Measurement, Instrumentation and Signal Analysis
This domain tests instrument class, calibration discipline, time and frequency weighting, and whether you can extract the right metric from a signal without artefacts.
Core rules
- Sound level meters conform to IEC 61672-1: Class 1 tolerances are tighter (±0.7 dB at 1 kHz reference conditions) than Class 2 (±1.0 dB); the class must be adequate for the standard being applied.
- Calibrate immediately before and after each measurement series with a pistonphone or acoustic calibrator, typically 94 dB at 1 kHz; a drift greater than about 0.5 dB invalidates the data.
- Microphone orientation matters: free-field microphones point at the source, pressure microphones are used flush-mounted or in couplers, and random-incidence types are used for diffuse fields. A windshield is mandatory outdoors.
- Time weightings: Fast (125 ms), Slow (1 s), Impulse (35 ms). Leq is an energy average over the whole period and is independent of time weighting.
- Digital analysis: sample above twice the highest frequency of interest (Nyquist), apply a window (Hann for continuous broadband signals) to limit leakage, and state the FFT line spacing and averaging type.
Worked example
A workroom is 90 dB for 10 minutes and 80 dB for the remaining 50 minutes. Leq = 10 log10[(10×10^9.0 + 50×10^8.0)/60] = 10 log10(2.5×10^8) = 84.0 dB. Note how the loud ten minutes pull the hourly average 4 dB above the quiet background.
Traps that cost marks
- Leaving an over-range or under-range indicator unchecked, so clipped data are reported as valid.
- Reporting a Fast-weighted reading as if it were Leq.
- Ignoring aliasing, or using a rectangular window on a non-stationary signal.
- Failing to correct for background noise or to report the residual when the source–background difference is under 3 dB.