January 20, 2021
by Kevin Herreman
If silence is golden, good acoustics may be priceless. Acoustic challenges have become very common place as millions have transitioned to working from home during the pandemic. As parents worked in one room while children attended their online classes in another, and doorbells and dogs became standard background noise, millions of workers got a crash course on how to use the mute button during video conferences.
The repercussions of the pandemic will no doubt play out in adaptations to the building enclosure and interior spaces. While social distancing, access to outdoor air, and filtration are projected to be likely areas of focus in future enclosures, acoustic design is also likely to be given more attention. From interior wall assemblies to open plan and exposed structure designs, architects must consider how materials specified for the interior and enclosure will enhance or detract from the quality of the sound in the space. Planning for the quality of the sound at the design stage can result in a building that delivers occupant comfort and drives positive outcomes, including patient satisfaction, speech privacy, and employee productivity.
The sounds of change
Even prior to the pandemic, trends in the choice of construction materials and preferences for interior furnishings were changing the fundamental sound profile of buildings.
The specification of lighter weight materials, such as laminate claddings and more vision glass, has resulted in less sound reduction from outdoor to indoor environments since these do not block sound as well as more opaque materials. The choice of an innovative material to solve one problem can result in a tradeoff between different performance attributes. For example, laminate façades are easier to install, but can allow more outside noise to infiltrate a space compared to traditional masonry. On the interior, lightweight drywall use can result in an increase in measurable sound transmission between partitions. It is easier to work with during the construction of the spaces, but with a lower density, it can make it possible to hear a conversation in the next room.
Interior design preferences are changing, too, with many commercial buildings eschewing upholstered furniture and carpeting in favor of hard surfaces that wear longer and are easier to clean and sanitize. Not only does an increase in hard surfaces reduce sound absorption, but also can create a new kind of noise (think classroom chair legs dragging across laminate flooring on the story above).
By considering noise issues during the design phase, architects and designers can make material and design decisions that reduce the need for expensive, after-the-build sound mitigation strategies. This approach can not only save costs on a building’s interior finishes, but also avoid issues that could be cost-prohibitive to completely solve once the structure is established.
How sound affects occupants
Some studies have correlated negative health effects like stress and rising blood pressure with exposure to loud, persistent industrial noises. Sound concerns in non-industrial spaces, particularly in workplace settings, are more subtle and may also lead to long-term health issues. The sound of water flushing through pipes in an adjacent room or the transmission of sound through a ceiling plenum space or air ducts can lead to problems with communication, comfort, and privacy.
In hospitals, noise is the number one complaint of patients, staff, and visitors. Studies have shown noise interferes with patient sleep and even impacts the healing process. For children, a noisy learning environment that makes it harder to hear the teacher can be associated with measurable cognitive effects. Not all sounds provoke the same response; high-frequency noises, such as fire alarms or whistles, tend to be immediately distracting, but low-frequency noise like mechanical systems powering up or the rumble of thunder outside is much more insidious.
The effects of low-frequency sounds are often subjective—symptoms such as headaches, fatigue, and loss of concentration may be reported. However, many study subjects report a sense of relief when a low-frequency noise is removed—even if they had not noticed it was there.
Commercial noise culprits
In commercial applications, low-frequency noise is concerning because sources can be difficult to spot and correct. Common sources in commercial structures include air handling systems, refrigeration compressors, climate systems, and even vibrations within the structure itself. Low-frequency noises are also harder to mitigate, as they propagate over longer distances than high-frequency noises, and are less likely to be weakened by passing through structural components like walls and windows, if those components are not insulated for sound.
The basics of acoustics
To understand how to mitigate noise in designs, it helps to understand how sound behaves in a building.
Sound is produced when a vibrating object causes the air around it to vibrate. The moving air particles collide and transfer the vibration into a pressure wave that bounces around the available space. In a room, sound will bounce off multiple surfaces, continuing to bounce around until their energy is expended. Therefore, empty spaces with lots of hard surfaces can quickly become noisy. When sound encounters something absorptive, such as insulation, carpet, soft furniture, or sound panels, the waves lose energy from interaction with the absorbing material.
In a building assembly, walls, floors, and ceilings can act as a proxy for sound speakers. In transmission through walls, most of the sound energy travels through the stud from one side to the other. The acoustic energy vibrates the wall covering and it, in turn, causes vibration in the wall stud. The sound energy transmits efficiently at some frequencies and less so at others as it travels through to vibrate the covering on the other side, re-radiating the sound into the adjacent space. The acoustic performance of wall assemblies is dependent on the construction detail. The measure of the ability of a wall to block transmission of sound is its sound transmission class (STC) or its outside-inside transmission class (OITC). These ratings describe how well a wall system reduces airborne sound transmission. The main difference between the two is the STC rating is determined over the frequency range from 125 to 4000 Hz while OITC extends the range lower to 80 Hz to account for transportation sound sources.
Sound traveling through walls tends to be airborne. Between floors, airborne sound travels through the floor assembly to the space below much the same way it travels through walls. The performance measurement for airborne sound transmission through a floor assembly is also the STC rating. Sound transmitted to the space below can be caused by impact to the floor itself, like people walking, dropping of items, or the switching on and off of rooftop mechanical equipment. Impact isolation class (IIC) measures how well a floor or ceiling reduces structure-borne impact noise. An assembly that is good at reducing airborne noise is not necessarily good at reducing impact noise, so both measurements are important to consider.
Sound transmission class
The STC value provides an assessment of a system’s ability to block sound. A higher STC rating means a better wall design. The STC value is usually determined by testing per ASTM E90-09(2016), Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements, and calculated per ASTM E413-16, Classification for Rating Sound Insulation. ASTM E336-20, Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings, and ASTM E413-16 are used for field testing. These standards provide a relative rating of how much sound transmits from one room through a partition and into another room. As an example, an architect who must design a room with a sound source emitting 100 dB and isolate an adjoining room for office workers will need to choose between various STC values when selecting a wall construction. A wall construction with an STC of 38 would not perform as well as a wall with an STC rating of 50. The latter will provide approximately 12 dB more sound-blocking performance. It is important to bear in mind a 10 dB reduction is viewed as a 50-percent reduction in loudness.
Outside-inside transmission class
OITC value provides an assessment of a system’s ability to block sound outside the building. Like STC, a higher OITC means a better performing enclosure wall. The OITC value is usually determined by testing per ASTM E90-09(2016) and calculated per ASTM E1332-16, Standard Classification for Rating Outdoor-indoor Sound Attenuation. ASTM E336-20 and ASTM E966-18a, Standard Guide for Field Measurements of Airborne Sound Attenuation of Building Facades and Facade Elements, are used for field testing.
Impact isolation class
The IIC value provides an assessment of a system’s ability to resist transmitting impact sound from a space above to an area below. A higher IIC rating means less noise from walking or impacts to the floor will be transmitted to the room below. The IIC value is usually determined by testing per ASTM E492-09(2016)e1, Standard Test Method for Laboratory Measurement of Impact Sound Transmission Through Floor-ceiling Assemblies Using the Tapping Machine, and calculated per ASTM E989-18, Standard Classification for Determination of Single-number Metrics for Impact Noise. ASTM E1007-19, Standard Test Method for Field Measurement of Tapping Machine Impact Sound Transmission Through Floor-ceiling Assemblies and Associated Support Structures, and ASTM E989-18 are used for field testing.
Noise reduction coefficient
Material absorption performance also matters when it comes to noise control, specifically the noise reduction coefficient (NRC) and its ability to absorb sound. NRC values are determined by testing per ASTM C423-17, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, and calculating the average of the sound absorption at the 250, 500, 1000, and 2000 Hz one-third octave bands. The lower the NRC, the less absorptive the material. For example, an NRC of 0.75 means roughly 75 percent of sound energy impacting the material is absorbed/dissipated and 25 percent is returned to the space. When it comes to whether a material’s thickness or density has a greater influence on its sound absorption, Figure 1 shows a material’s thickness generally has a greater impact on NRC value compared to a material’s density.
Single number ratings: for screening only
Specifiers should not assume a partition with a one-point higher STC rating is functionally any better than a partition with a lower score. Testing repeatability can be plus or minus two STC points. Like the NRC rating for sound absorption, STC should not be used for design or calculation purposes. It is intended only as a quick screening tool to compare different construction assemblies. The designer should use the actual laboratory sound transmission loss values at the frequencies of interest when determining the reduction of sound between two areas. By subtracting the sound transmission loss values from the dB levels of the noise in one room for each one-third octave band and accounting for flanking in the field, the designer can estimate what the resultant noise level should be in an adjacent room at each one-third octave band.
Like water or air, sound will always flow across the path of least resistance. A small opening, as small as 0.7 mm (1/32 in.), between a floor and wall can allow sound energy to flow from one space to another and reduce the wall’s STC. This flow of sound is called flanking noise. It occurs when sound finds its way around walls and floors via junctions and other openings. To prevent flanking noise, sound should be addressed in the design phase as part of a holistic design approach. Post-construction measures like furnishings and fixtures can help reduce sound levels in a room, but do not address flanking. For commercial buildings requiring STC/IIC of at least 50/50 (meaning, typical speech and walking in an adjacent room cannot be heard and walking in the room above will not be heard in the space below) managing flanking noise is a major contributor to meeting this standard.
Strategies for mitigating flanking noise
Managing flanking noise in commercial enclosures can be accomplished through four primary strategies: absorb, block, break, and isolate sound. These strategies can be combined within a structure to optimize the building’s sound profile and involve a mix of sound-forward design thinking, as well as the use of right materials to achieve the necessary mitigation.
To better understand how new, lightweight drywall types, studs and insulation options work with different assemblies and materials, the author’s firm evaluated close to 300 walls, examining how these new variables and wall configurations affect STC performance. A separate study also examined 18 combinations of pipe insulation to study how improved material combinations influence insertion loss performance. Materials tested included a variety of jacketing, the combination of cellular glass insulation with mineral wool and layering of mass loaded vinyl materials. These studies provided critical data to understand how designs can be optimized to absorb, block, break, and isolate flanking noise and other sound issues.
Sound strategy #1: Absorb
Absorption removes the acoustic energy of a sound wave. It can be accomplished by incorporating various types of materials, both structurally and as part of furnishings and fixtures. Sound absorption of materials is measured using ASTM C423-17 and ASTM E795-16, Standard Practices for Mounting Test Specimens During Sound Absorption Tests, and is expressed as NRC (read Psychoacoustics- Facts and Models by H. Fastl and E. Zwicker (2007).
The NRC rating is not perfect, so it is important to understand NRC ratings are not actually percentages but rather a ratio of the absorption to the area of the material tested. The NRC rating depends on the thickness, surface area, and perimeter of the material along with material characteristics. Small differences in NRC rating have little impact on overall sound absorption in a space. For example, a material with an NRC of 0.75 is not significantly better than one with an NRC of 0.70 when used in a space. In smaller rooms, with a limited amount of surface area for the absorption material, both would perform similarly at most frequencies. At high frequencies, most of the sound is absorbed and at lower frequencies the resulting difference in sound absorbed is not likely to be noticed. With more commercial buildings transitioning to harder surfaces, including sound absorption in the early design is a more holistic approach to managing flanking noise in commercial buildings.
Sound strategy #2: Block
Blocking sound involves creating a physical barrier to acoustic energy. Typically, this is a layered approach to a wall design that includes insulating a wall or floor assembly. Architects have many insulating options to choose from. For walls with STC values lesser than 50, there is no significant difference in acoustic performance between mineral wool and fiberglass insulation. In selecting an insulation material, both fiberglass and mineral wool offer similar levels of sound reduction. This frees up designers to select an insulating material based on other performance needs. For example, a designer might choose mineral wool to achieve a higher hourly rated fire wall and to help support life-safety objectives. Other properties, such as a material’s ability to manage vapor and liquid moisture or its resiliency or compressive strength can drive the choice of insulating material used in an assembly. Although fiberglass insulation performs well in all acoustic applications, in high-performing walls mineral wool insulation can provide a slightly higher level of STC performance.
Sound Strategy #3: Break
Breaking sound is achieved by creating a physical disconnect between vibrating components, so the sound wave cannot travel between them. Breaking strategies are often used in multi-family structures, where it is important to reduce the amount of noise that can be heard between units. An example of this approach can be seen in double-wall construction, where each wall covering has its own studs instead of sharing them. This approach eliminates the opportunity for sound energy to use the studs as conductors of noise from one space into another. Installing insulation in the cavity between two stud walls further reduces the flow of sound energy by absorbing it, reducing energy flow from the source space into the adjacent space.
Sound strategy #4: Isolate
A fourth strategy to reduce flanking noise is isolation. It is commonly used to manage mechanical vibration such as rooftop HVAC equipment or to improve sound transmission through walls and floors. Isolation enables one area to vibrate without that energy traveling to the adjacent rooms. In the case of HVAC equipment placed atop a roof, vibration-absorbing curbs help prevent the vibration and sound energy from transmitting to the building structure where it can then pass into the spaces below. A similar strategy can be used in floor assemblies where soundproofing clips absorb the vibration from walking and impact on the story above. In walls, isolation can be achieved by using resilient channels, changing the flow of sound energy to a path with higher resistance, such as an insulated cavity, rather than along the stud path where sound is more easily conducted.
Design best practices
While flanking noise can be managed with the four strategies described above, some considerations during the design stage can help avoid noise issues from arising once construction is complete. Following are four best practices to consider at the design stage.
1. Consider the space’s purpose
What function does the space provide? Will noisy equipment be in the space that might disrupt conversations? Will private conversations, such as those between a healthcare provider and patient, occur in the space? Isolating areas with heavier traffic away from consulting or meeting areas can protect privacy.
2. Look for cost-effective strategies
Identify interventions that are easy to introduce into the design and economical to implement. As an example, installing solid doors and drop-down door seals reduces the ability of sound to infiltrate. Adding insulation batts above walls that do not extend to the ceiling deck can improve privacy at a lower cost compared to retrofitting the walls or ducts. Adding caulking around the wall perimeter and specifying outlets in different cavities supports privacy measures.
3. Spec full-height walls
Specifying walls that extend up to the ceiling deck defends against flanking noise that travels from space-to-space via a ceiling plenum. If a full-height wall is not practical (this is more of a concern with modular environments), adding a row of insulation 150-mm (6-in.) thick and 610-mm (24-in.) wide will help reduce flanking noise.
Other design options for reducing ceiling plenum flanking include installing caulking around the wall perimeter and adding crown molding at the intersection of the wall and ceiling. Lining HVAC ducts also supports speech privacy.
4. Consider mechanical systems
Adding pipe insulation with mass-loaded vinyl cladding and isolating the pipe from the structure can defend against noise from a building’s mechanical pipes while also providing thermal insulation. Of course, insulation installed in the wall assembly of the mechanical room and surrounding areas can further reduce the mechanical system noise. While good design typically locates these spaces away from more occupied areas of the building, the noise from mechanical pipes has led to a common observation in mechanical spaces with steam-powered operations, “The pipes are coming up.”
Even the most sophisticated industrial or commercial insulation specified for an enclosure will not deliver if it is not properly installed. A range of resources, including materials published by the National Insulation Association (NIA), can help contractors ensure materials are installed to deliver optimal performance.
However, at the end of the day, sound performance begins with setting the spec. Specifying the right insulating materials for the job during the design stage can help owners and occupants achieve the most desirable result in a chaotic world—quiet, comfortable spaces. In this light, sound acoustic performance is priceless.
Kevin Herreman is a member of the senior technical leadership at Owens Corning with over 30 years of experience in acoustics. His technical expertise includes design, modeling, and testing of materials and systems for acoustics and noise control. Herreman holds multiple patents for noise control materials and systems. Herreman is chair of the Institute of Noise Control Engineering (INCE) Technical Activities Committee for Product Noise Emission.
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