Acoustic Design Requirements: STC, NRC, and NC Ratings by Space Type

There is no single code table that assigns one STC, NRC, and NC value to every space type. First identify the adopted building code, owner or agency standard, rating-system requirements, and any occupancy-specific criteria. Then coordinate three different parts of the room: sound isolation between spaces, sound absorption and reverberation within each space, and background sound from HVAC and other building systems.

The values below are selected design targets from the cited GSA and ASHRAE sources. They are not universal minimums for private-sector offices, healthcare, schools, housing, or projects outside the United States.

Use this acoustic-criteria sequence

STC is not the same as installed room-to-room isolation

ASTM E90 measures airborne sound-transmission loss of a partition or building element in a laboratory. ASTM E413 turns the frequency-band results into a single-number Sound Transmission Class (STC). ASTM states that laboratory results do not directly represent a building because real construction has flanking paths; the tested assembly represents an upper limit to what may be measured in the field. ASTM E336 provides field measurement procedures, and E413 assigns names such as field sound transmission class and noise isolation class to applicable field results.

Therefore, specify the required metric as well as the number. “STC 45 in the field” is ambiguous. A project might instead require a laboratory-rated STC assembly plus detailing and quality control, or a separately stated field isolation target verified after construction.

Doors, perimeter seals, glazing, outlets, above-ceiling paths, ductwork, structureborne paths, and workmanship can all control the result. A tested gypsum-board wall description alone does not establish the performance of the completed room.

Selected GSA office recommendations

GSA’s 2011 Sound Matters is office-planning guidance for contemporary federal workplaces. Its “Ten Steps” summary provides these selected recommendations:

These are source-specific design recommendations, not IBC requirements and not automatic targets for every office. Where a GSA project, lease, owner standard, or specification adopts them, capture that requirement explicitly. For other projects, use the applicable project criteria rather than copying the table without authority.

An STC number also does not prescribe one construction. Do not infer that every STC 45 partition requires resilient channel, or that every STC 53 partition requires one particular decoupled wall. Select a tested assembly suitable for the actual framing, layers, insulation, openings, structure, and fire or structural requirements, then detail its interfaces and field-quality controls.

NRC describes absorption, not sound isolation

ASTM C423 measures sound absorption in a reverberation room and reports frequency-band absorption coefficients and single-number ratings including Noise Reduction Coefficient (NRC). A high-NRC ceiling can reduce reflected sound and support reverberation control, but it does not give the enclosing wall a higher STC or stop sound traveling through an open plenum.

Material NRC is also not the same as a complete room reverberation-time result. Room volume, finish areas, furnishings, occupancy, distribution of absorption, and frequency all matter. If the governing standard specifies reverberation time, calculate or measure that criterion directly rather than substituting a material NRC value.

Selected ASHRAE HVAC background-sound targets

The 2023 ASHRAE Handbook—HVAC Applications, Chapter 49, Table 1 gives recommended targets for HVAC-related background sound in unoccupied, fully built-out and furnished rooms under normal or full-load design operation. Selected values include:

ASHRAE describes these as experience-based guidelines, not exact limits for every user or project, and notes that a project-selected target may vary by approximately ±5 dB. It gives the example that an executive office or specialty conference room might warrant NC/RC 25, while making a space quieter does not always improve privacy. Acoustically critical rooms below NC/RC 30 and performing-arts spaces warrant specialist input.

NC uses octave-band criteria and a tangency procedure. RC adds an assessment of spectral balance and can identify low-frequency or other sound-quality problems that one number may obscure. Do not treat NC as an A-weighted decibel limit or assume that meeting one curve eliminates tones, rumble, throbbing, vibration, exterior noise, elevator noise, or sound transmitted between rooms.

Privacy must be defined by the project

“Normal” and “confidential” privacy are performance intents, not complete specifications. Privacy depends on the source level, room separation, background sound or masking, listener position, door and glazing performance, and leakage and flanking paths. If a project uses a privacy index or speech-privacy metric, state the measurement method, source and receiver conditions, and acceptance threshold.

HIPAA does not itself assign a universal STC, NC, or privacy-index value to examination rooms or reception desks. A healthcare project may have owner, FGI, certification, state, or risk-management criteria, but those authorities must be identified separately. Do not label a GSA office recommendation as a HIPAA acoustic minimum.

LEED is a rating-system check, not a generic code table

LEED acoustic requirements depend on the registered rating system, version, project type, prerequisite or credit, and current addenda. USGBC’s v4.1 materials distinguish categories such as HVAC background noise, sound transmission, and reverberation time, and the requirements vary among rating systems. Confirm the exact registered version in the LEED credit library and project documents. Do not combine school, healthcare, and office thresholds into one “LEED v4.1” table.

Coordinate the full source-path-receiver system

An acoustic review should follow the source, transmission path, and receiver. At minimum, coordinate:

• adjacency and zoning of loud, quiet, private, and vibration-sensitive spaces;
• complete wall, floor, ceiling, door, glazing, and penetration assemblies;
• above-ceiling, underfloor, façade, duct, structureborne, and plumbing paths;
• room volume, finish absorption, and reverberation criteria;
• equipment sound-power data, duct attenuation and regenerated noise, diffusers, and vibration isolation;
• sound-masking scope and commissioning where used; and
• laboratory reports, product substitutions, construction observations, and field testing required by the contract.

The same STC wall can produce different room performance when the door, ceiling, perimeter, duct path, or workmanship changes. The acoustic criteria matrix should therefore be coordinated with drawings, specifications, mechanical design, and commissioning rather than added as an isolated finish-schedule note.

What this guide can and cannot determine

This guide can help establish a project-specific acoustic criteria workflow and interpret selected GSA office and ASHRAE HVAC targets. It cannot determine the required criteria without the governing project documents, occupancy, owner and agency requirements, applicable code and certification, room geometry, assemblies, mechanical design, and measurement conditions. Specialist acoustic analysis is appropriate for critical listening, performing arts, recording, healthcare, education, high-noise, vibration-sensitive, and unusually stringent privacy spaces.

Primary sources

• GSA: Sound Matters—How to Achieve Acoustic Comfort in the Contemporary Office
• ASHRAE: 2023 Handbook—HVAC Applications, Chapter 49, Noise and Vibration Control
• ASTM E90: Laboratory Measurement of Airborne Sound Transmission Loss
• ASTM E413: Classification for Rating Sound Insulation
• ASTM E336: Field Measurement of Airborne Sound Attenuation Between Rooms
• ASTM C423: Sound Absorption and Sound Absorption Coefficients
• USGBC: LEED v4.1

Correction history

• September 1, 2026 — Revision 1: Removed the false universal room-requirements framing; separated laboratory STC from installed field isolation; bounded GSA targets to federal office guidance; updated HVAC background-sound values and method descriptions to the 2023 ASHRAE Handbook; removed unsupported assembly prescriptions, HIPAA thresholds, international comparisons, and causal claims; and reassigned the page from IBC to the AEC Wiki acoustics topic.