Sound Measurement Terminology
Sound Glossary for Sound Level Meters:
Basic Noise Terminology
Frequency weightings are commonly used frequency filters that adjust the amplitude of all parts of the frequency spectrum of the sound or vibration
- A-Weighting: A weighting filter that most closely matches how humans perceive sound, especially low to moderate levels. This weighting is most often used for evaluation of environmental sounds. Notated by the “A” in measurement parameters including dBA, LAeq, LAF, LAS, etc.
- C-Weighting: Commonly used filter that adjusts the levels of a frequency spectrum in the same way the human ear does when exposed to high or impulsive levels of sound. This weighting is most often used for evaluation of equipment sounds.
- Flat or Linear Weighting: No longer used in current standards. Flat weighting indicated that no filter was applied, across a stated frequency range. Since 2003, IEC 61672 standard notes use of Z-weighting.
- Z-Weighting: "Zero” or no frequency weighting applied: in actuality, a passband filter from 10 Hz to 20 kHz. Often shown as dBZ, LZeq, LZS, etc.
Leq is the level of a constant sound over a specific time period that has the same sound energy as the actual (unsteady) sound over the same period.
where p is the sound pressure and the Measurement Duration (specific time period) is T=T2-T1.
In other words, the actual sound pressure levels will vary all throughout a measurement period. Imagine creating a continuous sound throughout that same period with the same total sound energy as the actual varying sound levels. That continuous sound pressure level is Leq.
Time-weighted Sound Level
Time-weighted Sound Level is defined by the following equation:
- is the frequency weighting, commonly A,C, or Z
- is the exponential time constant in seconds for either the S (slow) or F (fast) time weighting
- is a dummy variable of time integration from some time in the past as indicated by -∞ for the lower limit of the integral to the time of observation, t
- is the frequency-weighted instantaneous sound pressure signal (where frequency weighting is A, C, or Z)
- is the reference value of 20 µPa (considered the threshold of human hearing)
Source for above equation: IEC 61672-1:2013
Think of time-weighted sound levels as a kind of running average of sound level.
Why do we use time weighted sound levels? There are two main reasons. First, it is a closer representation of how the human ear perceives sound. Second, it’s a holdover from the days of analog sound level meters. Back in the early days of measuring sound, everything was analog and there were no digital readouts, only a needle. When these early meters were conceptualized, the inventors quickly realized that if the needle went up and down with sound pressure level then it would move so fast that it would be a blur and couldn’t be seen. These designers figured out a way to slow down the needle in a way that could be easily realized using analog components while still reporting the energy levels correctly.
Two standards were defined: slow and fast, referring to how the needle on the meter generally moved. In simple terms, this method implemented a type of running average that today is called a “time weighting” or “detector.” Because so much early research was done and data were collected using time-weightings, these weightings continue to be widely used today. Slow weighting uses a time constant of 1 second. Fast weighting uses a time constant of 1/8 second or 125 milliseconds. There is also a less commonly used weighting know as impulse weighting. It is used to measure extremely short bursts of sounds and has a rise time of 35 milliseconds. Impulse time weighting has a fall time of 1.5 seconds, so it is asymmetric. It should be noted that "I" time weighting does not correctly report energy levels due to the asymmetry.
Peak Sound Pressure
The peak sound pressure (P, measured in Pascals) is the greatest absolute value of the instantaneous (non-time-weighted) sound pressure during a particular time interval.
Lpeak is the highest instantaneous sound level, in decibels, with no time weighting (see above). It is important to remember that sound is a wave – a vibration as pressure oscillates through a medium (most often in our everyday lives, that medium is air).
Lpeak is measured in decibels and is twenty times the logarithm to the base ten of the ratio of a peak sound pressure to the reference sound pressure.
Where P is the absolute value of peak sound pressure in Pascals and P0 is the reference sound pressure, -20 µPa.
Most of the time, Lpeak is not the most important consideration in characterizing a sound or its effect on the human ear. Our ears perceive sound as an overall level over a short period of time and are more affected by that time-weighted sound level than an instantaneous peak level.
Consider a pure tone such as that emanating from the acoustic calibrator that is used when field calibrating a sound level meter. The pressure generated from the calibrator is in the form of a sine wave. The peak sound pressure of that wave is 1.414 or 2√2 times higher than the RMS value of the wave.
Just a note here that the RMS value of the wave over a given time period is called Leq (more info on Leq elsewhere on this page). For the pure tone discussed above, Lmax equals Leq. In this case, Lpeak will be 3 dB above Lmax (which, again, for steady signals only is equal to the Root Mean Square of the wave). For varying signals, Lpeak and Lmax can be vastly different. In the case of a varying signal, Lpeak cannot be calculated from Lmax or any other measurement.
Lmax is the highest time-weighted sound level measured by the meter during a given period of time (the maximum of the output of the time weighted sound level equation above). The time constant used can be fast or slow. Often, this parameter will be described along with information about the weightings used (for example, LAFmax indicates the maximum level measured with fast detector and A-weighting). Once again, this is NOT the same as Lpeak!
Lmin is the lowest time-weighted sound level measured by the meter over a given period of time (the minimum of the output of the time weighted sound level equation above). Just like for Lmax, the value is based on the time weighted sound level in dB. The time constant used can be fast or slow.
The total time-weighted average sound exposure for an individual worker over an 8 hour work day. This is expressed in the form of a percentage of total allowable sound exposure. If the worker is exposed to varying noise levels throughout the day, it is convenient to measure noise dose with a noise dosimeter. Without a dosimeter, it can get complicated to compute overall noise dose for varying noise levels, because decibel levels can’t be simply averaged. (Noise levels are measured on a logarithmic scale.) Allowable noise dose is not universally agreed upon. There are specific standards in place in the EU, the US, Canada, Australia, and elsewhere. For more terminology related to occupational noise and worker noise dose, see Noise Dosimetry Terminology.