The two targets
Speech intelligibility wants a shorter reverberation time — 0.9–1.2 s. Music envelopment wants a longer RT60 — 1.6–2.0 s for classical, ~1.4 s for amplified. A dual-purpose hall aims for the middle and uses variable acoustics to swing either way.
Variable acoustics — the secret weapon
Retractable curtains, hinged panels, or rotatable side-wall elements can add or remove absorption. A single hall with 20% of surfaces variable can serve both speech and music targets.
Where the absorption goes
Side-wall gripper-fabric panels (with poly-wadding infill) — 60–80% coverage for speech mode. Rear-wall diffusers (never absorbers) to preserve envelopment. Ceiling baffles or clouds scaled to auditorium volume. Front wall + stage: minimal treatment.
Isolation from adjacent halls
Multi-hall complexes need MLV inside inter-hall walls. Target STC 60+ for cinema-adjacent halls, STC 55 for lecture-hall multi-plexes.