Impact Insulation Class
Beyond airborne sound, multi-story building designs need to address the resistance of structure borne sound, usually created by people walking or creating other impacts onto the floor/ceiling above a space. Similar to STC ratings which address airborne sound, floor/ceiling assemblies can be evaluated based on Impact Insulation Class (IIC) ratings. These IIC ratings reveal the ability of a floor-ceiling assembly to absorb or deflect impact/structure borne noise and keep it from being transmitted to the space below. A floor/ceiling assembly with a low IIC rating will allow distracting noise to be transmitted into the room below leading to the associated problems of distraction and hampered communication.

Reverberation Time and Speech Intelligibility
After the issue of ambient noise level is addressed by limiting the airborne and structure borne transmission of sound in a space, then the fourth significant aspect of acoustic design needs attention, namely Reverberation Time (RT). Sound reflections are created when noise reverberates and echoes around architectural spaces. RT is the acoustical concept which measures how long, in seconds, it takes for these echo noises to become inaudible. These echoes can impair what acoustical specialists call “speech intelligibility” since the echoes create garbled sounding words and impair verbal communication. Measuring Reverberation Time is important to determine the sound quality of speech and music in acoustical spaces. Instructional spaces, such as classrooms for example, are best with short RTs – less than 1 second to ensure clarity and high speech intelligibility. Speech generated in a space with a reverberation time of longer than 0.6 seconds is considered difficult to understand. Although some reverberation within a space can aide in speech distribution, longer reverberation times will cause a build-up of noise and thus degrade speech intelligibility. Auditoriums, theaters, and other musical spaces will typically benefit from longer RTs, typically greater than 1.5 seconds.

RT is determined by looking at both the room volume and sound absorption rate in an acoustical space. The volume of a space is proportional to the RT of that space; the greater the volume, the longer the RT. Inversely, the amount of sound-absorbing material in any space will have a negative effect on the RT. As an example, a large space with tiled floors and a gypsum board ceiling will have a long RT. Conversely, a small room with a low suspended ceiling and high-pile carpet will have a much shorter RT. The key is to find the right balance of noise reflective and absorptive surfaces for a particular interior space.

Directly related to RT is the amount of sound energy absorbed upon striking a particular surface. The more sound energy that is absorbed then the less that is reflected back as an echo or reverberation. The commonly used scale to record different levels of such sound absorption determined per the ASTM C 423 test method  is a Noise Reduction Coefficient (NRC) which ranges from zero to one: an NRC of 0 indicates perfect reflection while an NRC of 1 indicates perfect absorption. In actuality, it is the average of four sound absorption coefficients of the particular surface at specific frequencies of 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz. These are the typical frequencies of human speech, and, therefore, the NRC provides a standardized, simple quantification of how well the particular surface will absorb the human voice. A more broad frequency range should be considered for applications such as music or controlling mechanical noise. Acoustical materials manufacturers  may report NRC values higher than 1.0 for highly absorptive materials such as fiberglass ceiling panels due to the way the number is calculated in a laboratory which does not  account for diffraction effects caused by the sides of the test panel and can increase the sound absorption coefficients above 1.0. The Sound Absorption Average (SAA) is another single number rating of sound absorption defined in ASTM C 423 and it also ranges from 0.0 to 1.0.  SAA is the arithmetic average of the sound absorption coefficients from 200 Hz to 2500 Hz and it is often reported along with the NRC for acoustical products.

Acoustic Control Through Noise Reducing Gypsum Board
Traditionally it has been common to design wall and ceiling/floor assemblies with a variety of layers and materials to achieve the desired STC and IIC ratings for a space. Installing sound deadening insulation like fiberglass between framing certainly helps and likely always will as long as it is installed properly. The use of multiple layers of gypsum board, sometimes installed over noise dampening resilient channels or isolation clips, was effective but often costly and time consuming. Any openings in the gypsum board for things like electrical outlets would be an obvious breach in the acoustic quality of any wall, prompting the common practice to offset their locations when installed back to back in framed walls or use putty pads to seal the outlet boxes.

Noise reducing gypsum board is manufactured by using a viscoelastic polymer middle layer applied between two specifically formulated thin layers of gypsum board with the end product matching common gypsum board thicknesses of ½” or 5/8”. Photo courtesy of CertainTeed Gypsum

Noise reducing gypsum boards are intended as a replacement for some of the traditional acoustic control methods on interior walls and ceilings in residential, commercial or institutional applications. They can be used for new construction or renovations over wood or steel framing. Assemblies that use a single layer of noise reducing gypsum board have been tested and shown to meet or exceed the acoustic performance of assemblies that use double layers of traditional gypsum board. This makes it quite effective in situations where acoustic management is needed to dampen sound energy and significantly improve sound attenuation through walls and ceilings. These results are achieved through the manufacture of a single panel product that contains a viscoelastic polymer middle layer applied between two specifically formulated thin layers of gypsum board. The final product ends up being ½ - 5/8” thick or the same as traditional gypsum board thicknesses. This building product has the ability to dampen sound transmission caused by sound energy striking it by using the inner polymer layer as a kind of shock absorber that dampens board vibrations. This type of “constrained layer damping” board product performs well acoustically over an extended range of frequencies, resulting in increased Sound Transmission Class (STC) ratings for the systems.

Noise reducing gypsum board is thus an excellent acoustic solution for meeting STC specifications without complex techniques such as isolation clips or resilient channels. Clips and resilient channel can easily be short-circuited during the construction process. They can also be compromised afterwards, during picture hanging or pressing of heavy objects against the wall which will negatively affect acoustic performance. These risks are eliminated when using noise reducing gypsum board directly applied to framing thus providing more consistent and predictable acoustic performance. Note that noise reducing gypsum board can still be used in wall assemblies where resilient channels or clips are desired to achieve extra sound transmission control. In this case, the material helps reduce the negative effect of any short circuits.

Using single layer noise reducing gypsum board can also help reduce material usage versus traditional multi layered gypsum systems that might be otherwise required to achieve high sound attenuation. The high acoustic performance of the product makes it possible to build effective noise-reducing walls with less material, gaining valuable square footage, and saving both construction time and material cost. Less material used also means a more sustainable structure in keeping with green building practices.