The same sound. The same room. The same moment. Three different measurements, three different answers. The filter is the architecture.
The bars show a simulated sound — the kind produced by a motor, a pump, an elevator mechanism running behind a wall. The energy is concentrated in the low frequencies, below 100 Hz, exactly where you feel it more than hear it. The dashed line is the building's noise limit.
Switch between the three weighting curves and watch what happens to the low frequencies. A-weighting — the standard for environmental noise measurement in most building codes — attenuates low frequencies by 30–50 dB before reporting a single number. The sound is still there. The measurement just stopped looking at it.
C-weighting attenuates less at the low end. Flat measures what's actually in the room.
A-weighting was designed to approximate human hearing sensitivity at moderate sound levels — how loud everyday sounds seem to a healthy ear. It was never designed to assess low-frequency noise, vibration, or the kind of resonance that travels through building structures.
When a housing authority measures elevator noise using only dB(A), and the complaint is about low-frequency resonance they can feel through their floor at 3 AM, the methodology is structurally incapable of measuring the problem. That's not a flaw in the measurement. It's a feature of the choice.
The measurement doesn't say the sound doesn't exist. It says the sound doesn't matter. The institution decides what matters by choosing what to measure.
"Dan heeft het geen zin meer" — the measurement professional's own words, realizing the methodology can't capture what they came to measure. On tape.