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Open Plan Acoustics: Sound Decisions

By Lacey Muszynski, Assistant Editor - January 2007 - Ceilings, Furniture & Walls


Office acoustics is a concern for facility executives who want to provide a space for happy, productive building occupants. Employees need a space to work that takes into consideration opposing acoustical factors — appropriate sound privacy coupled with employees’ need to communicate frequently between workstations, for example — which can be difficult to accommodate. The problem is most pronounced in open plan offices.

With only partial height furniture panels separating workstations, sound — from conversations, speakerphones, computers, mechanical equipment and other sources — can often travel uninterrupted from the source to surrounding workstations.

Adding to the acoustical equation is the push from companies to increase productivity. While some productivity-boosting practices are ultimately good for the company, and theoretically for the employees as well, they might exacerbate the acoustical problems in open office spaces.

For example, a common real estate-saving practice — reducing workstation size to increase the number of workers in an office space — can hurt productivity if acoustics is not carefully examined. Bringing employees closer together often results in decreased speech privacy which can lead to distractions when normal speaking voices are intelligible to adjacent co-workers. Additionally, employees may be nearer to noisy office equipment, such as fax machines, copiers and printers, which multiplies the problem.

But an open plan office need not be an acoustic disaster. The key for facility executives is understanding how various interior products affect acoustics. “You’ll never achieve acoustically in an open office plan what you can in a room, but you can come close,” says Dave Adams, principal and founder, D. L. Adams Associates, an acoustical consulting group.

Key Components

Carpet has traditionally been the flooring of choice in open office spaces. “Acoustically speaking, hard flooring has never worked well in open office spaces,” says Kring Herbert, principal, Ostergaard Acoustical Associates.

The main advantages of carpeting are dulling footstep noise throughout the office and controlling sound reflections. Installed carpet should have a pile of at least one-half inch to provide acoustic advantages.

Carpeting isn’t an effective sound absorbing material, however. The frequencies absorbed are higher than the range of human speech; therefore carpeting doesn’t contribute to the speech privacy of an open office workstation.

Choosing the right furniture panels is also an extremely important decision in creating an acoustically sound space.

Panels should be at least 60 inches high. This is above the standard 48-inch seated mouth-ear height and will be sufficiently high to block most noise from entering or leaving individual workstations. Both Adams and Herbert say the most common problem in open office spaces is panels that are too low, and sometimes don’t even break the line of sight between neighbors. Where possible, panels should go to the ceiling.

Furniture panels need to absorb ambient sound and block sound transmission between workstations, says Herbert. Panels achieve this with either an inner septum (a hollow inner core space between outer panel materials) or a masonite or metal sheet (which will increase the weight of the panel) barrier to reflect sound. This inner layer is covered with one to two inches of fiberglass, a sound absorber.

Panels should have a noise reduction coefficient (NRC) of at least .75 to prevent sound from reflecting back over the head of employees when they are speaking towards the panel, says Adams. In addition, facility executives should be aware of potential weak spots in furniture panels, like joints, corners and places where panels meet the wall. These can decrease the overall NRC rating.

Facility executives should also consider ceiling panels as a component of sound management. Panels should have a NRC rating of at least 0.9 to provide sufficient absorption of sound and reduce unwanted reflections. However, even highly absorbent panels will reflect sound waves that strike the ceiling at an oblique angle, says Adams.

The articulation class (AC) rating of a ceiling panel measures the panel’s performance based on speech privacy, specifically in an open office setting. Panels should have as high an AC rating as possible, with 250 being the highest and most effective at controlling noise between adjacent workstations.

Most ceiling panels are made from mineral composites or fiberglass. Mineral fiber is most common, but as with furniture panels, fiberglass is the better sound absorber. On the other hand, mineral fiber blocks airborne sound transmission better than fiberglass. Mineral fiber panels are more suited to areas such as conference rooms or executive offices — spaces that are acoustically separate from open workstations.

Acoustical Perfume

Sometimes overlooked by facility executives, a sound masking system is invaluable in an open office space. “The whole idea behind sound masking is to control background sound to provide better privacy and hide intruding sound,” says Herbert. “A sound masking system provides an acoustical perfume.”

No matter how acoustically well-planned a space’s floor, ceiling, furniture panels and finishes are, a correctly configured sound masking system will always improve speech privacy. It provides a controlled level of background sound in the space at a constant, appropriate volume and frequency. Masking sound is unintelligible, so while employees are aware of the sound, it can be tuned out much more easily than speech.

Typically self-contained above the ceiling panels in the plenum space, sound masking systems utilize a series of loud speakers. Speakers are evenly spaced to assure proper sound distribution, with consideration given to full-height walls and other obstacles that would hinder the sound masking. The ceiling panels evenly distribute the sound, as well as hide the speakers and equipment.

“Be careful about hot spots in the ceiling, such as open air returns,” says Adams. These and other inconsistencies in the ceiling could mean some employees hear much louder masking sound than others, which could become more a hindrance than help.

To work around these obstacles, at least one manufacturer offers a sound masking system that is installed either on ceiling panels or walls. It can also be used in offices that don’t have drop ceilings or sufficient plenum space for an above-ceiling system.

One of the biggest mistakes Adams sees facility executives make is installing and tuning a sound masking system themselves. “Facility executives sometimes think that all that needs to be done is introduce sounds into the space,” says Adams. “They don’t think about the sound that masks speech frequencies. People buy canned units and don’t take time to address the whole open plan space. Masking needs to be something that’s thought out and planned carefully.”

Adams has also encountered instances where facility executives have assumed that the HVAC system noise would serve as a sound masking system. When that didn’t work because the frequency spectrum was wrong for speech, they installed a sound masking system and cranked that up to cover the HVAC noise. A plan like that does not make for happy or productive employees.

Installation can be done by a contractor, but configuration and tuning should be done by an acoustical consultant. They will ensure that the frequencies and volume are correct for each space in order to provide the optimum level of speech privacy.

Getting What’s Needed

Facility executives can follow a series of steps to determine which products are best for a particular open office space.

First, the facility executive will need to answer questions about occupants: Who will be using the space? What degree of privacy do they need? Do they need different degrees of speech privacy at different times or locations? These answers establish conceptual goals for the space.

The next step is to determine numerical goals based on these conceptual goals. This involves establishing speech privacy goals in various areas of the office using the articulation index (AI), and determining the various NRC, sound transmission class (STC) and AC ratings of office components in order to meet the speech privacy goals.

Establishing conceptual and numerical goals is sometimes aided by producing a mock-up of the space. This can help ensure that acoustical goals mesh with architectural and design goals. However, Adams warns that a mock-up will never give the exact same acoustical results as the actual space.

An acoustical consultant can aid the facility executive in all steps of the process. One good way to find an acoustical consultant is through word of mouth from other facility executives who have had positive (or negative) experiences in the past. If this returns no results, look for consultants who are members or fellows of the Acoustical Society of America (ASA) or the National Council of Acoustical Consultants. These organizations have standards for membership which relate to education, experience and peer references.

An acoustical consultant can be especially helpful in formulating numerical goals and developing product specs. “We’re not going to recommend a specific manufacturer, but we’re going to give you minimum performance specs,” says Adams. “This enables the facility executive to go out and competitively bid to get exactly what they need to meet their acoustic goals.”


Acoustic Terms and Standards

Many acoustical rating systems are used when evaluating spaces and products for acoustical performance. Here are a few of the most common:

Articulation Class (AC) – Rating system used to evaluate open office acoustical ceilings based on speech privacy. Values range from 100 to 250, with higher meaning better performance.

Articulation Index (AI) – Measurement of speech privacy between open office plan workstations. It is performed with a speaker reciting meaningless words while someone in the neighboring workstation writes down the intelligible content. Values range from one to zero, with lower meaning better performance.

Noise Reduction Coefficient (NRC) – Rating system to determine absorptivity of a material over the speech frequency range at 250, 500, 1,000 and 2,000 Hz. The higher the number, the more absorptive the product is.

Sound Transmission Class (STC) – Rating for construction elements (doors, walls, windows, etc.) based on their ability to reduce speech sounds. The higher the number, the more sound is blocked.


Degrees of Speech Privacy

Minimal
Average noise level, conversation in
neighboring workstations is easily intelligible

Normal
Low noise level, conversation in neighboring
workstations intelligible with concentration

Confidential
Minimal noise level, conversation in
neighboring workstations unintelligible





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