This article throws light upon the top four ways to reduce noise at office. The ways are: 1. Partitions 2. Screens 3. Plant Installations 4. Lift Installations.
Way # 1. Partitions:
The reduction of sound by any practical barrier is not constant at all significant frequencies in the audio-frequency range; but if the partition is non- permeable to air, the attenuation increases with the rise in frequency at a rate ranging from 3 dB (A) to 12 DB (A) for each octave increase in frequency.
Cavity constructions generally have the more rapidly rising sound-reduction frequency characteristics compared to the partitions of homogeneous construction.
The light weight demountable partitions constructed from chipboard, plywood, or plaster panels on a timber or alloy frame, often exhibit a phenomenon known as “Coincidence”. This is characterised by the presence of a region in which the reduction of sound may remain constant, or even decrease, within a certain range of frequency.
This effect is less prominent in the case of paneled partitions of cavity construction, since the attenuation provided by the cavity and the sound-absorbent material serves to minimise the attenuation dip so prominent in the partitions of homogeneous construction.
Ordinarily, it is not economic to try to obtain noise reduction much higher than 40-45 dB (A) from the demountable types of office partition. Other measures should be considered if higher values of sound reduction are required.
An office for the secretary, a store, filing room or waiting room, for example, may be interposed between the source of noise and the sensitive area. If this is done, an attenuation of 50-55 dB (A) may be obtained, provided a lower limit is not set by the mechanical ventilation system.
For sound reduction between adjoining rooms one should aim for a minimum attenuation of 45 db. (A), especially between the “noise-producing” and “quiet” rooms. This minimum reduction can normally be achieved by a 10-12 cm thick plastered brick wall.
Way # 2. Screens:
The primary function of screens, which are a significant feature of most open- plan offices, is to break up sightlines and thereby provide visual privacy. This can sometimes be provided by the furnishings such as filing cabinets, bookshelves or even large indoor plants.
More commonly, however, purpose-built, free-standing screens are provided in the offices. Such screens, like other rigid, solid objects in the furnishing scheme, can also provide acoustic screening.
Much dependence has been placed in the past on such screens as remedial measure for a lack of acoustic privacy; but the results have often been disappointing. In order to be effective acoustically, a screen should be located as close as possible to the source or to the recipient of sound.
A screen placed within 50 cm of a noisy typewriter, for example, could give up to 15 dB (A) reduction of noise for an office worker situated on the other side of the screen.
Screens erected in open offices to provide visual privacy seldom ensure acoustic privacy. Such screens are normally used for the demarcation of territorial boundaries within the open-plan offices. When this is the case, the screens tend to be placed mid-way between the worker and an identified source of noise.
If, for example, the noisy typewriter and the office worker referred to in the example cited earlier were separated by 8 m and a single, free-standing screen was placed midway on the right line, the reduction in sound level would be about 2 dB (A) – a reduction barely detectable by the human ear.
In order to be acoustically effective, screens should only be used in acoustically treated rooms. In untreated offices, transmission of sound via reflections at room surfaces will flank the screens. The screens should, moreover, always exceed a minimum size. Any screen less than 1.25 m high is of little use for noise reduction. In general, the larger the screen, the better it is acoustically.
Wherever possible, the screens should be linked together to form a continuous barrier. In this way, the sources of sound (voices, telephones, typewriters, etc.) are brought into close proximity with a screening surface and, similarly, any potential recipient of noise is also screened by ensuring that his or her head position is located close to the continuous vertical surface provided by the screen.
Further improvements can be made, if necessary, in the screening of known sources of noise (typewriters, telex machines and other office machinery). Small supplementary side screens can be added on to the work stations, for this purpose, or a three-sided carrel formed up from adjacent panel units.
The screens should be faced with sound-absorbing materials in order to ensure that sound is not reflected away from the work station. Wherever necessary, semi-enclosed desks may be arranged to ensure that open offices in relatively small rooms, or with a fairly small number of workers, are not too noisy.
Way # 3. Plant Installations:
Trouble often arises due to the direct transmission of noise from the Plant Room (containing the ventilation or air conditioning plant) into an adjacent area. Difficulties are multiplied when the Plant Room occupies the top floor of the office building, although the problem is not entirely absent even when the plant is located in the basement area.
Attention at the planning stage is again the best and cheapest solution for noise problems arising from plant installations. If other advantages make a roof boiler and Plant Room advisable, for example, one should plan the floor below for an acoustically insensitive usage (mechanical service offices, store rooms, filing rooms, etc.).
If possible, the Director’s Office, the Board Room or important Conference Rooms should be kept two floors below the Plant Room.
Plant Rooms usually have noise levels in the 80-95 dB(A) region. In Board Room areas, on the other hand, noise levels around 30-35 dB (A) are desirable; but the noise reduction of about 60 dB (A) to achieve this is almost impossible to obtain from a single floor or wall of any thickness, because of flanking transmission down the supporting walls.
It may, therefore, be advisable to install a heavy suspended ceiling in any acoustically sensitive room which is directly below a Plant Room. Lighter ceilings (those weighing less than 50-100 kg/m*) are not very effective for this purpose.
Moreover, the cavity above the ceiling should be as wide as possible. Cavities only a few cm wide are of little value; a space of at least 50 cm being the minimum required to produce a worthwhile improvement.
The choice of a quiter plant is an obvious precaution in this respect; but the average supplier rarely has the basic noise data on his products. Even when such data are available, they are often useless because of the lack of information on the conditions under which the figures were obtained.
The level of air-borne noise in the Plant Room and, therefore, in adjacent areas can be minimised by the addition of sound-absorbent treatment to the internal surfaces. A reduction of about 8 dB (A) and under favourable conditions even 10 dB (A), may be obtained by such treatment of the ceiling and part of the walls.
Plant Rooms have to be heavy in order to support the weight of the mechanical equipment, a 30 cm floor being not uncommon. This thickness of the base is quite adequate for mounting machines; but it rarely affords an adequate degree of isolation of machine vibration without the use of additional precautions.
Rotating or reciprocating plant should be isolated from the floor by anti-vibration mounts designed to deal with the vibration frequencies present in the equipment
Way # 4. Lift Installations:
As in the case of plant installations, attention at the preliminary planning stage is the least expensive precaution against the problem of noise arising from lift installations. In the first place, the lift shaft should be isolated from sensitive rooms on all four sides by corridors, stairs, lobbies, stores, toilets or other similar areas.
Secondly, the lift motor room and the motor-generator rooms should be out on the roof at least one or two floors above any sensitive rooms.
It is almost impossible to prevent manually-operated lift or floor gates from being slammed with totally unnecessary force. The resultant noise can easily travel two rooms along in every direction, producing complaints in all these areas. Fortunately, however, manually operated lift or floor gates are rapidly becoming obsolescent.
Trouble with an existing lift installation requires individual attention. The cost of remedial measures in such cases is quite large. In a particularly difficult case, it may involve having the lift out of action for several days to allow the headgear to be isolated from the building structure.
Complete elimination of lift noise may, however, be quite impossible when the building is occupied. That is why it is so important to deal with the problem of lift noise at the planning stage.