Level[ edit ] For labels, the first letter is always an L. This simply stands for Level, as in the sound pressure level measured through a microphone or the electronic signal level measured at the output from an audio component, such as a mixing desk. Main article:
Sound level meter
Level[ edit ] For labels, the first letter is always an L. This simply stands for Level, as in the sound pressure level measured through a microphone or the electronic signal level measured at the output from an audio component, such as a mixing desk. Main article: A-weighting A, C and Z frequency weightings for sound The second letter indicates the frequency weighting.
A-Weighting, with less lower and higher frequencies , and a slight boost in the mid-range, represents what humans are capable of hearing. C-Weighting, more sensitive to the lower frequencies, represents what humans hear when the sound is loud over dB.
The IEC 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. C-frequency weighting is however still used in the measurement of the peak value of a noise in some legislation, but B-frequency weighting — a halfway 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.
Time Weighting[ edit ] Graphs of Fast, Slow and Impulse time weightings applied so that sound levels measured are easier to read on a sound level meter. If the third letter is F, S or I, this represents the time weighting . Time weighting is applied so that levels measured are easier to read on a sound level meter. The time weighting dampens sudden changes in levels, thus creating a smoother display. The graph indicates how this works.
In this example, the input signal suddenly increases from 50 dB to 80 dB, stays there for 6 seconds, then disappears just as suddenly. A Slow measurement yellow line will take approximately 5 seconds attack time to reach 80 dB and around 6 seconds decay time to drop back down to 50 dB. S is appropriate when measuring a signal that fluctuates a lot. A Fast measurement green line is quicker to react.
It will take approximately 0. F may be more suitable where the signal is less impulsive. The decision to use Fast or Slow is often reached by what is prescribed in a standard or a law.
However following can be used as a guideline: The ‘Slow’ characteristic is mainly used in situations where the reading with the ‘Fast’ response fluctuates too much more than about 4dB to give a reasonably well-defined value.
Modern digital displays largely overcome the problem of fluctuating analogue meters by indicating the maximum r. The impulse response, I can be used in situations where there are sharp impulsive noises, such as when measuring fireworks or gunshots.
LAT or Leq: Equivalent values are a form of time weighting that is easier to read on a display than the instantaneous sound level. If you look at these graphs of sound level over time, the area under the blue curve represents the energy.
The horizontal red line drawn to represent the same area under the blue curve, gives us the LAeq. That is the equivalent value or average of the energy over the entire graph. LAeq is not always a straight line. If the LAeq is plotted as the equivalent from the beginning of the graph to each of the measurement points, the plot is shown in the second graph.
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 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. 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 the exponential sound level, displays 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 there were separate standards for exponential and linear integrating sound level meters, IEC and IEC —both now withdrawn , but since then the combined standard IEC 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 Graph of an LAFmax sound level measurement calculated every minute Lmax and Lmin[ edit ] If the words max or min appear in the label, this simply represents the maximum or minimum value measured over a certain period of time.
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