Published May 9, 2024
You can spend years in construction without learning what all the technical specifications mean in building materials.
But an understanding of the ratings can not only demystify the mindset of architects and designers, it can also lead to better problem-solving on the jobsite.
Here’s an overview of the important specifications and ratings for acoustic panels and ceiling materials, following the noise abatement strategies used in the structures you build.
Sound is transmitted in the form of excited air particles. When those particles hit a surface, they are either reflected (as echoes and reverberation) or absorbed.
NRC measures how effective a material is at absorbing sound; technically, it’s the process of transferring kinetic energy into heat energy.
Solid surfaces like glass and plaster reflect sound. Soft and porous surfaces, like carpet and drapes – and of course engineered acoustic materials – absorb sound.
NRC ratings range from 0 to 1; they represent an average of a room’s ability to absorb sound at different frequencies. The higher the rating, the more sound is absorbed. A closed window has a low NRC, while an open window has an NRC of 1.
An NRC of 0.75 is generally considered effective, and building materials with a 0.9-or-higher rating are considered excellent.
STC is the most commonly used sound rating across building materials in North America.
While NRC measures how well a material absorbs airborne sound, STC measures how well it blocks the transmission of sound. So while NRC relates to the acoustic qualities within a room, STC measures the transmission of sound between rooms.
A higher STC rating means better sound-blocking from one space to another. While STC is a specification more commonly associated with walls, it can also be relevant for ceilings in spaces where sound isolation between rooms or floors is a priority.
Measured in decibels (dB), STC is determined by using the material being tested to build a dividing wall within an acoustical chamber. A sound source is placed in one half and measuring equipment in the other. The difference in sound level between the two rooms determines the STC of the material being tested. If the sound source generates 75 dB and is measured at 25 dB in the other half of the chamber, the material’s STC is 50dB.
CAC is essentially the STC rating for a suspending ceiling system. It measures the ceiling’s ability to prevent transmission of airborne sound between rooms that share a common air plenum, according to Baswa Acoustic, manufacturer of acoustic ceiling materials. The range of CAC ratings is 25-50; CAC 35 and up is considered to be proactive mitigation of noise transmission.
A ceiling’s CAC rating can change over time as materials age and as building modifications are made in the plenum above the ceiling.
Architects and engineers follow four basic techniques to control noise in the structures they design, according to a sound transmission guidance document from the Department of Housing and Urban Development:
1. Elimination or reduction of noise sources
Operating infrastructure such as HVAC equipment and elevators can be specified with noise levels in mind, which is part of the give-and-take in designing to a budget. But outside noises like traffic need to be addressed through other strategies.
2. Noise absorption
This is indicated through specification of construction acoustical materials with high NRC ratings to reduce the sound within a room and high STC ratings to reduce transmission of sound between spaces.
It’s also a function of construction techniques that seal a room properly, such as:
3. Sound barriers
Mindful architecture can help prevent noise transmission. Closets, hallways and stairwells can be used as buffers from airborne sound transmission between separate areas of a structure. Solid partitions with very high STC ratings can be used to separate utility rooms and noisy public areas from spaces where sound levels need to be controlled.
4. Design factors
Beyond the integration of sound barriers in a building design, there are ways to lay out a building that reduce noise and restrict its transmission. For example, according to HUD, “Adjacent apartments can be arranged to have quiet areas abut, and have noisy areas (kitchens and bathrooms) next to similar noisy areas. Apartment door openings into the same hallway can be staggered … [so] some of the sound from one doorway would be absorbed or diffused into the wall building material of the unit directly across the hall. Windows should be placed as far away as possible from common walls. The closer the windows are to each other, the more sound will pass from one apartment to another.”
Best Supply offers a broad range of acoustical building materials, and our team has the know-how to use them effectively. Let us help with your next project. Request a quote here.