Courtesy: Pattern approval for measuring instruments
eq = equivalent. 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 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.
hort 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 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 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.
Graph of an LAFmax sound level measurement calculated every minute
Lmax and Lmin
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.
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 decibels 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.