Use Cementitious Wood Fiber for Great Acoustical Design
Introducing Cementitious Wood Fiber (CWF)
A powerful material for any architect's acoustical control arsenal is cementitious wood fiber (CWF). CWF is a cement-wood composite comprised of wood fibers, also referred to as excelsior, which are coated with an inorganic hydraulic cement binder and infused with several dissolved mineral compounds, including sodium silicate. The composite is formed with heat and pressure.
The resulting CWF material is often used in LEED-certified projects and other green buildings. The environmental credentials of CWF will be addressed in greater detail later in this article. But there are many other characteristics of CWF that make it an appealing building material. It is incredibly durable. During the manufacturing process, the cement and sodium silicate bond to the wood fibers and harden, giving the CWF products an impressive strength and durability.
There is much more to this material than its durable, sustainable nature. When the water in the mineral solution evaporates, the sodium silicate leaves behind an internal glass matrix in the structure that acts like the resin in fiberglass, fortifying the structure. Mold can't grow on it, bugs can't eat it and it is decay and fire resistant. But perhaps the most noteworthy characteristic of CWF is the exceptional acoustical properties of the material.
Acoustical Properties of CWF
CWF is an excellent sound absorber. This porous material is capable of absorbing low, midrange, and high frequencies using a single panel or product. This broad versatility is unique and it equips CWF to best accommodate a wide range of activities that may take place in multi-purpose areas, as well as providing a more comprehensive sound-absorbing solution for auditoriums, public spaces, and performance arenas.
The audible frequency range that is perceptible to the human ear spans from 20Hz to 20,000Hz. This significant spectrum can be effectively broken down into seven different categories or frequency bands. They are as follows: sub-bass (20-60Hz), bass (60-250Hz), low midrange (250-500Hz), midrange (500-2,000Hz), upper midrange (2,000-4,000Hz), presence (4,000-6,000Hz) and brilliance (6,000-20,000Hz).
These frequency categories are important to identify when discussing sound absorption, because the way that a material absorbs sound is dependent upon the frequency of the sound. This means that although two materials may look identical, one may be designed to absorb high frequencies exclusively, while the other is better suited to absorb low or midrange frequencies.
Luckily, there are metrics that measure how a material performs at different frequencies on the spectrum. The ability of a material to absorb sound at a specific frequency is quantified in a metric called the Sound Absorption Coefficient. The Sound Absorption Coefficient details the percentage of the sound energy that a material absorbs at a specific frequency. Oftentimes, a table of Sound Absorption Coefficients will detail how a specific product performs at 125Hz, 250Hz, 500Hz, 1,000Hz, 2,000Hz and 4,000Hz. The actual coefficient is the percentage of the sound that is absorbed at that specific frequency. The portion of the sound waves that are not absorbed are reflected back into the space. For example, a Sound Absorption Coefficient of 0.6 at 500Hz indicates that the material absorbs 60 percent of the sound waves at 500Hz and reflects the other 40 percent.
There is also a metric that describes how a material absorbs sound in general. The Noise Reduction Coefficient (NRC) represents the average of the sound absorption coefficients of a material at frequencies between 250Hz and 2,000Hz. NRC values range from 0.00 to 1.00. An acoustical material that absorbs 100 percent of the sound it contacts has an NRC value of 1.00.
Here are some examples of NRC values of typical building materials. Unpainted brick has an NRC value of 0.04, meaning it absorbs 4 percent of the sound waves between the frequency of 250 Hz and 2,000Hz and reflects 96 percent of those sound waves back into the space. Plywood paneling 3/8-inch thick absorbs 14 percent of the sound that reaches it with an NRC value of 0.14. Heavy carpet on concrete has a much higher NRC value of 0.30. CWF products can produce NRC values near or equal to 1.00, absorbing 100 percent of the sound waves between 250Hz and 2,000Hz with which it comes in contact.
These CWF products are available in 1-inch, 11/2-inch, and 2-inch thick panels. The thickness of the product affects its ability to absorb different frequencies. Typically, as the thickness increases so does the ability to absorb low frequency sounds. For example, at the bass frequency band of 250Hz, a 11/2 inch thick panel has an NRC value of .84 and a 2 inch panel has a higher NRC value of .89.
CWF Acoustical Product Solutions
When specifying the materials to manage the acoustical performance of a project, sound-absorbing products made from CWF are available in acoustical wall and ceiling panels, acoustical clouds and baffles, and structural acoustical roof deck solutions.
CWF panels can be shaped and painted in a variety of colors to create aesthetic, acoustical treatments and custom installations, as shown here at Tectum Inc. in Newark, Ohio.
Sound-absorbing products made from CWF are available in acoustical wall panels, acoustical ceiling panels, and acoustical roof decks.
The acoustical properties of CWF panels at Northside Christian in New Albany, Indiana, make it an unmatched solution for performance spaces.