Controlling Sound Transmission in Multifamily, Healthcare and Educational Environments

Although many building designers focus on visual aesthetics, acoustics play a vital role in every building environment. Owners expect homes to be quiet, so they don't hear the neighbors. In a classroom, students are expected to focus and learn without being distracted by sound or unable to hear what the teacher says because of competing noise. In healthcare facilities, quiet not only promotes healing, but privacy is mandated: Patients have the right to private conversations with their healthcare providers. In each of these settings, acoustics is the key to meeting basic owner expectations.

Acoustics 101

Before delving into the topic of acoustic controls, we need to know what we mean by “noise,” “sound,” and “control.”

Simply put, noise is unwanted sound. An air conditioner makes a soft, constant hum in the background. As such, this sound helps mask more objectionable noises, such as intermittent traffic from outside. For most people, sounds that are objectionable typically are:

• loud enough to be uncomfortable.

• intermittent rather than continuous.

Acoustics—the science of controlling sound—is made more challenging by the way people perceive sound. Sound is created by vibrating matter. A person speaks in one room, causing pressure waves to travel through the air like ripples on a pond. When these pressure waves strike the room's walls, ceilings, and floors, some of the sound is reflected back, creating a reverberation that continues to bounce around the room until it loses energy. Some of the sound waves can cause the wall, ceiling, and floor to vibrate. This vibration, in turn, transmits pressure waves on the building assemblies on the other side of the wall. These waves then strike the ears of occupants in the adjacent rooms and are heard, provided the frequency is in the range of 20 to 20,000 cycles per second (Hz), and the occupants have decent, healthy hearing. How much sound is reflected and how much is absorbed depends on the type of surface materials in the room.

A recycled rubber underlayment works with a direct application of most floor finishes, including ceramic tile. Photo courtesy of ECOSilence

Interior Room Acoustics vs. Transmitted Sound

The basic mechanics of sound that have just been described explain only part of the picture that building designers need to see. When a person speaks (or a radio or television blares), the sound is carried through the air. In buildings, however, there are also structure-borne sounds caused by direct impacts against the structure, such as doors slamming or people walking or jumping on floors. Depending on the building type, these structure-borne vibrations can be very intense. Equipment, machinery, high-volume traffic, rolling gurneys in a hospital, or basketball practice next to a classroom or library all impart vibrations that must be controlled.

There are two primary areas of architectural acoustics that address methods of controlling both air-borne and structure-borne sound transmission.

1) Interior room acoustics

2) Sound transmission

Interior Room Acoustics

Interior room acoustics addresses airborne-sounds that are reflected back from the walls, ceilings, and floors. In this case, designers are most concerned with controlling reverberation (the energy reflected and re-reflected in an enclosed space). Controlling room acoustics happens at the surfaces, beginning with finish materials and furnishings that either absorb or reflect sound, depending on their acoustic qualities. Rooms with very hard surfaces—such as concrete floors, tiled walls, and decorative tin ceilings—reflect a lot of sound. A room with these finishes would be considered “acoustically live,” which may or may not be desirable. Some restaurants and retail centers like an active, busy ambience, whereas in an office or dorm room this acoustic emphasis would be inappropriate. To create a quieter ambience, designers need to select finish materials that absorb sound, such as carpet, acoustic ceiling tile, or perhaps thick fabric wall coverings.

To take room acoustics to a higher level, designers might also wish to control the direction and quality of the reflected sound waves. A lecture hall or performance space will need a variety of engineered baffles and surface textures to deliberately direct, diffuse, and distribute the sound waves within the space. Acoustic design for large performance spaces can get quite involved and is beyond the scope of this course. For now, it's enough to understand that interior room acoustics are primarily controlled by the material properties of interior finish materials. The chief emphasis is on limiting and controlling reflected sound waves.

Sound transmission is another animal altogether. To control the sound and vibrations that are transmitted through building materials from one space to the next, the focus should be on dampening vibrations that pass through building materials, and isolating one room from another by installing absorptive, isolating, and resilient materials within the building assemblies.

Sound Transmission

Sound transmission depends on the partition types. A thick concrete slab, for example, is unlikely to vibrate from individual foot falls, but the impact of heavy equipment or machinery, or even the multiple impacts of a stadium audience, can transfer a significant vibrational load. In a residential multifamily building—one with uninsulated steel joists supporting hardwood floors and standard fire-rated drywall ceilings—foot traffic and perhaps even voices (or radio, television, etc.) will readily transfer to surrounding neighbors.

To control both interior room acoustics and sound transmission in buildings, it helps to understand some basic terms and definitions. In particular, designers need to understand the existing standards for evaluating the acoustical properties of building assemblies and materials.

In the assembly shown here, the key materials are the black recycled rubber underlayment, the pink fiberglass insulation, and the resilient channel. The underlayment and resilient channel isolate structural members, minimizing structure-borne sound, while the fiberglass insulation provides absorption to minimize airborne sound. Illustration courtesy of ECOSilence

Sound Pressure Levels

Loudness is a subjective measurement, but sound pressure can be quantified in decibels (dB), which is a logarithmic unit: Each increase of 3 is twice as powerful although humans generally perceive an increase of 10 dB as being twice as loud. Ninety dB sounds twice as loud as 80, which sounds twice as loud as 70, etc. See below for a table that correlates subjective interpretations of loudness with sound levels expressed in decibels, providing loudness in dB of some common noise sources.

Loudness is a subjective measure, but sound pressure can be measured quantitatively in decibels. The table above correlates subjective interpretations of loudness to measured decibel levels.

Room Acoustic Ratings

For interior room acoustics, key metrics are reverberation time and absorption.

Reverberation. When a person talks in a room, the sound of their voice travels through air. A portion of that sound strikes the room surfaces and is reflected back as reverberation. A long reverberation time and many reflections contribute to degraded speech intelligibility.

The time it takes for the sound to drop in energy, or “decay” to a specific sound level, is a key design factor for building spaces. Auditoriums, gymnasiums, lecture halls, theaters, and other performance areas are all designed with specific reverberation times to meet the expectations for the activities that will be performed there. Designers need to understand that with an increase in demand for certain materials, such as tile, marble, and stone, even small spaces may have intense reverberation that must be acoustically managed.

RT60. The standard measure for reverberation time is RT60—the time required for sound to decay 60 dB. Although reverberation time is frequently stated as a single value, it is typically measured in bands of different frequencies. The human ear is sensitive to frequencies from 20 Hertz (Hz) to 20,000 Hz. This broad range is typically broken into octave bands, each doubling the previous one. In the range of human hearing, the octave bands are broken down with these center frequencies: 31.5, 63, 125, 250, 500, 1,000, 2,000, 4,000, 8,000 and 16,000 Hz. Designers typically focus on a specific range of bands, depending on the performance requirements.

Sound absorption. When a sound wave strikes a surface, the amount of sound absorbed into that surface material will depend on its composition. The absorbed sound causes the particles of the absorbing material to vibrate, and this vibration creates tiny amounts of heat due to the friction. This conversion to heat energy dissipates the sound energy. Fibrous materials are efficient absorbers. The sound absorbing characteristics of any finish material varies significantly with frequency. In general, high-frequency sounds, which have small wavelengths, are more easily absorbed than low-frequency sounds, because the wavelength is larger than the absorptive material.

Additionally, the thicker a material is, the better it will generally be at absorbing sound. The acoustical performance of a porous surface material can often be enhanced by incorporating an air space behind the surface to improve the absorption of low-frequency sounds.

NRC. How absorptive a material is can be rated using a standard test method described in ASTM C423. This test defines the Noise Reduction Coefficient (NRC) for a given material, which is an average of results demonstrating the change in the amount of sound energy recorded at different frequencies during the test. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption.

ASTM C423 uses a reverberation room test method, in which sensitive recording equipment is used to measure the decay rates in a room before and after the specimen is installed. The final result of the calculations is reported as sound absorption coefficients (“alphas”) at octave band center frequencies from 125 Hz to 5,000 Hz. For a convenient, single-number rating, the alphas in different ranges may be averaged to give a Sound Absorption Average (SAA) or Noise Reduction Coefficient (NRC). The higher the number, the more sound is absorbed.

Sound Transmission Ratings

To travel, sound needs a source, a path, and a receptor. The path is the only factor a building designer can control. Sound, like water, seeks the path of least resistance. Airborne sound between rooms travels most easily through gaps, doors, windows, and solid walls—in that order. Most barriers contain all of these sound paths.

Sound Transmission Class (STC). An STC rating is a single-number rating of a partition's ability to resist airborne sound transfer in one-third octave bands with counter frequencies from 100 to 5,000 Hz. The higher the STC rating, the more sound a partition will block. When evaluating the transmission of an airborne sound, such as the voice of a neighbor or their television, an STC rating provides a useful way to rate the effectiveness of a sound barrier. (See Table 2 for a general comparison of different STC ratings.) An STC rating is not suitable for evaluating acoustical performance in all applications, however. Because the frequency range is limited, different acoustical analyses should be used when designing mechanical equipment rooms, large conference rooms, recording studios, and music practice and performance spaces.

Note: This table provides a general indication of what can be heard on the other side of a partition wall. It is assumed that there it is a quiet 30 dB of background noise on the “listening” side.

An STC rating is derived by a laboratory test and calculation (per ASTM E90 and E413). This test involves a standardized sound source in the source room and a microphone system on the other side of the partition to be tested in the receiving room. Measurements are taken in specified frequency bands in the receiving room. Those measurements are then used to calculate a single number rating. The higher the number, the higher the resistance the building assembly has to airborne sound transmission.

In addition, an STC rating may be derived for a specific building partition using a field test (FSTC per ASTM E336 and E413). This field test uses the same general testing methods in the actual building after construction is completed.

There are two key things to understand about STC ratings: they only refer to airborne sound, and they only apply to the tested partition, not an individual element of the tested assembly.

Also keep in mind that while a partition might have a high STC rating, there are many ways airborne sound can travel. Through the building assembly is only one path. However, there are numerous flanking paths, including the air path over walls and through ductwork, under doors, through outlet penetrations (especially back-to-back outlets in a wall), open vents, louvered doors, and wall/floor connections to name a few. Certainly, one of the best ways to reduce sound transmission is by filling open gaps in building assemblies.

Impact Insulation Class (IIC). While STC ratings evaluate the transfer of airborne sound, IIC ratings evaluate the transfer of structure-borne sounds through floor-ceiling assemblies. These sounds result from impacts, such as someone walking in high heels, dropping items on a floor, or sliding heavy objects. As with an STC rating, the higher the IIC value of a floor-ceiling assembly, the better its ability to control impact sound transmission.

Like an STC rating, IIC results are derived from standardized tests: ASTM E492 details the lab test; ASTM E1007 for the field test and the single number ratings are calculated per E989. These tests are similar to STC testing. The difference is that the sound source in the source room or upper chamber is a tapping machine that impacts the floor with several hammers.

Here is an example of the effect different materials can have on the IIC rating of an assembly: A floor constructed with an 8-inch concrete slab attains an IIC rating of 32. Adding an acoustic underlayment and finished floor (hardwood) may improve the rating to 54.

Here, too, the important thing to remember about an IIC rating is it only describes the entire tested building assembly, not a single material.

ΔIIC. The “Delta IIC” test is another test that can be used to compare the impact insulation class characteristics of individual materials. This test is detailed in ASTM E2179 and consists of two IIC tests conducted over the same concrete slab that is 6-inches (150mm) thick. One test is over the bare concrete subfloor (without any flooring materials) to define a base value for the assembly. This actual assembly should be close to an IIC 28, the reference floor defined in the ASTM standard. Then, another test is conducted over the concrete subfloor with a small sample of the floor covering materials and any proposed underlayments. The ΔIIC, or improvement of impact sound insulation, is obtained by subtracting 28 (the reference floor) from the adjusted IIC of the whole assembly.

The key thing to remember is that ΔIIC measures the effectiveness of the floor covering in reducing impact sound transmission through 6-inch concrete floors and only tests a small specimen, not an entire floor. As with any IIC test, the results pertain only to the assembly. It can be used as a basis for comparison when evaluating one material over another, but should not be used beyond that.

Detailed description of assembly required. When evaluating an STC or IIC report, designers need to pay attention to how the report is detailed. Remember that IIC and STC tests (both lab and field) are not tests of single components, but evaluations of entire floor-ceiling assemblies. A report for any of these sound tests should include a detailed description of the floor-ceiling assembly used in the test. Without accounting for the entire assembly, the results are meaningless. Using IIC and STC results to represent the sound deadening ability of an underlayment without describing, in detail, the whole floor-ceiling assembly causes confusion at all levels.

A 5-mm-thick recycled rubber underlayment with 19 dB—more than most flooring underlayment materials. Recycled rubber provides excellent resilience and does not age or harden. Photo courtesy of ECOSilence

Acoustical Control Strategies

The main ways to minimize sound transmission from one space to another are by adding mass and breaking the path of transmission materials within the building assembly. Some examples of breaking the path of transmission include decoupling structural components using an air space (e.g. double wall with an air space between), or adding resilient isolators (e.g. isolation rubber beneath wall plates to decouple the wall from the floor it bears on). This latter approach is not as effective as a true separation with airspace, as some vibration may still transfer through the isolating material. However, it is the only way to achieve some isolation of structurally bearing components.

Interior Room Acoustics

When a person speaks inside an empty room finished in drywall with a wood floor, the sound carries very well. All surfaces are smooth and hard, so sound bounces from one hard surface to another until all of the sound energy is absorbed into the structure. Because the materials are relatively dense, this absorption happens slowly. Acoustic engineers call this an acoustically “live” room. It will get noticeably deader (quiet) when carpet, furniture, and acoustic ceiling tile are installed. These finishes cut reverberation by absorbing sound waves into their soft, cellular surfaces.

Carpeting the floor, for instance, will deaden most rooms enough for most applications; however, carpet is not always desirable or practical.

Controlling the acoustics within a room is more of an issue in large spaces, such as schools and hospitals, than it is in homes. In homes, particularly multifamily dwellings, client noise problems are more likely to involve sound transmission from one unit to another, than the deadening of sound levels within the dwelling unit.

Sound Transmission

Simply installing soft, sound-absorbent materials over the surfaces of one room doesn't do much to stem the transmission of airborne sound to another. Acoustical ceiling tile, for example, is often misconstrued as a way to reduce inter-floor sound transfer. It has little effect on sound transmission. It was originally developed as a means to help deaden noises within office spaces, and had some effect on muffling noises from HVAC ductwork passing through the space above a dropped ceiling.

To reduce sound transmission, designers need to focus on sound isolation—breaking vibration pathways through structural materials. Adding sound absorbing materials, such as insulation in building cavities, will also help; but, it's the separation that really counts when blocking both airborne and impact sounds is required.

In multi-family dwellings, the focus is usually on the floor-and-ceiling assemblies and wall partitions. In educational and healthcare facilities, floor-ceiling sound transfer is part of the problem, but special attention to wall partitions for sound-isolated classrooms, libraries, in-patient rooms, consulting rooms, counseling offices, and maternity wards is needed. Sound isolation is also key for intensive care units and surgical rooms where quiet is requisite.

Floor-ceiling assemblies present one of the biggest challenges, because the floor needs to support the live loads within the occupied space. Tools left to the designer include:

• Using soft finish flooring materials that absorb impact vibration (e.g. carpet)

• Adding mass (e.g. using concrete over steel pan or pouring a gypsum-concrete subfloor topping over a plywood subfloor)

• Installing acoustical underlayments beneath the finished floor

• Decoupling the ceiling using resilient channels or isolation clips

When carpet is not an option, unwanted impact sound transmitted through a floor-ceiling assembly can be reduced with the use of an acoustical underlayment. The primary purpose of this material is to isolate the flooring and any flooring impacts from the building structure.

The chief property of an effective acoustic underlayment is resilience. To reduce impact transfer, the material must be able to absorb impacts by compressing and then returning to its original thickness. It does not need to be soft or fibrous; rather it needs to behave like a spring. In fact, soft, fibrous materials tend to absorb water, either from the subsurface below or from wet-use conditions above, making them unsuitable in many applications. A good, resilient underlayment should be water-resistant and capable of supporting the finished flooring.

There are a number of acoustic underlayment materials on the market. The most common materials used in sound-rated building assemblies include:

• Foam

• Cork

• Mass-Loaded Vinyl

• Asphalt Roofing Membranes

• Felt

• Chlorinated Polyethylene Sheets

• Re-bonded Recycled Rubber

Table 3 provides a comparison of attributes a designer can expect from each type of material. Selecting which one to use should be based on a number of factors, including the type of sounds expected in the building and how well the underlayment will support the finished flooring materials. Remember, to be acoustically effective, the underlayment must break the path of vibration. If the finished floor is installed over battens attached to the subfloor, and the material fills the spaces between battens, it won't be effective. Impact vibrations will simply transfer from the finish floor through the battens. Some materials, such as cork, and to some extent felt can be relatively effective at blocking sound, but there is limited structural evaluation of how well it can support tile. The material that is most effective at blocking sound with a proven record of supporting a wide range of finish materials, including tile, is re-bonded recycled rubber. It is also water-resistant, so it can be used in a wide variety of applications.

This table provides a comparison of a variety of floor underlayments commonly used in sound blocking

With all the materials commonly used as an acoustic underlayment the most important thing to review is the published test data. An example is this statement for an acoustical underlayment:

“Test construction included a sound rated ceiling, concrete substrate 6” x 6” quarry tile, and latex modified thin-set... IIC = 62, STC = 59... ASTM E2179 ΔIIC = 12”

While this sounds impressive, there is no mention of the thickness of the concrete substrate or the details of the sound rated ceiling, both of which can have a significant impact on both IIC and STC ratings. The delta IIC rating doesn't specify which floor finish was tested. Quarry tile on a 6-inch slab alone can achieve a 12 dB improvement.

The only reliable criteria designers have to evaluate the acoustical properties of any one material are the IIC and STC ratings. Make sure that the referenced standards match the actual ASTM standards, using Table 4 as a reference.

This table lists the acceptable ASTM standards for evaluating sound-rated materials for floor-ceiling assemblies. Keep in mind that all of the tests rate a building assembly, not an individual material. To be useful, any published reference to one of these Standards should detail the components of that assembly.

Acoustics in Multifamily Dwellings

For multifamily dwellings, the International Building Code (IBC) references both the IIC and STC laboratory and field tests. The code specifies that IIC and STC ratings be at a minimum of 50 if tested in the lab, and 45 if tested in the field.

These requirements are often satisfied when carpet is used as the floor covering. However, hard-surfaced flooring creates real sound transmission problems. The trend towards stone and tile flooring, in particular, means that designers of multifamily dwellings can no longer depend on carpeting to solve the problem. Designers need to pay more attention to the design of floor-ceiling assemblies to meet specific standards.

The code is simply a minimum. Owner's expectations, particularly in high-end and luxury housing, need to reach beyond this standard. In its Guide to Airborne, Impact, and Structure: Borne Noise Control in Multifamily Dwellings, the U.S. Dept. of Housing and Urban Development has largely set the standards for controlling noise in multifamily dwellings (see Table 5). These guidelines establish three grades for acoustic environments. The minimum grade reflects the IIC requirements of the IBC for floor-ceiling assemblies.

The U.S. Dept. of Housing and Urban Development (HUD) provides descriptive definitions of three grades of acoustic environments, which largely set the standards for controlling noise in multifamily dwellings. *The Minimum grade reflects the IIC requirements of the International Building Code (IBC) for floor/ceiling assemblies.

Acoustics in Healthcare Facilities

Healthcare facilities feature a broad range of buildings. This includes hospitals, nursing homes, hospice facilities, assisted living facilities, independent living settings, adult day care facilities, wellness centers, and outpatient rehabilitation centers. These environments must be designed to meet published standards described in the Sound and Vibration Design Guidelines for Hospital and Healthcare Settings. These standards are laid out to support the Health Insurance Portability and Accountability Act (HIPAA) and set guidelines for how to ensure sound isolation and speech privacy in healthcare facilities. HIPAA is meant to prevent intentional or unintentional privacy breaches, and the privacy standards it establishes apply to both new construction and healthcare renovations of all types. The HIPAA guidelines serve as the reference standard for the acoustics section of the 2010 FGI/ASHE Guidelines for Design and Construction for Healthcare Facilities; the Green Guide for Healthcare version 2.2; and LEED for Healthcare, version 4, which is currently under development.

Recommended daytime noise levels for patient areas in healthcare facilities is 35 dB(A)—a target that is difficult to achieve. According to a 2005 study of noise levels in hospitals, noise levels at Johns Hopkins Hospital, a top-ranked U.S. hospital, exceeded 45 to 50 dB(A). This is higher than the typical conversational speech level.

As in other institutional buildings, sound is transmitted through common partitions, common suspended ceilings, and flanking paths, such as open doors. HVAC systems and ductwork add to the sound levels. In healthcare facilities, mechanical sounds include a wide assortment of patient-care equipment, emergency alarms, and other equipment, such as service carts. Other sound sources that must be managed by designers of healthcare facilities include amplified paging and intercom systems and conversational speech during patient/doctor communications and family visits. All these sounds will require the use of acoustically absorptive materials, both as finishes on interior surfaces to control room acoustics and as underlayments, isolation membranes, and gaskets to reduce sound transmission. Because of the clean-room requirements, carpet is rarely acceptable as a floor finish.

Nursing stations and staff rooms have proven to be a major source of noise. While this noise may be unavoidable, efforts should be made to isolate nursing stations from patient rooms, counseling rooms, and other areas of the facility where privacy and sustained periods of quiet are needed. The use of acoustic underlayments and soft floor finishes in nursing stations not only helps this isolation but provides some joint relief for nurses.

Designers should also be aware that expansion breaks and transitions between floor surfaces increase the vibrations from rolling equipment when they pass over these transitions. Smooth, continuous, cleanable floor surfaces should be the goal.

Classroom Acoustics

Consider a young student that has a tenuous grasp on vocabulary and linguistic sounds. Moderate levels of noise and poor room acoustics can impair this student's ability to understand words, even when they are clearly spoken. The problems become more acute for hearing-impaired and second-language listeners.

Much of the acoustic management comes from school teachers and staff (e.g. keeping classroom noise to a minimum, cooperation with scheduling activities in adjacent spaces, etc.). However, building designers have an enormous impact on how successful those efforts will be. The goal should be to create learning spaces capable of isolating outdoor noise and achieving indoor levels below 40 dB (A). This is a quiet space.

General strategies for accomplishing this include:

• Locate buildings away from noisy roads and other noise sources.

• Provide better sound isolation in exterior walls and windows to limit the intrusion of outdoor noises.

• Include high-quality acoustical floor underlayments in floor-ceiling assemblies

• In addition to the use of acoustical underlayments, floor-ceiling assemblies also need 2-hour rated ceilings on resilient channel and sound-isolated wall plates.

• Provide quieter ducted central ventilation systems and locate HVAC units away from classrooms and learning centers. Rooftop HVAC systems are recommended.

Controlling sound transmission alone is not enough for controlling noises in educational settings. Efforts to control room acoustics, such as providing sound absorbing surface finishes have proven successful. It is also recommended that classroom sizes be limited. In rooms greater than 1,500-square-feet, speech can reverberate a second or longer, degrading speech intelligibility.

Bottom Line

Regardless of what building type a designer may be detailing, there are only two ways to evaluate the sound transmission characteristics of building assemblies:

• STC – Sound Transmission Class; measures airborne sound

• IIC – Impact Insulation Class; measures structure-borne sound between floors.

The IBC specifies that IIC and STC levels be at a minimum of 50 in the lab and 45 in the field. To reach these levels in multifamily, healthcare, and educational facilities, sound transmission can be reduced in the floor-ceiling assembly using an impact sound attenuation material. But, be careful when specifying these materials. Building designers must pay close attention to the product evaluation reports to make sure that laboratory and field tests are not misused or the wrong tests cited.

 

ECOsilence is proud to offer cutting-edge sound control underlayments, accessories, and solutions. ECOsilence underlayments and accessories work together to create ideal flooring systems and solutions. All ECOsilence underlayment is made from recycled rubber and holds a limited lifetime warranty against product defects and workmanship. www.ecorecommercialflooring.com