June 6, 2017
by Jerry Heid, AHC
When discussing how to secure the openings of a building, physical security commands the lion’s share of attention, with concerns about unauthorized access and fire typically Being at the forefront of any list of potential threats. However, a growing number of buildings are now being designed or retrofitted to protect occupants against a menace far more difficult to deter than any criminal. Hospitals, schools, office buildings, hotels, and even private residences are all taking steps to fortify their defenses against the pernicious effects of noise.
When compared to the dramatic impacts of a fire, it is easy to understand why noise has long been regarded as more of a nuisance than a serious threat. That said, in recent years, there has been a decided shift in awareness of the potential risks that can result from inadequate soundproofing, ranging from impaired learning in schools to slowed rates of patient recovery in hospitals. Privacy concerns have also become a major issue, particularly in hospitals required to comply with the Health Insurance Portability and Accountability Act (HIPAA) and ensure the confidentiality of communications with individuals.
As a result, several standards and code organizations have been incorporating noise mitigation into model building codes, and customers across a growing spectrum of industries are demanding better and more reliable acoustical control. However, as with any architectural specifications, the devil is in the details. Preventing the transfer of sound through door openings—specifically through clearances and gaps around doors—requires specialized expertise and quality materials to be successful.
Understanding sound transmission
To fully appreciate why sound control is so difficult to achieve, it is necessary to first understand the mechanics of sound transmission. Sound is simply vibrations moving in waves through the air. When these waves contact a door, some of the energy from the vibrations transfers to it. The resulting vibrations in the door itself set the air in motion on the other side, creating more sound vibrations.
The volume of those sounds is measured in decibels (dB), and each 10-dB increase doubles the sound reception. In other words, a noise registering at 60 dB is twice as loud as a noise at 50 dB, but only half as loud as it would be at 70 dB. As a result, even the smallest increase in sound levels would be perceived as a dramatic change by occupants.
Another characteristic of sound waves that makes them difficult to minimize is their ability to travel through even tiny openings with very minimal loss. As illustrated in Figure 1, a gap of less than 3 mm (1/8 in.) may only allow a small amount of air to pass through, but it is still large enough to completely cancel out any noise-reduction benefits of an acoustic door.
The key to minimizing sound transmission is mass, particularly for lower-frequency sounds. The greater the mass, the less sound is transmitted through the barrier. A variety of materials are used to accomplish this in acoustical doors, and their effectiveness is measured in sound transmission loss (TL). The TL of a door is measured over 16 different frequencies, and the average of those values is used to determine the sound transmission class (STC) rating of the door. The higher the STC value, the better the rating, with 30 considered to be the industry minimum for an acoustical door (Figure 2).
It is important to understand STC values are not proportionate units of measurement. To continue reducing sound transmission—that is, to achieve increasingly higher levels of sound control—each 10-dB increment requires 10 times as much improvement as the one before. While door openings rated in the range from STC 30 to 40 are common, achieving a rating of STC 50 or higher is extremely difficult.
As sound doors are tested under optimal conditions, actual field performance varies. The testing process takes place in two different phases—fixed and operable. For the former, the doors are sealed in place, and sound is applied and recorded. This step is used to determine the STC rating of the door itself. However, the second phase of testing requires the door to be operable, and all sealants are removed before sound is applied. This results in a more-realistic assessment of the door’s performance in the field. Professionals should be aware some manufacturers will advertise their sealed-in-place STC ratings rather than this more-realistic operable rating. Selections based solely on the sealed-in-place ratings are likely to yield disappointing results for the end-user.
Mind the gaps
Installing an acoustical door with a sufficient STC rating is an important first step toward achieving acoustical control, but it is only part of the solution. As mentioned, the smallest opening can allow an enormous amount of sound to travel through even the highest-rated acoustical door. Put simply, gaps guarantee noise.
Successful sound control requires any gaps around a door’s perimeter must be sealed with gasketing providing a complete, uninterrupted airtight seal around the head, jamb, and sill. Head and jamb gaskets must work together with sound-rated door bottoms, which should close on solid, flat surfaces like marble or aluminum thresholds.
One of the most frequently overlooked sources of gaps is carpeting, as it allows sound penetration through its fibers. Imperfect alignment is another common cause, even in newly installed gasketing. Problems can also arise over time as buildings shift and settle and doors are exposed to seasonal changes in temperature and humidity. One solution is the use of adjustable gasketing, which allows easy modification using a screwdriver to close any gaps in clearances and restore sound-tight seals. Doorframes can also be treated with a special coating inside the frame to improve total STC performance, but it is more critical they are installed correctly to avoid sound transmission. Flanking noise—noise that indirectly penetrates the opening through and around walls, floors, ductwork, and similar—is also an issue. These types of noise sources should be addressed in the building design and are another reason sealed-in-place STC ratings are far less accurate than what is likely to be measured in the field.
Use of appropriate gaskets is critical for achieving an effective STC rating as close as possible to the door’s published rating. However, as with any product, there are significant variations in quality of materials and designs. Lower-quality gaskets will deliver lower-quality results, measured in disappointing STC performance. Designers and consultants should ensure only high-quality gaskets are used with their sound-control systems. Compression seals extruded from high-quality neoprene are essential for basic gasketing integrity, and supplemental seals can be invaluable in helping compensate for the effects of common variances in installed clearances.
The ultimate goal is to ensure that sound-door perimeters are sealed systems that can continue to function properly through thousands of cycles over many years. In assessing gasketing quality and design for optimal durability and service life, as well as STC performance, it is important to seek:
In all cases, providing precise specifications for required elements minimizes potential for errors due to inferior materials or mismatched components. This raises the somewhat controversial question of where exactly gasketing should be specified. The most basic answer is it depends on the manufacturer and needed sound rating. If the project requires an STC in the 30s range, there is flexibility in selection of gasketing manufacturers, and the gasketing may be specified in section 8710. However, if STC ratings are in the high 40s and 50s, they are then typically furnished by the door manufacturer, and the relevant specifications should read “gasketing by door supplier.”
No matter where the gasketing is ultimately specified, the most important factor is confirming it has been tested with the unit to ensure test documentation is available. However, the most-effective way to ensure proper performance in the field is to minimize the variables that can affect door functionality. Engineered assemblies with acoustical gasketing, tested as a unit, provide full accountability through a single manufacturer and its installers, as all components are engineered and tested as one system. This can be particularly useful for high-demand sound control, as well as all applications where performance to precise STC levels is needed.
No matter how detailed the specifications or how high-quality the materials, a product can offer no protection against improper installation. Using the wrong gaskets or cutting them too short, extending a door bottom too far into the perimeter seal, and allowing edges that do not line up are all common mistakes that compromise system performance and promised sound ratings. Specifiers would be wise to include the manufacturer guidelines for installation, along with appropriate product features, to minimize and correct sound-barrier gaps caused by installation mistakes.
When working on jobs requiring higher STC-rated openings, it is helpful to rely on the recommendations of leading acoustical consultants. These include:
Acoustical control is rapidly progressing from a consumer demand to a code mandate. In response to a number of studies by the Acoustical Society of America (ASA) and others that document the adverse impact of noise on student learning, some states and jurisdictions have incorporated American National Standards Institute (ANSI) S12.60-2002, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, either referenced as part of building code updates or adopted separately. This standard details acoustical performance criteria for learning spaces, requiring a minimum STC 35 rating for classroom doors. It also spells out minimum STC ratings for specific locations and conditions, which will be incorporated into accessibility guidelines under the Americans with Disabilities Act (ADA) in order to accommodate those who are hearing-impaired.
It is important to be aware of other conditions and circumstances that might necessitate effective sound-control measures, even though they may not be code-driven. For instance, sound control has become an increasingly vital security issue for corporations and other private entities, as well as military and government facilities. Further, in settings such as houses of worship, doctors’ and therapists’ offices, and corporate human resources (HR) quarters, there is a significant risk of legal liability if confidentiality cannot be effectively maintained.
Knowledge is key
Given the number of variables that can negate even the most thorough sound-control measures, it is easy to see why providing effective acoustic control continues to be one of the most demanding challenges for door openings. Although proper performance depends on a number of different factors, when installed assemblies fail to deliver the specified level of sound control, designers, consultants, and suppliers may all share responsibility or even potential legal liability as well as the expense of correcting the problem.
Knowledge and proper training are crucial to avoiding those risks, which is why it is important to know when to turn to acoustical experts for support. Their understanding of the science of sound can help ensure a successful sound-control solution in any facility.
Jerry Heid, AHC, is the national sales manager for Zero International, an Allegion Brand Company. He has worked in the door and hardware industry for 37 years, specializing in gasketing and sound-control products for almost two decades. Heid is also a current member of the Door and Hardware Institute’s (DHI’s) Board of Governors. He can be reached via e-mail at firstname.lastname@example.org.
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