A typical plaster on lath assembly rates around STC 40. That lets loud speech through, so it is often worth upgrading between bedrooms, offices and meeting rooms.
Most effective upgrade
Seal cracks and gaps, then add a resilient-mounted drywall layer if needed.
Acoustic panels on the surface add only 1–3 STC points. They are for the sound inside the room. See STC vs NRC and flanking paths.