Making low level measurements like of the background noise is always a challenge. There is always someone or something you don’t want to measure that decides to interfere.
In the past when sound level meters had little memory and processing power, a simple back-erase was a solution. For attended, non-logging measurements, the operator would stand with sound level meter waiting to press a button if an unwanted noise source ruined a measurement. If he or she was attentive to any external noise sources and could remember where the back erase button was, they could remove the previous 5 seconds from the measurement. Typically the back erase would then pause the measurement. The attentive operator could then un-pause to continue the measurement once the unwanted sound source had quieted down.
Modern sound level meters can log the sound every second and also simultaneous record the audio to a WAV file. Back at the office, a noise consultant can examine the logged data and listen to the audio. The consultant can then make the professional judgement which parts should be excluded from the total.
This way of working produces more reliable results and does not rely on the operator in the field:
A. Being close to the meter – possible compromising the sound field and adding their own, self-generate noise
B. Recognizing that a new sound is something that needs to be excluded within the time limit of the back erase function, typically 5 seconds
C. Remembering there is a back erase function and how to initiate it
D. Re-starting the measurement once the unwanted noise source has passed
Removing unwanted sound sources in the field
Sometimes an initial answer, with the unwanted noise source removed, is needed in the field.
The parameter L90 (often measured as LAF90), describes the level which was exceeded for 90% of the time is often used as an approximation of background noise, see https://www.nti-audio.com/en/support/know-how/how-are-percentile-statistics-measured . L90 automatically removes short transient events without the operator needing to act. It is good practice to compare the LAFmax, LAeq and LAF90 to see how the noise varied over the measurement period.
Example showing simultaneous measurement of LAFmax, LAeq and LAF90 by the XL2 with installed optional Extended Acoustic Pack