Every operable partition brochure leads with an acoustic rating. Very few explain what the number means, how it was measured, or — most importantly — whether the wall will perform that way in your building. This guide covers the three ratings that matter and how to use them when you specify.
STC — the laboratory number
Sound Transmission Class measures how much airborne sound a partition blocks under controlled laboratory conditions. A higher number means better isolation. Advanced Equipment's ALPHA® all-welded steel panels rate up to STC 54, the SIGMA® series reaches STC 53, and the economical GAMMA® series spans STC 45 to 52.
The key word is laboratory. STC is tested with the partition sealed into an ideal opening, with no ductwork above it, no gaps at the floor, and no adjoining structure for sound to travel around. Real buildings are not laboratories.
NIC — the number your occupants actually live with
Noise Isolation Class is measured in the finished building under ASTM E 336 — seals, track, floor conditions, and the surrounding construction all included. Because sound flanks around a partition through ceilings, plenums, and floors, a field NIC is normally lower than the laboratory STC of the same wall.
This gap is where specifications go wrong: a wall sold on its lab STC can disappoint in the field, and the manufacturer can point at the building. Advanced Equipment takes the opposite position — we guarantee field sound ratings on our large operable walls, with guaranteed NIC values published by panel series in our field sound testing specification (Form #2036, available in the document library). ALPHA panels field-test at NIC 42.
NRC — absorption inside the room
STC and NIC measure sound passing through the wall. Noise Reduction Coefficient measures how much sound the panel face absorbs rather than reflects back into the room. For spaces where echo and speech clarity matter — sanctuaries, music rooms, lecture halls — Advanced Equipment offers acoustically absorptive perforated panel faces rated NRC 0.65.
At Congregation Rodef Shalom in San Rafael, ALPHA 'P' perforated acoustic panels at NRC .65 keep spoken word clear in every room configuration — a good example of specifying absorption and isolation together.
The one question to ask
When a manufacturer quotes a sound rating, ask: "Is that a laboratory STC or a guaranteed field NIC — and will you put the field number in writing?" If the answer is only a lab number, plan for lower real-world performance. If the manufacturer guarantees the field rating, the risk shifts off your project.
- STC: laboratory isolation rating — ALPHA to 54, SIGMA to 53, GAMMA 45–52
- NIC: field-tested isolation under ASTM E 336 — AEC guarantees these values by panel series
- NRC: in-room absorption — perforated faces available at 0.65
- Always request the guaranteed field value in writing
Specifying for a project now? Our sound test documentation is in the document library, and DWspec® writes the acoustic requirement into a complete specification in six steps.
