STC and NIC Explained: How to Choose the Right Acoustic Rating for Sound Isolation and Building Privacy
In architectural acoustics, the terms STC (Sound Transmission Class) and NIC (Noise Isolation Class) are often used interchangeably — but they measure different things. For architects, developers, interior designers, and building professionals working to ensure acoustic comfort and privacy in multi‑unit residential, commercial, or institutional buildings, understanding the distinction between these ratings is essential for designing effective sound isolation.Though both STC and NIC are measures of how well a partition attenuates airborne sound transmitting directly through a partition, they differ in their methodology, context, and reliability as predictors of acoustic performance in the built environment.
How Airborne Sound Travels Between Rooms
- Sound is Generated in Room 1 — A sound source (for example, a TV or conversation) radiates sound waves within Room 1. Unless the room is highly absorptive or the sound is very low frequency, reflections from walls, ceiling, and floor create a relatively even (diffuse) sound field.
- The Sound Transmits Through a Separation Boundary — The sound energy reaches the demising assembly (e.g., wall or floor/ceiling) between spaces. Some sound is absorbed at this boundary, but a portion still passes through the materials — and some may also travel around the separation via flanking paths such as shared ductwork, structure, or gaps.
- The Sound Radiates into the Adjacent Room 2 — The transmitted sound enters Room 2. Like with Room 1, it reflects off the room’s surfaces, which may absorb or scatter portions of the sound depending on the finishes and furnishings.
- The Sound Level in Room 2 is Measured at a Specific Location — The resulting sound level in Room 2 is what occupants actually hear after the sound energy has passed through the boundary and has been reflected and/or absorbed by elements in the new room.
Understanding NR, TL, STC, and NIC
Noise Reduction (NR) — Is defined as the difference in sound levels between Room 1 and Room 2 in the process described above. This reflects what people actually hear in Room 1 versus Room 2 after walking between them.
Transmission Loss (TL) — The difference in sound level measured on each side of the demising assembly before reflections or absorptions occur in Room 2. TL represents more of a targeted descriptor of how much sound the partition boundary itself blocks between rooms.
From these measurements come the standardized ratings:
STC / ASTC (Apparent Sound Transmission Class): Derived from TL (or Apparent Transmission Loss – ATL). These ratings only depend on how sound transmits through (or around) the demising separation.
NIC (Noise Isolation Class): Derived from NR. This rating depends not only on the demising wall/ceiling/floor construction, but also on the absorptive qualities of Room 2 (such as sound reduction panels, carpets, furniture).
This means that STC/ASTC are more accurate in terms of defining the performance of the demising separation, while NIC is more accurate in terms of defining the difference in sound level between two specific rooms.
Noise Isolation Class (NIC) Criteria Chart
NIC Rating Guide
| NIC Rating |
Subjective Performance |
Typical Measured Performance Characteristics |
| NIC 30–34 |
Very poor – Speech and everyday activity heard clearly and easily |
Indicates a system that provides very little airborne noise isolation. The measured result reflects that sound passes readily through or around the partition as installed, regardless of why. |
| NIC 35–39 |
Poor – Normal speech clearly audible; low privacy |
Indicates a measurement in which the partition system offers limited reduction of speech and mid frequency noise. The measured isolation is low enough that typical conversation remains easily heard. |
| NIC 40–44 |
Fair – Loud speech intelligible; everyday noise still noticeable |
Reflects a system providing moderate airborne isolation. Normal speech becomes less distinct, but raised voices, televisions, and daily activity remain clearly perceivable through the partition. |
| NIC 45–49 |
Fair – Loud speech heard but not understood; moderate separation |
Indicates a system providing noticeable noise reduction. Normal speech is muffled, but louder activity still produces audible cues. Provides some practical privacy, though not high performance. |
| NIC 50–54 |
Good – Loud speech faint; normal activity rarely distracting |
Reflects a system performing at a solid level of airborne separation. Speech and day‑to‑day activity from the adjacent space become faint and often unobtrusive. |
| NIC 55–59 |
Very good – Very loud sounds faint or barely audible |
Indicates strong delivered airborne isolation. Only intense or unusual sounds are transmitted at low levels, and most routine activity is effectively controlled. |
| NIC 60+ |
Excellent – Even shouting generally inaudible; strong acoustic separation |
Represents a system with very high measured airborne isolation. Achieves a strong sense of acoustic separation between spaces even under demanding conditions. |
