Statistics from the Data Management Association suggests that a lack of acoustical privacy in open plan offices can reduce productivity by as much as 40% and increase errors by as much as 27%!

Basic properties of sound

There are two types of sound:

• Direct Sound – Sound travels directly to the ear from where it originates.


• Indirect Sound – Sound reaches the ear after being reflected one or more times from the room surfaces or is transmitted from one room to another.

When sound strikes any surface a part of its energy is absorbed, part is transmitted, and the remainder is reflected back into the room.

Hard surfaces such as drywall, plaster, concrete, and glass absorb very little sound. Acoustical materials are designed to absorb a proportion of the reflected sound energy.

Absorption of Sound

Hard massive non-porous interior finishes such as drywall, plaster, masonry, glass, wood and concrete absorb generally less than 5% of incident sound waves and reflect 95% or more.

Acoustical materials include: carpets, draperies, upholstered furniture, clothing of people, acoustical ceiling tile and panels. These may have coefficients approaching 1.00.

Reverberation

Prolongation of sound after the source has stopped due to continued multiple reflection is termed reverberation.

If sounds dies out slowly, a room is described as live or excessively reverberant and if it dies out very rapidly a room is called dead.

Reverberation is an important factor governing hearing conditions and it has an important bearing on the noisiness of working areas.

Reverberation Time

Reverberation time is a basic acoustical property of a room, which depends only on it’s dimensions, and the absorbing properties of its surfaces and contents. This may vary typically from a fraction of a second in a very dead room to the order of 5-10 seconds in a live room.

Sound Transmission

Problems of room acoustics are encountered when sound or noise is transmitted from sources in adjoining rooms or corridors through walls, floors, and ceilings. The noise reduction between rooms depends on several factors: room absorption, transmission loss of each path and frequency.

Usual sound transmitters can be walls, floors, or ceilings. In addition, actual openings in the interior room boundaries such as the crack around a door, a hole in the wall around pipes or utilities, open plenums above suspended ceilings, recessed lighting fixtures, or a connecting ventilating duct can also act as a pathway for sound. Like water sound follows the path of least resistance.

Sound Absorption

A material with a noise reduction coefficient of .50 or greater is normally considered to offer significant sound absorbing properties. In theory the maximum sound absorption coefficient is 1.00.

All surfaces and finishes of a room, which absorb sound, must be taken into consideration in determining the sound absorption characteristics of the room.

Hard non-porous surfaces such as drywall plaster, glass and wood are poor because they absorb less than 20%. Porous materials such as drapes, upholstered seats, carpets, furniture and people themselves provide some level of sound absorption.

Ceiling Sound Attenuation

Ceiling attenuation can be defined as the noise reduction measured in decibels for the ceiling-plenum path between two rooms. The plenum space above acoustical ceilings for ducts, piping and utilities, serves as another sound transmission path, which parallels the transmission directly through the dividing partition.

If the ceiling is extremely light or porous, or if it is suspended in such a way as to allow numerous leakage paths the plenum may have a greater influence on the sound transmission from room to room than the partitions.

Improved attenuation via the plenum can be obtained by extending the partition or other suitable barrier through the ceiling and blocking virtually all of the plenum. Another approach would be to place a sound-attenuating blanket over the top of the acoustical ceiling extended at least four feet on both sides of the partition line. Close attention must be paid to all other sound paths such as air ducts, electrical outlets, and closure panels at exterior walls.

One of the weak points in the ceiling plenum transmission path is the joint or seal between the dividing partition and the ceiling, especially if the ceiling is continuous over the top of the partition. Special closure panels, caulking, and special suspension systems can provide a better sound at this point.

The number type and location of recessed light fixtures may significantly affect the ceiling performance. Fixture attenuation boxes should be considered to reduce the adverse effect of these fixtures.

Air grilles into return air plenums generally provide direct sound paths negating ceiling performances. Return air plenums should be avoided in situations where sound is a major concern.

Sound pressure levels of common sounds and noises

• 0-20 decibels – rustle of leaves, faint whisper
• 20-40 decibels – quiet conversation, quiet home or private office
• 40-60 decibels – noisy homes, average conversation
• 60-80 decibels – average street noises, noisy office, average radio
• 80-100 decibels – loud street noises, noisy factory, truck unmuffled
• 100-120 decibels – Thunder, Artillery, boiler factory

*Zero on the decibel scale represents the minimum sound that we can hear.

Some experts suggest that most workers won’t tolerate levels much higher than 48dB of steady background noise.

Open Plan Acoustics

The prime advantage of an open plan office is flexibility. Changes can be implemented with minor effort and at a cost lower than in conventional offices. Fabric work module panels serve both as a sound attenuator and as a sound absorber. Ceilings should be highly sound absorbing and floors should be carpeted primarily to eliminate impact or scraping noises. The final component of an open office plan is the masking sound system, which controls the level of background sound.

One of the most important elements in minimizing reflected sound is a highly absorbent acoustical ceiling. Ceilings with a high STC rating are typically constructed of a high-density mineral fiber that does a good job of absorbing sound and provides a barrier to sound traveling through the tile. As a rule the higher the ceiling is placed the better the acoustical result. Many authorities suggest a nine-foot minimum for the open office.

Landscape Screens

Partial height landscape screen is a basic element in most open office plans, The larger the screen and the closer it is to the speaker or listener the less sound will be able to go around the barrier. Some experts recommend panels with an NRC rating of at least .80 and an STC of at least .23 when the speaker is 3’ or more away form the panel. If the speaker were a few inches from the panel an STC of 26 would be better. A drywall partition will typically have an STC of 35. A drywall partition can be built with an STC of 45 or higher, but the costs and efforts increase as well. Glass typically has an STC of 26 or lower. While drywall has an STC of 35 its NRC is very poor, generally absorbing only 5% of sound waves.

Carpeting

Carpeting is usually used in open plan offices because surface noise levels are seven to twelve times higher with vinyl or tile on the floor.

General Layout

Orientation of adjacent workers (even a 30 degree rotation of a speaker) away from the listener will result in a decrease of the sound heard by the listener. A maximum decrease of 9 dB occurs when a speaker turns his back to the listener. Some experts say that workers may be placed about 12 feet apart on the average to achieve normal privacy.

Acoustic Recommendations

• Use acoustical ceiling tiles that have an STC of 35 or higher, a minimum NRC of 65 and are made of mineral fiber.


• Use carpeting instead of wood or tile flooring.


• Use indirect HVAC ducts where possible.


• Use parabolic louvered light fixtures.


• Place recessed lighting fixtures far apart on opposite sides of the wall.


• Implement a sound masking system.


• Minimize use of glazing between offices.


• Orient adjacent workers away from each other.

Glossary

Articulation Class – Rates a ceiling’s suitability for achieving normal speech privacy in open office spaces by absorbing noise reflected at an angle off the ceiling into adjacent cubicles.

Ceiling Attenuation Factor – The difference in noise level measured in decibels between a source room and an adjacent receiving room when it is assumed that all the sound entering the receiving room travels by the common ceiling.

Noise Reduction Coefficient (NRC) – The average percentage of noise that a material absorbs in the mid frequency range. An NRC of 1.0 would denote total sound absorption. A material is typically not classified as a sound absorber unless it has an NRC value of at least .40.

Noise Criteria (NC) – The measurement of background noise in an environment. Too much quiet can be as big a distraction as too much noise. NC for a boardroom is 30, NC open plan office 38-40, NC for a cafeteria 45.

Sound Transmission Class (STC) – A single number system used to rate the airborne sound transmission performance of a wall panel, ceiling, etc. The higher the STC number the better the product’s ability to block sound transmission. A two or three point difference in STC rating is not detectable to the human ear.

References

CISCA Acoustical Ceilings Use and Practice 1999

Trendwall Acoustics Guide 2000

Armstrong Acoustical Performance 2000

Interior Construction Oct/Nov 1986