Enhanced acoustical design

Ambient noise levels in some facility types are trending up and becoming a barrier to clear communication between building occupants. A 2005 study conducted by Ilene Busch-Vishniac, PhD, a dean and professor of mechanical engineering at John Hopkins University, Baltimore, discovered that sound pressure levels in hospitals have risen an average of 0.40 decibels per year since 1960. In fact, the volume of noise transmission has increased so much that average sound levels at Johns Hopkins Hospital, as measured through Busch-Vishniac’s research (http://www.aip.org/asa_laypapers2011/Mahapatra.html), had exceeded the 45 dB(A) to 50 dB(A) range that is the volume at which normal conversation takes place.

Another study, this one conducted by University of Kansas researchers in 2000, found that the average speech intelligibility rating in U.S. classrooms was 75% or less. That means that a quarter of what those students were meant to hear actually went unheard. It is likely that such conditions are prevalent in many schools.

Fortunately, good acoustics as a component of indoor environmental quality (IEQ )are now receiving more attention in today’s buildings. However, this complex discipline requires an understanding of how sound is transmitted, acoustical properties of different materials and systems, and where and when particular products and solutions make sense.

THE CRUCIAL ROLE OF CEILING SYSTEMS

While all interior surfaces factor into the resulting acoustical performance in a particular space, ceiling system design plays a most significant role. With its large, exposed surface area, the ceiling will usually either absorb or isolate sound, although sometimes it is expected to do both.

When speech intelligibility is a priority, such as in classrooms or conference rooms, or where reverberant noise buildup needs to be controlled—in atriums or cafeterias, for example—sound absorption is the goal. In such cases, products with a high noise reduction coefficient (NRC) of more than 0.5 on the NRC scale of 0.0 to 1.0 are recommended. (The higher the NRC coefficient, the greater the noise reduction.)

Popular options include fiberglass ceiling tiles, which rank high at around 0.90, and mineral fiber tiles, which offer NRC values in the range of 0.70, according to Ryan Bessey, PEng, an acoustical engineer in Stantec’s (www.stantec.com) Toronto office. The porous qualities of these materials are what produce such high levels of sound absorption.

However, some designers have concerns about the look of ceiling tiles. “From an aesthetic perspective, designers are often seeking materials to provide smooth and monolithic surfaces that look like drywall,” says Jeffrey L. Fullerton, INCE, LEED AP, director of architectural acoustics at Acentech (www.acentech.com), Cambridge, Mass. “These days, there are a number of surface-applied products that might satisfy this visual objective. These products consist of troweled-on finishes that are able to be installed over large areas with minimal joints and architectural reveals, looking much like drywall.”

The downside of these surface-applied solutions is the price, which often tops $25/sf of installed system. As an alternative, Fullerton, who contributed to the development of acoustical credits in the LEED 3.0 Commercial Interiors rating system, recommends perforated stretch membrane and fabric products, or—at even lower cost—tiles that can be suspended individually from the ceiling to break away from the typical ceiling grid look.

Vertically hung baffles also do a creditable job of sound absorption in that both the front and back surfaces are exposed, absorbing noise. Many Building Teams also appreciate the aesthetic value that vertical baffles can bring to a project. 

Yet another sound-savvy design strategy is fully suspending the ceiling, either via ceiling tiles or by using drywall. “By having a combination of an acoustically absorptive ceiling layer and large air gap behind, acoustic absorption occurs at lower sound frequencies, or longer sound wavelengths, and across a wider range of frequencies,” explains Chris Field, PhD, a senior associate and acoustic and theater consulting practice leader with Arup (www.arup.com), based in Sydney, N.S.W., Australia.

One approach, for instance, is to use vibration isolation hangers, which use neoprene or spring components (or both) to decouple the ceiling from other building elements, according to Andrew Mitchell, a consultant with Acoustic Dimensions (www.acousticdimensions.com) in Addison, Texas. But Field, recipient of an Australian Acoustical Society Excellence in Acoustics Award, points out that suspended ceilings can create acoustic “short-circuits” that transmit sounds across wall partitions. This being the case, it is important to specify full-height, slab-to-slab wall partitions to ensure speech privacy.

“A typical lightweight suspended ceiling, such as acoustical ceiling tile on a grid, can block or contain only a limited amount of noise, especially at the low frequencies typical of noise from major HVAC systems,” explains Jonathan Lally, owner of Lally Acoustical Consulting (www.lallyacoustics.com), New York, N.Y. “Although this performance is normally sufficient for low-volume ductwork and small HVAC terminal units, a common mistake is to hide noisier components in the plenum above an occupied room, as high noise levels can pass directly through a lightweight ceiling.”

Instead, heavier, rigid materials such as thick gypsum wallboard on a steel grid, or multiple layers of gypsum board for increased mass, are commonly used for their optimized sound isolation properties.

Of course, the real trick is figuring out how to design a space that both absorbs and isolates sound.

“For example, the suspended ceiling in a hospital neonatal intensive care may need to both absorb sound generated within the nursery and block the noise generated by airflow valves located in the ceiling void,” notes Nathan Sevener, PE, LEED AP, INCE, a principal consultant with Soundscape Engineering (www.soundscapeengineering.com), Chicago. “This generally means compromising and selecting a ceiling tile that has a mid-level Ceiling Attenuation Class [a measure of the tile’s ability to block sound] and a mid-level noise reduction coefficient (NRC), or using a specialty ceiling tile comprised of a high-NRC tile backed with mass-loaded vinyl or gypsum board to increase the tile’s Ceiling Attenuation Class.”

As an alternative, Lally recommends a heavy, rigid ceiling finished with a light, porous treatment, such as a drywall suspended ceiling with an acoustical plaster system exposed to the room.

WALL SYSTEMS: STRIKING A BALANCE

If you boil down the ideal sound isolation design for walls to a formula, the three main ingredients would be mass, air space, and air tightness. But striking that balance is not easily done.

For instance, concrete and concrete masonry unit (CMU) block walls offer more mass than stud-framed walls, but double-stud wall construction allows for larger cavity depths—in other words, air space—and also produces structural decoupling, which significantly increases sound isolation, according to Acoustic Dimensions’ Mitchell.

Even when dealing with wall mass itself, balanced design is paramount. “Increasing the mass of the construction can be a cost-effective factor to start with, but will exhibit the law of diminishing returns after a certain level of increased mass—three layers of gypsum wall board or eight inches of masonry—is incorporated,” says Fullerton.

Other techniques for improving acoustical performance, according to Sevener, who has consulted on more than 300 projects throughout his career, include: 

• Adding fibrous insulation to the wall cavity

• Using lighter gage metal studs

• Adding layers of gypsum board

• Using double rows of studs

While the double rows of studs do a great job of low-frequency sound isolation, if floor space is at a premium it may be better to substitute resilient clips for the second row of studs.

But even where the mass and air space are carefully calibrated for optimal acoustic performance, walls that are not carefully sealed will undermine the entire acoustical system. “Ensuring air tightness is perhaps the most important component to achieving good sound isolation,” says Mitchell. “All too often, field conditions result in gaps around duct or pipe penetrations, poor seals at the top and bottom of partitions, gaps at the intersection of window mullions, or leakages around electrical outlet boxes.” He recommends a thorough inspection of wall systems during construction to “head off a number of issues with sound isolation.”

As for the actual sealing, caulk products—either squeezed from tubes, spray applied, or troweled on—are fairly inexpensive, although the labor can be rather tedious and time-consuming.

Other acoustical wall design strategies include insulation and damping, says Fullerton. While insulation’s main purpose of thermal performance isn’t always compatible with sound isolation, there are a number of products, such as fiberglass, mineral fiber, and spray-in open-cell foams, which provide both.

Another system, closed-cell foam, does not offer sound absorption, as its closed pores inhibit sound from passing through. Field also  points out that thermal insulation is usually low in density with a foil insulation layer, whereas acoustic insulation is typically a higher-density product and must have a layer exposed to the sound in order to absorb it.

As for damping, which describes a material’s ability to minimize sound resonation, one product that has traditionally worked well in this regard is laminated glass, according to Fullerton. That’s because the layer between the two panes of glass diminishes sound vibration. Now, with the availability of selected laminated and damped gypsum board panels, these products are also effective at reducing resonation and vibration.

Another major factor that impacts the STC rating of a wall assembly is fenestration. Because glass is light and stiff, it  is considered to be the weakest link in the wall. Fenestration such as windows and storefronts allows sound to travel more easily through the wall assembly. In fact, if the window area exceeds 10% of the total wall area, then the window and frame will generally dictate the STC or sound transmission loss performance for the entire wall. But even in cases where the overall STC is relatively high, localized sound at the fenestration may still be of concern, according to Justin Stout, a consultant with Acoustic Dimensions. 

To work fenestration into the acoustical design package, Mei Wu, PhD, INCE, of Mei Wu Acoustics (www.mei-wu.com), Redwood City, Calif., recommends solid-core doors, laminated glass for windows and sidelights, and sealing the gaps around the doors. “For better sound isolation, double-glazing windows and sidelights can be used, and for the most demanding spaces, acoustical windows and doors are available that can offer an STC rating as high as 50, even into the 60s,” he says.

The junction between the curtain wall mullion and interior walls is also a locus of concern for Building Teams. “As the curtain wall expands and contracts with thermal loading, this junction is notoriously difficult to seal properly,” says Stantec’s Bessey. “If a gap occurs, the sound-isolation performance of the wall will decrease significantly. This is a very common problem in newer buildings utilizing curtain wall.”

FLOOR SYSTEMS: GETTING THE MOST FROM MATERIALS, FINISHES, AND UNDERLYING STRUCTURE

Moving on to the next significant interior surface—flooring—it’s the material, finish, and underlying structure which most directly impact the dynamics of sound. Generally speaking, concrete performs better than wood thanks to its mass. Concrete floor slabs are also less prone to construction defects, leaks, and flanking, all of which can significantly degrade sound isolation performance, says Bessey, who is involved with a number of professional acoustical associations.

One obvious drawback with concrete is that it effectively transmits high-frequency impact noise such as high heels tapping on the floor. To overcome this problem, designers usually need to include resilient floor elements, such as mats, foams, and felts either beneath or on the floor’s surface, adds Bessey.

Arguably the most effective sound buffer for flooring is carpeting, if this element makes sense for a particular space. Another common approach is to create an air space between the ceiling and the underside of the structure above to muffle impact noise coming from upstairs.

Whatever resilient underlayment is chosen for a concrete slab, the application can usually be fairly minimal in thickness. On the other hand, says Fullerton, “Where the floor consists of a wood structure, it may be necessary to incorporate a gypsum floor underlayment or a thicker resilient underlayment, or both, to achieve effective impact isolation.”

In fact, Lally describes achieving adequate impact noise insulation in wood-framed buildings as “extremely challenging.” Such designs often require multiple treatments such as floated floor platforms, thin concrete topping slabs, and isolated “sound barrier” ceilings below. “Nonhardening acoustical sealants and batt insulation must also be used throughout to prevent airborne sound leaks, especially since these buildings may settle or shift over time,” he explains.

CONTROLLING NOISE FROM MEP SYSTEMS

Mechanical, electrical, and plumbing (MEP) systems are often the primary source of noise within a building. “HVAC noise control is a world all its own,” confirms Gregory A. Miller, PE, INCE, principal of Pin Drop Acoustics (www.pindropacoustics.com), Chicago. “We probably spend at least a third of our efforts on any given project making sure that they are at the right level of quiet.”

For example, air-handling units are famous for radiating noise through both the mechanical room and ductwork, not to mention sounds coming from terminal units such as variable-air-volume and fan-powered boxes.

Fortunately, there are a number of steps that can be taken to significantly mitigate these noise issues. For starters, consultants recommend selecting quiet equipment and correctly sizing the systems. Similarly, limiting air velocity will also reduce ductwork noise. 

“Once the equipment is selected, we then look at how it is mounted in the building and make sure that there is some element of vibration isolation between the equipment and the building structure,” says Miller, who serves on the Acoustical Society of America’s Committee on Architectural Acoustics.

For instance, equipment room walls and slabs can muffle equipment sounds; vibration isolators can be used to limit vibrations coming from equipment such as air-handling units. Of course, siting HVAC equipment as remotely as possible from occupied spaces is also a logical strategy. “For HVAC fan noise traveling down ducts, we still rely on internal duct lining to do the lion’s share of the noise reduction,” says Miller. “When lining isn’t permitted—as is the case in many schools and hospitals—we rely on a combination of duct silencers and the use of acoustically rated flex duct at diffusers.”

In sum, Stantec’s Bessey points out that sound coming from mechanical equipment can travel through multiple pathways. “Failure to account for any one of these pathways could lead to significant complaints and costly retrofitted noise controls,” he cautions.

ACOUSTICS AND SUSTAINABLE DESIGN

Although it may seem counterintuitive, a number of sustainable design practices can lead to problems with building acoustics, according to say some Building Teams. For example, while daylighting is a desirable feature in most buildings, providing larger glazed areas and lowering partitions for optimal daylight harvesting can reduce wall STC ratings and provide open paths for sound to travel.

Light shelves and light-reflecting surfaces can also reflect sound. Furthermore, operable windows welcome in significant levels of outdoor noise, especially in urban settings.

While these conflicts are significant, there are a number of strategies that can help enable sustainable design and good acoustics to coexist harmoniously.

For instance, designers can specify transparent, sound-limiting barriers in workspaces that only minimally impede daylighting. Alternatively, certain dividers can be strategically lowered while other partitions remain on high in order to preserve sightlines while providing acoustical separation, suggests Lally.

And while some lighter surfaces do reflect sound, there are also products which absorb sound, thereby accomplishing the dual purpose of supporting natural light while controlling sound.

Natural ventilation, on the other hand, is somewhat more nettlesome as it can be difficult to justify increased internal noise levels that exceed recommended international standards limits. Addressing this issue, Arup’s Field questions whether naturally ventilated buildings should be judged by the same standards as sealed, air-conditioned spaces. Citing research published in an industry report, “Natural Ventilation in the Urban Environment: Assessment and Design” (edited by Francis Allard and Christian Ghiaus), Field states that decibel levels ranging from 55 dBA to 60 dBA, and sometimes as high as 65 dBA, are considered acceptable in open-plan offices.

Another way to make natural ventilation work is to give building occupants control over opening and closing the windows or ventilation openings. With a mixed-mode system, mechanical ventilation kicks in at times when outdoor noise is exceeding comfortable levels for occupants and workers.

Yet another sustainability hurdle for designers is the fact that some fiberglass acoustical products are not considered eco-friendly by some in the AEC industry. While specifiers may opt for greener alternatives such as packless silencers and closed-cell foam duct liners, cost increases while acoustic performance decreases, according to Field.

LEED AND OTHER RATING SYSTEMS AND STANDARDS

Even though a few acoustical products may have a tough time being green, the good news is that since acoustics are a key component of indoor environmental quality, good acoustical design is recognized and rewarded by green building standards.

For example, LEED for Schools and LEED for Healthcare both provide credits for meeting sound isolation, room noise, acoustical finishes, and site exterior noise requirements. In addition, these LEED systems reference nationally adopted standards such as ANSI’s Classroom Acoustics Standard, ANSI S12.60, and the Facility Guidelines Institute/American Society for Healthcare Engineering (FGI/ASHE) Guidelines for Design and Construction of Health Care Facilities, which has been codified by 42 states and seven federal agencies, according to Lally.

Now that the 2010 edition of FGI/ASHE’s guidelines has begun making the rounds, Sevener anticipates that this updated document will have an even greater impact on acoustical design in healthcare facilities.

“These new criteria are intended to help quicken patients’ recovery time, improve patient privacy, reduce the acoustical stressors for the staff, and reduce the acoustical impact of the hospital on the local environment and community,” says Fullerton.

While consultants are pleased with LEED’s inclusion of acoustic requirements, they do see room for improvement. For instance, Bessey points out that LEED for Healthcare (http://www.usgbc.org/DisplayPage.aspx?CMSPageID=1765) offers only two credits for considering acoustics. “While this is great first step, on some projects the expenditure required to achieve these credits is a tough sell to clients,” says Bessey. “Hopefully, in future versions of LEED, more credits will be available for optimizing the acoustic environment in healthcare facilities.”

LEED for Schools (http://www.usgbc.org/DisplayPage.aspx?CMSPageID=1586) does include acoustic prerequisites for background noise levels and reverberation times in learning spaces, but in Miller’s view, “We would argue that the LEED standards are the bare minimum for acoustic performance in schools.” At the same time, he notes that many school districts were not designing to these standards until the LEED requirements emerged, so LEED has begun to make a difference.

Similarly, Field gives a nod to ASHRAE for the 2007 Handbook for HVAC Application’s section on sound and vibration control, which offers noise and room criteria levels for acceptable background noise in different spaces.

BALANCING THEORY AND PRACTICE

While such standards and design recommendations can certainly provide much needed guidance, sound isolation design remains a very complex issue. “Anyone can pick an acoustic wall construction out of a book, but STC ratings are determined in laboratories under ideal conditions, and walls can perform differently in the field,” says Bessey. “There’s no standardized approach.”

That is one reason why experienced building acoustics professionals should be consulted whenever acoustical design is a priority on a project. Relying on measurement data, statistics, computer modeling, and years of experience, these experts can help bring optimized acoustical designs to the building space.