LAS. LAF. LCS. LCF. These are common terms you probably see when taking a noise measurement with your sound level meter. The “F” and “S” indicate the different Time Weightings used for that measurement but what do they mean?
Fast, Slow and Impulse Time Weightings originate from a time before integrating sound level meters and way before digital sound level meters.
When sound level meters were first developed, they used analogueย meters withย needles thatย moved back and forth to give theย user a reading. As noise levels almost never remain constant, the needle was always moving. A shorter needle would be able to move quicker than a longer, heavier needle and so the information given by two different instrumentsย could be very different.
A set of standards were created to ensure that sound level meters from different manufacturers wouldย display the same and these are known as Time Weightings. By effectively applying different weights to the needle, the speed at which the needle moved for a known noise level could be calibrated and measurements could be compared with one another.
Fast, Slow & Impulse Time Weightings
Fast Time Weighting
Fast corresponds to 125 milliseconds up and down. The needle would be moving much faster, showing you the varying noise levels quickly.
The Fast Time Weighting is usually used for measuring all the sound in your environment, which may vary widely overย time. Therefore, you would need to report the range of noise fluctuations e.g. 58 to 64 dB (A). Fast is generally the chosen Time Weighting for noise measurements.
Slow Time Weighting
Slow corresponds to 1000 milliseconds up and down. By slowing down the needle, the noise measurements are easier to read as the data is smoothed out.
By using theย Slow Time Weighting, you are recording sound levels at 1 second intervals. This smoother level history can give you a better indication of the average noise level in an environment where it is constantly changing.
Impulse Time Weighting
Impulse is the least common of the Time Weightings. It’s about four times faster than Fast (35 milliseconds)ย and isย usually used to measure quick bursts of impulsive noise, generated by pile drivers or explosions. Impulseย Time Weighting has often been used for the measurement from, for example, clay pigeon shooting ranges where there is a need to assess the short, impulsive noise created when a shotgun is fired.
The Impulse Time Weighting has a fast rise but a slower decay, which was designed to mimic the ear’s response to these types of sound. However, theย Peak Time Weightingย isย now used to measure the true Peak values of impulsive sounds. It is often specified for measurement in Regulations, such as the 2005 Control of Noise at Work Regulations. Peak is the maximum sound pressure reached at any instant during a measurement period.
Time Weightings in Digital Sound Level Meters
Even though we’ve moved into theร digital age, Time Weightings are still calculated and simulated by sound level meters for the end user. Depending on the function you choose, the decibel reading on your LED display will change faster or slower.
The reason we still use Time Weightings is that they were set out in the Standards for acoustic equipment, which haven’t changed to this day. This means that no matter what generation of sound level meter you are using, you can clearly indicate theย parameters you used to take your noise measurement.
Time Weightings are usually displayed alongsideย A, C or Z Frequency Weightings to give you the noise level you need for your assessment. The abbreviations used at the beginning of this article express how the sound level was measured:
- LAS – Slow, A-Weighted Sound Level
- LAF – Fast, A-Weighted Sound Level
- LCSย – Slow, C-Weighted Sound Level
- LCF – Fast, C-Weighted Sound Level
Why do we note downย these terms when measuring sound levels? It’s so you can ask anyone to measure the sound level using the same parameters to hopefully achieveย same results. You also have to report them for the various Regulations that govern noise measurement for different applications.
Time Weightingsย are still an important parameter forย environment noise monitoring rather than workplace noise monitoring. In the UK for example, you only need to measure using LEQ with the A-Frequency Weighting to comply with the 2005 Control of Noise At Work Regulations, so Time Weighting isn’t necessarily required. However, I always recommend you refer to the Regulations that you are required to meet first andย check what noise level data you require for compliance.
Finding the Right Sound Level Meter
Time Weightings and Frequency Weightings are standard parameters in our Optimusยฎย Sound Level Meters. The Fast and Slow Time Weighting readings are calculated and stored simultaneously for any noise measurement you make, so you can always refer back to them.ย
