Understanding Acoustics and Sound Control
Excess noise in commercial buildings is an ongoing problem. Sound control is vital in ensuring occupant comfort and well-being.
By Stanley D. Gatland II, CertainTeed Corp.
In our building science series, we have covered such important topics as the control of heat, air, and moisture flow. These have a significant impact on the health of a building and its occupants. Our series now brings us to the topic of acoustics and sound control, an issue that has a large impact on the well-being, happiness, and productivity of building occupants. This installment addresses the science of sound, acoustical-performance testing standards, and designing environments for effective sound control.
Unwanted, excess noise is an ongoing problem in many commercial buildings today, and its sources go far beyond the expected noise of traffic, airplanes, and construction crews. Noise can originate from within a building as well, and can often have an adverse effect on building occupants.
Noise can negatively affect people's hearing ability and diminish worker productivity. Because of this, the concept of sound control is vital in ensuring the comfort and well-being of building occupants.
We'll first take a look at the physics of sound to gain an understanding of how sound works. With a better grasp of its properties, building and design professionals have an easier time controlling sound.
The science of sound
Acoustics is the science of sound, including its production, transmission, and effects. When we hear sound, it is actually our eardrums vibrating due to sound energy in the air. Sound waves can travel through many different types of material, including air, water, wood, and metals. Any sound that is unwanted, or in excess, is referred to as "noise." The first part of understanding the science of sound is finding the sound paths-sound's sources and modes of transmission.
There are two types of sound paths: airborne sound and structure-borne sound. Airborne sound is what we hear when sounds radiate from a source directly into the air. Examples emanating from a building's exterior include passing traffic, aircraft, highway, and industrial noise.
On the interior of a building, depending on individual decibel levels, voices, music, motors, machinery, and office equipment are often sources of airborne noise. Another leading cause of unwanted interior airborne noise is conditioned airflow through uninsulated HVAC ductwork.
Structure-borne sound, also known as "impact noise," is sound that travels through solid building materials. Examples are the sound of footsteps on floors, door knocks and slams, plumbing and mechanical equipment vibrations, and the impact of rain on a building. Think of the disturbances created by steady rain on a metal roof over a typically quiet building, such as a church or library, and one can understand what a disruption structure-borne noise can be.
How is sound transmitted through a building? Sound waves propagate, or spread, in three dimensions as expanding spheres of pressure waves. Imagine blowing a soap bubble, and then another inside it, and then another inside of the second, and so on, until they expand out infinitely through the air.
Sound waves radiate directly around the source and decrease in amplitude, or loudness, as they get farther from the source. Sound energy is reduced by half as the distance from the source doubles.
There are three properties of sound: frequency, wavelength, and amplitude. Frequency, also known as pitch, is the number of cycles/sec. produced by a sound wave generated by a sound source. Its unit of measurement is the hertz, and the human hearing range is 16 to 20,000 Hz.
The second property, wavelength, is exactly what it looks like: the distance between the start and the end of a sound-wave cycle. Amplitude, or loudness, is the third property. Figure 1 shows two sounds with the same wavelength, but with different amplitudes.
Measuring Sound
How is sound measured? When we measure sound, we're actually measuring the sound pressure of a wave, which is the relative amplitude of that sound wave. The resulting amount is measured in decibels. Figure 2 is a table of typical sound pressure levels in decibels (dB), ranging from a whisper-about 20 dB-to the threshold for pain-about 120 dB. Most activities among people, such as conversations in offices and homes, are in the 50- to 80-dB range. For example, in an office, restaurant, or other public place, once sound rises above 80 dB, it becomes difficult to communicate effectively. Higher decibel levels will likely elicit a reaction from people in a room.
The table in Figure 3 demonstrates subjective human response to sound level changes. A difference of as much as 2 dB is not noticeable. Changes are only obvious when sound-level changes reach the 5- to 8-dB range. Also, sound-level combinations are not linearly additive-they are logarithmic. So, measuring sounds and sound control gets a bit complex.
Acoustical performance
Acoustical test methods for various building materials and systems take many forms. The most common method, ASTM testing, falls into four categories: sound absorption, airborne sound transmission, impact sound transmission, and airborne sound transmission through suspended ceilings.
Sound Absorption (ASTM C 423)
The first property, sound absorption, is the ability of a material to absorb sound waves rather than reflect sound waves. Most building materials are measured for their noise reduction coefficient (NRC), but sound absorption can also be calculated with the sound absorption average (SAA). Sound absorption has to do with controlling sound energy within rooms and enclosed spaces.
The two standards for calculating sound absorption, the NRC and the SAA, are single-decimal ratings between 0 and 1, used to express the absorption properties of materials. One example of this rating is an unfaced R-19 fiberglass insulation batt-its NRC is 0.95. The higher the NRC or SAA value, the better the material absorbs sound energy.
The table in Figure 4 provides examples of common building materials and furnishings in an occupied space. Most of these materials have fairly low NRCs. Materials such as brick, concrete block, wood floors, gypsum board, and tile do not absorb much sound, while carpet with foam padding and velour fabric are better at absorbing sound.
Airborne Sound Transmission (ASTM E 90)
Sound transmission loss is the decrease in sound energy, expressed in decibels of airborne sound, as it passes through a building element or envelope. The metric used to quantify that reduction is sound transmission classification (STC). The STC value indicates how well sound is controlled from room to room, or outdoor to indoor, including through walls or through floor/ceiling assemblies.
ASTM E 90 is the standard addressing airborne STC. This is a single number rating that evaluates the efficiency of systems in reducing the transmission of airborne noise. In this class, the higher the STC rating, the better. The rule of thumb is that a 10-point increase in STC means a decrease in the perceived noise by half.
Impact Sound Transmission (ASTM E 492)
Impact sound-transmission loss is expressed in decibels of airborne sound. This decrease in sound energy is measured after the impact noise that is generated above transfers through the floor-ceiling assembly and is transmitted into the air below. Imagine someone hopping around upstairs. That is impact sound transmission, rated using an impact insulation class (IIC) number.
The standard for impact sound transmission measurement is ASTM E 492. The IIC number is a single number rating that estimates the impact sound insulation performance of floor and ceiling systems. The number is an estimate of how much the sound energy is reduced. The higher the number, the better the system.
Airborne Sound Transmission Through Ceilings (ASTM E 1414)
Typically, airborne sound transmission through ceilings occurs when there are adjacent spaces connected by a common air plenum. The specification is the ceiling attenuation class (CAC). It is similar to STC, but in this case, the measurement is specific to controlling sound transmission from one space to another through the ceiling.
Designing for Sound Control
There are many ways to control sound within building assemblies. Important considerations include:
Using Sound Absorptive Materials
One way to control sound is to use absorptive building materials and systems. Design spaces to use a minimum of hard, sound-reflecting surfaces and install acoustical ceiling systems. Cover floor and wall surfaces with absorptive finishing materials, such as carpet, fabrics, and draperies. Isolate HVAC equipment and line ductwork with sound-absorbing insulation.
Acoustical ceiling-tile systems, especially those with fiberglass, are designed to help reduce reflected sound energy and are very effective at reducing overall room sound levels. Acoustical ceilings absorb rather than reflect airborne noise and improve conversational privacy in open spaces.
There is also a role for fiberglass and perforated metal panels, so consider hanging acoustical baffles with sound-absorptive properties. Cover walls and ceilings in large open spaces. Fiberglass and perforated metal panels on walls are especially effective in high-ceiling areas, where a sound-absorptive ceiling will not be as effective.
Reducing Airborne Sound Transmission
Steel-stud partition walls, wood-stud partition walls, wood joist floor-ceiling assemblies, and metal buildings all demand different components for optimum acoustic control.
- Steel-stud partition walls: Light-gauge steel studs, typically used in walls, are acoustically resilient, so they reduce more sound energy transmission on their own than concrete or wood. Air sealing always improves sound control, regardless of the structure involved. By adding sound-absorbing cavity insulation to a steel-framed wall, the sound performance of that assembly will greatly improve. For example, a similar increase in acoustical performance is achieved if either the amount of drywall is doubled or a single layer of drywall is kept on each side and insulation added. Drywall plus fiberglass insulation is the most cost-effective option. Figure 5 shows a comparison of three different assemblies. Wall 1, standard construction, has an STC of 38. Steel wall 2 with a double layer of drywall, no cavity insulation, and air sealing has an STC of 50, a substantial improvement. Wall 3 also has an STC of 50 and is the most cost-effective. It has a single layer of drywall, fiberglass insulation in the cavity, and is air-sealed.
- Wood-stud partition walls: As opposed to steel, wood studs are acoustically stiff, as sound transmits readily through wood. Air sealing can significantly improve sound control. By adding cavity insulation, such as fiberglass, the STC can be improved by two to three points. A significant increase in acoustical performance can be achieved by mounting drywall to resilient channel and adding sound-absorbing cavity insulation, as shown in the improved-wall illustration (Figure 6). Data for three wall constructions include: wood wall 1 is a standard wood-framed wall, nominal 2 x 4-in., 16-in. studs on center, and no cavity insulation or air seal. The STC for that assembly is 29. If the assembly on wood wall 2 is airtight and cavity insulation is added, the STC increases by 10, to 39. With wood wall 3, cavity insulation has been added, the drywall mounted to resilient channel, and the assembly air-sealed. This is a dramatic improvement in STC with a value of 46.
- Wood joist floor-ceiling systems: Wood joists are acoustically stiff, easily transmitting sound, so air sealing does improve sound control to an extent. Wood joists have more mass and are typically deeper than wall studs. So, adding cavity insulation to this space limits the improvement to one to two points because of the inherent qualities of wood construction. A more significant increase in acoustical performance is achieved when sound-absorbing cavity insulation is used and the structural tie between the ceiling and the framing system is broken with a resilient channel or by using hanger wire (Figure 7). Figure 8 introduces five wood-joist floor-ceiling assemblies for comparison. Floor 1 is standard construction, air-sealed, STC 33. Floor 2 achieves STC 34 after adding fiberglass insulation 6 1/4- in. thick. Floor 3 eliminates the cavity insulation and instead adds drywall mounted to resilient channel, breaking the structural tie, for STC 43. Floor 4 achieves a higher value of STC 52 by adding fiberglass insulation. Floor 5 has the highest value and best performance with STC 54. This floor includes fiberglass insulation and a wire-suspended drywall ceiling for the best acoustical treatment and a 21-point improvement over standard construction. Metal buildings present another challenge for sound control, and a variety of acoustical treatments with fiberglass insulation are available, resulting in STCs ranging from 28 to 54. View metal wall comparisons in Figure 9. Again, breaking the structural tie and adding specialized metal building insulation creates maximum benefits and there are several options. For more information on wall assemblies for metal buildings, consult the website of the North American Insulation Manufacturers Association (NAIMA), Alexandria, VA, at www.naima.org.
Reducing Impact Sound Transmission
IIC ratings apply to all floor-ceiling systems, including lightweight concrete floating floors and wood floor-joist ceiling systems. Again, since wood joists are acoustically stiff, air sealing will help with sound control. The strong structural tie will transmit impact sound through the structure because the sound vibrations are physically entering the structure. So, adding cavity insulation limits IIC increases to one to two points, due to that strong structural tie between the finishing materials and the frame. There is a significant increase in acoustical performance by mounting drywall to resilient channel and adding sound-absorbing cavity insulation.
- Wood joist floor-ceiling systems: A pattern has emerged. In Figure 10, floor 1 is standard construction, air-sealed, with an IIC of 28. Adding fiberglass insulation helps. Floor 3 is good, but floor 4 achieves an IIC rating of 46. Floor 5 controls impact sound with the highest rating of IIC 49 by using standard construction, adding 6 1/4 in. of fiberglass insulation, and a wire-suspended drywall ceiling.
- Lightweight concrete flooring: Lightweight concrete floating floors are common in commercial construction. Typically, concrete slabs are good at reducing airborne sound transmission, but poor at reducing impact sound transmission. For a 4-in. homogeneous concrete slab, the STC is typically greater than 50, but the impact insulation class value is less than 25 (Figure 11). To improve sound control, resilient underlayments under floating floors can be used to isolate the finished floor from the concrete slab and/or suspended finished ceilings used to break the structural tie between the ceiling and the slab to improve the IIC (Figure 12). The use of both underlayments and suspended ceilings does the most to improve the IIC of the flooring.
- Sound Flanking: In addition to controlling sound within building assemblies, there is also the issue of sound flanking-isolating assemblies for optimum acoustical performance. ASTM E 497 covers sound-isolating partitions. Acoustical performance can be substantially increased by controlling air leakage, isolating structure-borne sound paths, and with compartmentalization.
Sound Flanking Paths
Like air and moisture, sound energy "leaks" through paths of least resistance. Acoustical partitions are effective, but sound does transmit around them. Blocking above, between, and under partitions limits sound leaks for maximum sound control.
- Blocking above steel partitions: Add a barrier to stop sound energy from flanking. Caulking is important.
- Steel truss cavity: Add a barrier to stop sound energy from flanking. Again, caulking is important.
- Partition wall height: Extend the wall all the way through the ceiling plenum. Again, caulking to air seal will maximize results.
- Blocking between floors: Add sound-blocking material to minimize sound transmission between floors. This breaks the continuous path for sound energy to transmit through the plenum.
- Air sealing: The importance of air sealing cannot be stressed enough.
- Underfloor barrier: In underfloor spaces, put in a barrier material and attach it both to the ground and to the joists. Be sure the blocking material will withstand moisture if wet conditions exist.
- Electrical outlets: Airtight electrical boxes block sound or can be air-sealed with caulking. If possible, stagger the outlets so there is no direct path for sound. If that is not possible, block the direct path by creating a double stud wall and adding a gypsum board baffle.
- Potential negative impact: There is potential negative impact with improperly attached drywall fasteners. Be sure the screws are not long enough to reach the studs. This will short-circuit the sound-control function by creating a direct sound path.
- Wall corner and floor/ceiling: Architects can design corners and T-intersections to be more acoustically beneficial. One way is to minimize direct contact between adjacent studs and staggering drywall joints. Gypsum board attachment: When using two layers of gypsum board, make sure that the board seams are staggered. Seams that are directly on top of each other can be efficient sound paths.
- Plumbing and conduits: Plumbing and conduits can vibrate due to water movement and electrical energy. Make sure mounting methods do not short-circuit acoustically isolated building elements. Mount plumbing and conduits directly to stud members with gasketed resilient hangers.
Additional References
There are a number of great resources that offer good advice on sound control in building design. One is ASTM E 1374, Standard Guide for Open Office Acoustics and Applicable ASTM Standards. The National Research Council of Canada, Ottawa, is an excellent source of data on STC and IIC values. The California Office of Noise Control's Catalog of STC and IIC Ratings for Walls and Floor/Ceiling Assemblies, an excellent historical reference document, is available on the NAHB Research Center's, Upper Marlboro, MD, website (www.nahb.org) and at ToolBase Services', Upper Marlboro, MD, website (www.toolbase.org). Also, The Gypsum Association's, Washington, manual GA-600 is quite comprehensive. The NAIMA guide can be downloaded at no charge and has data on a wide variety of sound control solutions, using various insulation products. For a thorough indoctrination, with guidelines for builders and homeowners, go to an online resource: www.mysoundchek.com. Finally, take a look at www.certainteed.com, for a wealth of information on the use of insulation, gypsum board, and acoustical ceilings.
Author
Stanley D. Gatland II is the manager of building science technology for CertainTeed Corp.'s Valley Forge, PA, Insulation Group. He is responsible for generating and providing technical information to architects, engineers, builders, trade contractors, building-envelope consultants, building scientists, and building-code officials on the system performance of new and existing building-envelope materials, as well as building-science educational training. Stan has expertise in the areas of building science and architectural acoustics. He is a graduate of the Univ. of Massachusetts, Amherst, with BS and MS degrees in mechanical engineering. He is a member of ASHRAE, ASTM, ASME, and BETEC.
Editor's note: This print series is based on the CertainTeed video series "Commercial Building Science: Concepts and Practices."
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