page_title: Sound level meter
section_title: Short Leq
hierarchical_section_title: Sound level meter / Measurements / Short Leq
caption_attribution_description: English: Graph of an LAFmax sound level measurement calculated every minute
context_page_description: A sound level meter is used for acoustic measurements. It is commonly a hand-held instrument with a microphone. The best type of microphone for sound level meters is the condenser microphone, which combines precision with stability and reliability. 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 Meter. This movement of the diaphragm, i.e. the sound pressure deviation, is converted into an electrical signal. While describing sound in terms of sound pressure is possible, a logarithmic conversion is usually applied and the sound pressure level is stated instead, with 0 dB SPL equal to 20 micropascals.
  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.
context_section_description: 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.
caption: Graph of an LAFmax sound level measurement calculated every minute
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