What Rw actually measures
Rw, the weighted sound reduction index, is a single number summarising how much airborne sound a partition stops when it is tested in a laboratory. A higher number means more reduction. It is a property of the tested panel and seal system on its own, in an opening with no flanking paths.
Why the room never matches it
Your room is not a laboratory. Sound travels over the top of the wall through a ceiling void, under it through a floor that is not flat, and around it through the junctions at each jamb. A wall rated at Rw 50 installed over an unsealed ceiling void can behave like Rw 35, and nothing about the wall is at fault.
The ceiling is the most common problem. A suspended grid with a continuous void above it is an open path between the two halves of the room no matter what the wall does. Sealing that void is usually a builder item, and it has to be in the scope before the wall is worth its rating.
Do you need a rating at all
Only if both halves of the room will be in use at the same time. A wall closed for privacy, or to make a large room feel smaller, does not need one, and an unrated system costs less. A wall closed so a meeting can run while a function is on next door does.
Picking a band
Three questions get most rooms to a sensible target: what is happening on the loud side, what has to happen on the quiet side, and whether the ceiling and floor can be sealed. Speech next to speech needs less than amplified music next to a lecture. The selector on the operable walls page walks through those questions and gives a band to write into a spec, including when the answer is that no rating is warranted.
Over-specifying costs money that did not need spending, and the heavier panels are harder for one person to move. Under-specifying costs the room. Aim for the band the building can actually deliver rather than the highest number on a datasheet.
- Speech on both sides, occasional use: the lower bands
- Meetings that must not be overheard, regular use: the middle bands
- Amplified sound on one side, quiet work on the other: the upper bands, and the ceiling void must be sealed
Seals are the rating
On a rated wall the panels do part of the work and the seals do the rest. Retractable seals drop from the top and bottom of each panel when it is set, pressing against the head and floor. A wall run with the seals left up is not a rated wall, which is why the operating instructions matter as much as the specification, and why servicing keeps the rating rather than just the movement.