Open Offices

Open offices create a multitude of problems for achieving speech privacy. Open office spaces are more densely populated with office workers, with fewer buffering spaces like storerooms between people. The trend to

Sound Open Offices

Direct sound path through partition

Diffracted sound path over top of partition

Reflected sound path from ceiling over partition

Figure 54-1 Sound paths in an open office.

Direct sound path through partition

Diffracted sound path over top of partition

Reflected sound path from ceiling over partition

Figure 54-1 Sound paths in an open office.

ward more employees working at computers or desks results in open office plans with increasingly smaller cubicles for one or two people.

The amount of speech privacy required within an open office varies. Acoustic consultants identify three levels of speech privacy: normal, confidential, and transitional. Normal speech privacy levels allow normal voice levels from an adjacent cubicle to be heard, but without intelligibility unless the listener concentrates on the sound. Raised voices will generally be intelligible. Where overall noise levels are low, background noise levels should remain within 6 dB of the intruding sound so that speech doesn't stand out.

Confidential speech privacy requires that normal voice levels be audible but generally unintelligible. Raised voices may be partially intelligible. The noise level should be minimal. The background noise level should be no more than 2 dB less than the intruding sound, and a maximum of 3 dB more. In such a space, 95 percent of the people don't sense sound as intrusive and disturbing, and are able to concentrate on most types of work.

Transitional speech privacy levels are also referred to as minimal or marginal privacy. Transitional speech privacy levels are considered intolerable by around 40 percent of people, and the number whose productivity would suffer is even higher. Speech at normal voice levels in adjoining open offices is readily understood most of the time, and the overall noise level is average. Intruding speech levels may be 10 dB or more than the background level. Offices with two occupants or one office receiving noise from three adjacent offices have transitional levels of speech privacy. It is almost impossible to have adequate speech privacy in a cubicle shared by two people.

Ky Sound in open offices can travel directly from the source to the listener (Fig. 54-1). It may also be dif fracted by objects in its path, or reflected off ceilings or walls. The architectural arrangement of the space has a great impact on speech privacy. When designing an open office, group spaces according to their speech privacy requirements. Confidential areas should be at the edge of open areas that serve as a buffer zone, with low overall sound levels including any background noise. Speech in perimeter offices with reflective surfaces may bounce out into areas occupied by other workers (Fig. 54-2). High noise production areas should be grouped and placed on the perimeter at a maximum distance from confidential areas.

Open areas within an open office plan should be as large as possible with acoustic insulation on the perimeter walls. Ceiling height should be no lower than

Figure 54-2 Speech privacy issues in a corner office.

2.75 meters (9 ft), with a 914-cm (3-ft) plenum above. Ductwork should be acoustically treated.

Try to keep individual office cubicles as enclosed as possible, with the maximum possible partition height. Occupants should be at least 3 meters (10 ft) apart, which increases to 3.7 meters (12 ft) for normal privacy and 4.9 meters (16 ft) for confidential privacy. Check desk locations for speech path privacy; desk orientations need not be the same in all cubicles (Fig. 54-3).

The absorption characteristics of the ceiling are the most important factor in designing open office speech privacy. Metal air pan diffusers and flat lighting fixtures provide strongly reflective speech paths, which bounce sound rising from one cubicle down into another. Highly absorptive baffle strips on equipment perimeters help to block sound paths.

The angles of reflection of sound waves from the ceiling depend on the location and height of the source of the sound, and on the ceiling height. The majority of angles formed by sound hitting and bouncing off the ceiling are between 30 and 60 degrees. Ceiling materials are available with high absorption at these angles. Minimal absorption coefficients at angles between 30 and 60 degrees should be 0.65 for 250-Hz frequency sounds (around the lower range of a male voice), 0.65 to 0.75 at 500 Hz, 0.85 at 1000 Hz (both of these encompass men's and women's voices), 0.90 at 2000 Hz (women's voices), and 0.90 at 4000 Hz (electric office equipment).

An articulation class (AC) rating indicates the absorption efficiency at angles of incidence between 45 and 55 degrees, and AC ratings should be at least 220, with the higher the number the better. Ceiling material manufacturers have tested and can supply accurate absorption data on their products. Light fiberglass ceiling tiles have an absorption coefficient at voice frequencies of 0.95. Mineral fiber tiles range from 0.8 to 0.85.

If you place flat-bottom lighting fixtures directly over low office partitions, they provide a speech path between the offices. The best fixture from an acoustic standpoint has deep reflector cells with parabolic bottom surfaces (which is what reflects both light and sound) in a 31 by 122 cm (1 by 4 ft) or 61 by 122 cm (2 by 4 ft) format.

Because sound always finds the path of least resistance, very little sound actually passes through low office partitions, as it usually goes over the top. Where a seated speaker is 112 to 122 cm (44 to 48 in.) high and about a meter (3 ft) away from the partition, the partition should have an STC of 25 to 26. With greater distance and a higher partition, an STC of 20 to 22 is permissible. The AC ratings should be in the 200 to 220

Sound bounces off corridor wall and into adjacent office.
Sound bounces off office wall, then into corridor, before entering adjacent cubicle. Result: Less sound transmitted

Front closure partition eliminates reflection off corridor wall.

Figure 54-3 Open office layouts.

range. Joints between partitions should be carefully sealed, as even small openings lower efficiency. For acoustic isolation, partitions should reach the floor, although lower areas don't have to be insulated in low speech privacy areas.

Partitions must be high enough to block direct line-of-sight voice transmissions. The median height of the mouth of a standing American male is 160 cm (63 in.), so partitions should not be lower than 165 cm (65 in.), and preferably 168 to 183 cm (66-72 in.) when located between cubicles. Because the greater 183-cm height also blocks vision, tall partitions are generally only used between departments, with 160 to 168 cm high partitions between cubicles.

Team work areas should be located away from normal working spaces, or be completely enclosed in full-height fixed or demountable partitions. Areas where raised voice levels would be common, like video conferencing rooms, telecommunications spaces, and areas with speakerphones or voice activated computers, should be sited carefully or completely enclosed.

Glass is very reflective of sound, and windows are often located in managers' offices where confidential discussions routinely take place. Windows and walls that lack absorptive treatment will reflect sound out of the space at an angle. To preserve privacy in these offices, use full-height partitions and fixed glass vision panels, with doors in openings. If windows are present, heavy drapes can be used to eliminate reflections. Locate confidential spaces in groups, and buffer them from open office spaces with unoccupied storage areas.

Floors in open offices do not affect the overall sound absorption very much. However, cushioned floors do greatly reduce the noise of chair movements and footfalls. For this reason, all floors in open office areas should be carpeted, but pile depth makes only a minimal difference. Using a polyurethane pad rather than jute gives the same positive difference as a thicker pile.

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