By: Henry Ashburn, MCH Acoustical Consultant
Have you ever wondered what all those letters stand for when looking at technical acoustical data for certain products or construction assemblies?
Acoustics, like many other fields, is full of acronyms. Our goal is to explain some of the most common acronyms for a smoother design and decision-making process.
NRC: Noise Reduction Coefficient
NRC is a measure of how much sound a material absorbs in the speech frequencies. The greater the number, the better the material is at absorbing sound. NRC tends to range from 0 (no absorption) to 1 (total absorption). NRC is measured in a laboratory under ideal conditions.
Example - something highly reflective (such as gypsum board) might have an NRC of 0.10, absorbing roughly 10% of the sound that hits the surface. On the other hand, something highly absorptive (such as a glass fiber or polyester panel) might have an NRC of 0.90, absorbing roughly 90% of the sound that hits the surface.
STC: Sound Transmission Class
STC is a measure of how well a wall, floor, window, or other assembly can block airborne sound in the speech frequencies. The greater the number, the better the assembly is at blocking sound. STC does not directly address low-frequency sound energy, which is typically of keen interest on performing arts projects, cinemas, or for noisy mechanical rooms. STC values for common constructions are typically between 30 and 60. STC is measured in a laboratory under ideal conditions.
Example - a typical wood stud wall might have an STC of 35, while a 6” concrete wall might have an STC of 55.
For a more in-depth look at STC and how it is calculated, please check out our blog post “What is STC?”
IIC: Impact Insulation Class
IIC is a measure of how well a floor/ceiling assembly can block impact noise, such as from footsteps or dropped objects. The greater the number, the better the floor/ceiling assembly is at blocking impact noise. IIC values for common constructions are typically between 20 and 65. Carpet and resilient materials such as recycled crumb rubber underlayments are often used to increase IIC performance of floor assemblies. If IIC cannot be improved from the top-side of the assembly, a common solution is to resiliently suspend a continuous hard lid ceiling (gypsum board) below the overhead floor assembly. However, in some cases, a floated concrete floor is necessary to achieve sufficient levels of isolation. IIC is measured in a laboratory under ideal conditions.
Example – a 6” concrete slab might have an IIC of 25, while a 6” concrete slab with carpet tiles and a specific 3mm thick crumb rubber underlayment might have an IIC of 58.
CAC: Ceiling Attenuation Class
CAC is a measure of how much airborne sound a ceiling can block between adjacent rooms. The metric is typically used for ACT (acoustic ceiling tiles). The greater the CAC, the better the ceiling is at blocking sound between two horizontally-adjacent rooms with a shared ceiling plenum. Unlike the other single-pass metrics discussed in this article, CAC is a two-pass metric, meaning the measurement method quantifies how much sound makes its way up through the ceiling on one side of a common partition, and then back down through the ceiling on the other side of the common partition. CAC values are typically between 20 and 50, and typically go hand-in-hand for maintaining STC performance between rooms.
DTC: Door Transmission Class
DTC is a measure of how well an operable door assembly can block airborne noise. The greater the number, the better the door assembly is at blocking noise. The measurement process involves measuring loud pink noise on both sides of the door, in both the open and closed positions. DTC applies to doors not exceeding 64 sq. ft. and can be used to quantify sound isolation of both interior and exterior doors. DTC is a field measurement under real-world conditions. DTC is often 5-10 points lower than the associated STC for the same door assembly. It is important that all gasketing is adjusted for a proper airtight seal upon door closure prior to any measurements of DTC.
NC: Noise Criteria
NC is a measure of the steady continuous background noise level in a room, usually HVAC noise. The lower the number, the quieter the room. NC are frequency spectrum curves that range from NC 15 to NC 70. Each curve encompasses frequencies between 31.5Hz and 8kHz, and therefore include frequencies lower than speech, such as from rumble. NC is a field measurement and should be taken with the room in its normal operating condition after HVAC Test & Balance has been completed.
Example – recording studios and concert halls are often NC 15-20 (max), while classrooms are NC 30 (max), and lobbies can be NC 40 (max).
Takeaway
There are a lot of different acronyms, and we hope this blog offers a better understanding of some of the most common ones in acoustics that you may encounter on your next project.
One word of caution is that single number ratings such as those above can be useful for general reference, but often fail to tell the entire story. It is important to address project goals with your acoustical consultant when designing assemblies and treatments to ensure all frequencies of concern are addressed appropriately.
If you have any questions or need more guidance on how to choose the best products, construction assemblies, or solutions, please feel free to contact us on our contact page, email us at info@mchinc.com, or call us at 818-991-9300. We are always eager to assist you and your team to find cost effective and reliable solutions.
