The Speech Intelligibility Index fails for sharply filtered stimuli.

Evidence

The SII is then not expected to predict the data presented here but other values for the ceiling and floor SNRs than those proposed by the standard are needed to describe speech intelligibility of sharply filtered signals. — Dijk, Pim van, Physiology, Psychoacoustics and…, 3582

Categorization

• DEFINE: Clarifies the boundary of SII applicability — it was not designed for sharply filtered speech or noise.