A-weighted measurements ignore low-frequency energy that unweighted SPL captures.

Evidence

Unweighted sound pressure levels (SPL) and A-weighted sound levels are not equivalent unless the sound source is narrow band, falling within the A-weighting contour. For example: if a sound has a strong low-frequency component it will be ignored by an A-weighted measurement, but will influence or may even dominate an SPL measurement. — Toole, Floyd E, Sound reproduction- the acoustics and…, 891

Categorization

• DEFINE: Clarifies exactly when A-weighting and unweighted SPL diverge, which is a core conceptual distinction in psychoacoustic measurement.