Time Weightings

Time Weightings Explained – Fast, Slow and Impulse

Time weightings are a common specification provided on most sound level meters. Categorised as ‘Slow’, ‘Fast’ and ‘Impulse’ it is not always clear exactly what they are or how they are represented. Thankfully, as with most Jargon, these are easier to understand than you think. The easiest way to understand them is with a little bit of history.

A brief history…

When sound level meters were first created, as with most instruments, digital technology was not readily available and readings were presented to the user via analogue displays. In the case of sound meters this was a needle that moved back and forth across a scale to give a reading. A major issue soon came to light with these needles due to different manufacturers producing them to different specifications. As noise very rarely stays at a constant level, the needle was always in a constant state of motion. Longer and shorter needs would react at different speeds to the same amount of sound producing different results.

A set of standards known as Time Weightings were created. These weighting specified the speed at which the needle had to move. This ensured that different sound meters could be calibrated to these weightings making measurements comparable with each other.

Digital Sound Level Meters – The Next Evolution

Although sound meters now feature high-tech digital displays and analogue needles are no longer in use. Time Weighting remains a measurable variable in sound. Digital meters simulate the delay, changing the reading slower or faster depending on the chosen weighting. This ensures that no matter what generation of sound meter you use, you can always replicate the results with a different meter.

Fast, Slow and Impulse Time Weighting

Fast, Slow and Impulse time weightings are outlined in IEC-61672:2013 determining the speed at which the instrument responds to changing noise levels.

Fast Time Weighting

Fast time weighing is typically the selected weighting for most noise measurements. It is about 8 times quicker the slow weighting and corresponds to roughly 125 milliseconds.

Slow Time Weighting

Slow time weighting records sounds at 1 second internals. Slowing the readings down makes any measurements easier to read and the data is smoothed out. This generally gives a better indication of the average noise level.

Impulse Time Weighting

Impulse is a bit of a niche time weighting and not commonly found on many sound meters. It equates to roughly 4 times faster than the Fast weighting and is most commonly used to measure short bursts of sounds. With its fast rise time and slower decay its original design was to mimic the response of the ear. This job has since been taken over by Peak measurements.

Time Weighting Representation

Time weightings are still represented by their appropriate measurements to allow repeatability. Anybody with an instrument set to the same parameters should be able to replicate your recorded results. Abbreviations indicating how the measurements were taken would be shown as

  • LAS : A = ‘A’ Frequency Weighting, S = Slow Time Weighting
  • LCF : C = ‘C’ Frequency Weighing, S = Fast Time Weighting


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