There has been significant work done on the acoustics of landscaped offices, especially in Europe in the mid to late sixties (Zeller 1964, Gottschalk 1968 and Boyce 1974). This work shows that other people's conversations are the most bothersome noise source, followed by office machinery, telephones and air-conditioning. Of interest, many workers surveyed reported that it was the ability to discern the content of these conversations rather than the loudness that was disruptive.

A couple of other interesting points have come out of this work. In general, executive and managerial groups were more disturbed by noise than clerks and secretarial staff. The provision of space per person is also an important factor as overcrowding produces tension and therefore a heightened awareness of the environment. Past work (Boyce 1974 and Croome 1977) suggests 8.5 - 14 m² per person as a satisfactory range, with around 10 m² being preferable. However, below 12.6 m² there will often be a small number of people who still feel overcrowded. The influence of occupation density on sound levels is quite noticeable as there is a tendency for people to raise their voices when the room they are in is more densely occupied, particularly when talking on the telephone.

Design Aims in Open Offices

When we come to design an open office, what we ideally require is perfect communication out to a specific distance and zero intelligibility beyond that. Landscaped offices are distinguished by a high width-to-height ratio so the walls play a negligible role in sound distribution. Floors are usually carpeted and, together with furniture and people, the sound absorption is relatively fixed. The only real variable within the office is the ceiling.

Before considering ceiling design, there are some other points to note. The effects of multiple sources and lack of diffuse reflections within the office produces a non-uniform, non-diffuse sound field, thus rendering the use of classical acoustical equations invalid. This also means that spatial attenuation per doubling of distance can be shown to vary significantly from the inverse square law.

As a general pattern, three distinct regions of attenuation can be seen. Near the source equates to about 6 dB; this decreases to 3 dB at about 0.5h to 3h from the source; attenuation beyond 3h really depends upon the amount of absorption present but can often exceed 6 dB. Here, h refers to the ceiling height.

Ceiling Design

It is obvious from the above discussion that the role of the ceiling is crucial. Basically it should by highly reflective for reflection angles near normal incidence and highly absorbent for angles near grazing.

Separated Spaces

Baranek (1971) gives a method of calculating privacy between offices separated by full partitions based on what he terms Sound Excess. For satisfaction, the sound excess should be less than or equal to zero.

Excess = (Source Level + Room Effect)
       - (Screen Effect)
       - (Noise Level + Privacy Factor)

       = (68dB + F) - (SRI + K) - (NR + P)

Where:
68dB = the level of a reasonable load conversation,
F = the Room effect factor of an office space (given in Diagram 3),
SRI = the sound reduction index (in dB) of the intervening panel,
K = the panel area factor (given in Diagram 3),
NR = the noise rating for both spaces (in dB) and
P = the privacy factor.

For example, consider two very quiet (NR28) furnished managerial offices each having a floor area of 18 m2 . If they are separated by a partition having an SRI of 35 dB and an area of 18 m² can a confidential privacy factor (P = 6) be achieved ?

Excess = (68 + 5(diag. 2)) - (35 + 0(diag. 2)) - (28 + 6)
       = 73 - 35 - 34
       = 4

Therefore a confidential conversation will probably be audible in the adjoining office. Obviously a couple of options exist. The first of these is to increase the SRI of the panel. Another option may be to increase the level of background noise through the use of piped music or an air-conditioning unit. Many commercial air-conditioning units have been designed not to cause disturbance in an office environment. In many office settings it is actually desirable to introduce air-conditioning noise as it assists in the maintenance of acoustic privacy. The tailing off at both low and high frequencies in this noise prevents excessive hiss and rumble.