore.iec.ch/webstore/webstore.nsf/artnum/048670!opendocument
  8. ↑ http://www.nti-audio.com/Portals/0/data/en/XL2-TA-Type-Approval.pdf
  9. ↑ "American National Standard for Sound Level Meters". American National Standards Institute. 2006. Retrieved 2013-04-29.
  10. ↑ "IEC 61672-1, Electroacoustics - Sound level meters - Part 1: Specifications". International Electrotechnical Commission (IEC). May 2002. Retrieved 2013-04-29.
  11. ↑ Barringer, Felicity (21 February 2011). "Shhh, and Not Because the Fauna Are Sleeping". The New York Times. Retrieved 2 August 2014.
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