Making sense of acoustical ratings alphabet soup

Acoustical design plays a major role in helping people feel comfortable and relaxed at work, school and home. With noise pollution cited by the World Health Organization as one of the leading environmental causes of health problems such as high blood pressure, increased stress and poor sleep, it has never been more important to understand the alphabet soup that makes up acoustical ratings.

This article will define each rating system and when each is relevant during the design process, as well as lay out the basics behind the laboratory testing used to generate these ratings.

Why are acoustics important?

Privacy is essential to maintaining a high-quality and healthy indoor environment in offices, schools, medical facilities and homes. Highly rated acoustical assemblies provide:

• Enhanced productivity: By reducing extraneous noise, spaces with enhanced acoustics decrease stress and frustration while promoting productivity and performance — whether it’s a student taking a test or an office worker analyzing data.
• Privacy: Keeping conversations in the rooms where they’re happening provides peace of mind for patients confiding in doctors or business leaders discussing strategic plans.
• Freedom to collaborate: Today’s most successful companies encourage employees to collaborate and sometimes that creative process gets boisterous. Enhanced acoustics gives people the freedom to go all-in during collaboration sessions without bothering colleagues in neighboring rooms.
• Code compliance: Building codes often encompass acoustical performance standards that include sound ratings for wall, floor-ceiling and roof-ceiling assemblies.
• Quiet spaces: From facility managers to homeowners, many clients have high expectations for quiet spaces and privacy that can require acoustical assemblies with ratings that exceed building code requirements.
  

Get to know acoustical ratings and corresponding test methods

Independent testing facilities, such as NGC Testing Services, conduct full-scale acoustical testing in compliance with established test standards. National Gypsum Company, which provides high-value products manufactured by its affiliate companies Gold Bond Building Products, PermaBASE Building Products and ProForm Finishing Products, publishes The SoundBook®, a compilation of tested wall, floor-ceiling and roof-ceiling acoustical assemblies designed to streamline the design process.

What is Sound Transmission Class?

The STC represents a wall or floor-ceiling assembly’s effectiveness at diminishing airborne sound transmission. It’s important to consider STC ratings when designing interior spaces. These ratings are derived from testing assemblies in accordance with ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.

STC testing includes:

1. Mounting a wall or floor-ceiling assembly between a source room and a receiving room.
2. Playing sounds in the source room over 16 frequencies between 125 Hz and 4,000 Hz to replicate people talking.
3. Measuring the reduction of the decibel level for each sound in the receiving room.
4. Plotting the transmission loss values on a graph to form a curve which is positioned in relation to a standard reference curve. The STC is the value of the reference curve at 500 Hz.

Pro Tip: While STC was originally developed for interior walls, you’ll also see architects specify it for exterior walls. As a result, be sure to check the STC rating for all components, such as windows and doors that will be installed in the wall assembly. And keep reading to learn about Outdoor-Indoor Transmission Class, which takes transportation noises into account for building materials used on exterior assemblies.

Learn about the six variables that affect STC ratings.

What is Ceiling Attenuation Class?

CAC is a measure of the ability of a suspended ceiling system to reduce sound transmission over the wall separating adjacent spaces through a shared plenum. It is essential to consider the CAC rating of the ceiling systems where noise is a concern and the walls do not extend to the deck above.

When walls with a high STC do not extend to the floor above, sound can travel through the ceiling, over the wall, and down through the ceiling of the adjacent space. To prevent this, the ceiling should be designed to achieve a CAC that closely matches the STC. The CAC value of acoustical ceiling panels typically range from 25 to 40 while 5/8” Type X gypsum board has a CAC of over 50. Like STC, a higher number represents greater sound reduction.

CAC values are generated by testing assemblies according to ASTM E1414, Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing a Common Ceiling Plenum. This test method is similar to that used to generate STC ratings and includes:

1. Installing the suspended ceiling assembly over two adjacent rooms — one designated as a source room and one designated as the receiving room.
2. Playing sounds in the source room over 16 frequencies between 125 Hz and 4,000 Hz to replicate people talking.
3. Recording the sound levels in decibels with microphones positioned throughout the receiving room.
4. Plotting the transmission loss values on a graph to form a curve which is positioned in relation to a standard reference curve. The CAC is the value of the reference curve at 500 Hz.

Resource Alert: Find ceiling assemblies featuring Gold Bond® Gridstone® Gypsum Ceiling Panels or 5/8” Gold Bond Fire-Shield Gypsum Board with a CAC of 41 or higher on pages 115–116 of The SoundBook.

What is an Impact Insulation Class?

Pay attention to the Impact Insulation Class when selecting floor-ceiling assemblies, especially in apartments and condos where residential units are stacked. Where STC and CAC ratings measure airborne sound transmission, IIC ratings measure how well a floor system reduces impact sounds carried by vibrations through the structure. Foot traffic and furniture moving are two examples of activities that create impact sounds. Floor-ceiling assemblies carry both STC and IIC ratings.

Floor-ceiling assemblies are laboratory tested according to ASTM E492, Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-Ceiling Assemblies Using the Tapping Machine.

1. As with STC and CAC tests, a floor-ceiling assembly is installed between two chambers — one designated as source and the other as receiving.
2. A machine taps on the floor at 16 standard frequencies between 100 Hz and 3,150 Hz in four locations in the source chamber.
3. Microphones measure sound-pressure levels in the chamber below.
4. The sound levels from each tap in the receiving chamber are plotted on a graph and compared to a standard reference graph to find the IIC rating of the assembly.
   

IIC ratings typically range between 25 and 85, with higher numbers indicating better noise reduction. Remember that floor-ceiling assemblies must perform on two levels. On the floor side, impact sounds are mostly mitigated by floor coverings and sound control mats. On the ceiling side, building materials such as the Gold Bond SoundBreak® XP® family of drywall products contribute to airborne sound attenuation.

Resource Alert: Find IIC and STC ratings for floor-ceiling assemblies on pages 103–108 of The SoundBook.

What is Outdoor-Indoor Transmission Class?

OITC is a newer rating system designed to measure how well a roof assembly, wall assembly, window or door inhibits outside noises from coming inside. This rating system focuses on lower-frequency sounds representative of transportation noises.

ASTM E90, the same test method used for STC ratings, is used to calculate OITC ratings, but the OITC test includes a wider range of decibels between 80 Hz and 4,000 Hz to represent outdoor sounds such as car wheels on a road and engine noises.

Resource Alert: Find OITC and STC ratings for roof-ceiling assemblies on pages 111–112 of The SoundBook.

What is Noise Reduction Coefficient?

NRC is a single number rating between 0.0 and 1.0 that measures how well objects and surfaces in a room absorb airborne sound energy to prevent reverberation and echo. Soft materials absorb more sound and have higher NRC values than hard materials. The NRC of wall, floor, and ceiling finishes is most important when designing spaces for large groups of people, such as restaurants, offices, and schools. NRC ratings apply to individual materials tested in accordance with ASTM C423, Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, whereas STC, CAC, IIC and OITC ratings apply only to assemblies.

Get sound advice from the National Gypsum technical support team

Our team of experts is ready to provide project-specific insights and assistance. Call 1-800-NATIONAL for technical support or reach out to your regional construction design manager for architectural design assistance.