Sound Measurement
What this domain tests
The Sound Measurement domain checks whether you can produce a defensible acoustic measurement and a reportable number: correct instrument class, weighting, time constant and bandwidth, calibration traceability, and the arithmetic that turns raw readings into the value compared against a criterion. Items are scenario-based - you are given a setup or a set of readings and asked what is valid, what correction applies, or what the answer is.
Instrumentation and calibration
- IEC 61672 / ANSI S1.4 Type 1 tolerance is about ±0.7 dB versus ±1.0 dB for Type 2; Type 1 is expected for design verification and low-level work.
- Calibrate with an acoustic calibrator, normally 94 dB at 1 kHz, before and after the run; record drift and serial numbers.
- Confirm the coupler matches the microphone diameter (1/2 in vs 1/4 in).
Weighting and time response
A-weighting approximates low-level hearing response and is used for dBA criteria. C-weighting retains low frequency for high-level and equipment work; Z (flat) is used for band data. Fast is a 125 ms exponential time constant, Slow 1 s, Impulse 35 ms with fast decay. Leq is an energy average over the stated interval and is the correct descriptor whenever the criterion is written in Leq.
Bands and corrections
Octave and one-third-octave filters follow IEC 61260 Class 1; NC and RC ratings are read from octave-band data, never from a single dBA number. Background is removed in the energy domain: L = 10 log(10^(Ltotal/10) − 10^(Lbg/10)). A 10 dB or greater difference needs no correction, 3–9 dB requires it, under 3 dB the measurement is invalid.
Worked example
Meter shows 68.0 dBA with the HVAC running, 62.0 dBA with it off. Difference 6.0 dB, so correct it. 10^6.8 = 6,309,573; 10^6.2 = 1,584,893; difference = 4,724,680. 10 log(4,724,680) = 66.7 dBA. Report 66.7, not 68, and not "6 dBA of HVAC".
Traps
Adding band levels arithmetically instead of energetically; A-weighting twice (meter already did it); quoting Fast for an impulsive source when the criterion is Leq; measuring closer than about twice the largest source dimension; omitting the windscreen or the note about reflective surfaces; answering NC with a dBA value.