After reading this article you will learn about:- 1. Introduction to Office Noise 2. Sources of Office Noise 3. Background Noise 4. Open-Plan Offices 5. Reverberation Noise Time 6. Articulation Index 7. Noise Control.
- Introduction to Office Noise
- Sources of Office Noise
- Background Noise at Office
- Noise Level at Open-Plan Offices
- Reverberation Noise Time
- Articulation Index of Office Noise
- Office Noise Control
1. Introduction to Office Noise:
The modern commercial building or office block is usually designed to provide maximum accommodation on minimum land area. In order to keep the loads on the foundations within economic limits, the use of light weight structures is favoured for commercial buildings.
Such materials, however, provide relatively lower attenuation and damping for noise and vibration, and this places a heavy premium on careful planning. This is especially true in the case of open-plan offices.
The open-plan offices offer many advantages, for example:
(a) The cost per work station is acceptably low;
(b) They can reduce many administrative and management problems; and
(c) When correctly designed, they provide each worker with optimized heating, lighting and working facilities.
Open-plan offices often fail, however, because of unsatisfactory acoustics resulting in the problem of noise. In order to avoid this problem, sufficient attention has to be paid, during the design stages, to certain acoustic requirements.
The basic requirements for satisfactory work in an office are;
(i) Good communication conditions;
(ii) Privacy outside the group work area;
(iii) Comfortable and non-intrusive background noise levels; and
(iv) Occupational noise levels free from irritating sounds.
More specifically, there should be a steady background noise in the office, and it should be pleasant and easy on the ear. This background noise should be capable of masking distant conversations so that they are relatively unintelligible. The long-distance propagation of speech and noise should be controlled by the room acoustics or the room furnishings.
2. Sources of Office Noise:
The main sources of noise within an office building may be categorized as follows:
(a) Ambient background noise;
(b) Noise from external sources; and
(c) Occupational noise.
Ambient background noise is the irreducible noise level in an unoccupied office normally associated with the air-conditioning system. This noise is usually constant in its level and frequency content.
Noise from external sources includes the traffic noise outside the building and the occupation noise in adjacent rooms. The proportion of such external noise which enters an office is moderately variable in the level and spectral content.
Occupational noise, on the other hand, is the sound generated in the office itself during the normal working day. A single typewriter For example, has a typical noise level of 73 dB(A) for a manual machine and.66 dB(A) for a manual machine and 66 dB (A) for an electric machine.
Usually, it is a steady “hubbub” arising from conversations, office machinery, personal movements, etc. It can also include some undesirable components such as bangs and thumps from accidental impacts, loud talking, or the monotonous whine of a fan or cooler Occupational noise in the office can be highly variable in its level, character and duration.
The sources of office occupational noise include:
(a) Conversations in adjacent offices;
(c) Office machines;
(d) Telephone conversations;
(e) Footsteps, voices, slamming of doors in corridors, etc.; and
(f) Heating and ventilating plant.
The effects of noise generated within the building can be minimised by subdividing the floor area into individual offices. If this plan is adopted, however, it incurs a penalty in weight, and this has to be considered closely.
Traditional brick walls automatically provide adequate acoustic isolation between adjacent offices and, moreover, they usually have a large safety factor Where the use of brick walls is not possible, it becomes necessary to make a more sophisticated approach to the noise problem.
In order to deal with the Problem of noise at the planning stage, the following information should be collected and utilised in the design:
(i) What noise levels are acceptable;
(ii) What noise levels are generated by people and equipment;
(iii) What factors affect the efficiency of a barrier in impeding the passage of noise; and,
(iv) How the sound reduction of light-weight barriers can be maximised at minimum cost.
3. Background Noise at Office:
The acceptability of the background noise level is essentially a subjective response. It is closely related, however, to the working environment. In a busy office (including typewriters), for example, a relatively high level of noise would be regarded not only as acceptable, but also a normal working environment.
On the other hand, a high noise level would be regarded as intrusive if it interferes with a particular working requirement, e.g., making use of a telephone. In fact, the overall background noise level may be higher than desirable, but the one to which the individual occupants become habituated.
It is desirable, in this connection, to set the maximum recommended background noise levels, since conditioning to unduly high levels may lead to loss of working efficiency in the office. Such recommended maximum noise levels are, therefore, set with regard to maintaining good working efficiency. At the same time, however, these must be realistic with respect to what can be achieved economically.
The background noise levels which are “acceptable” or “typical” in various types of offices are shown in Table 1. The degree of background noise normally present is important in determining the attenuation of sound required from dividing walls, ceilings and floors. An intruding noise will not be noticeable (or objectionable) until it equals, or exceeds, the background noise.
This, however, is modified in practice by the presence of opening windows. On noisy sites, the windows only offer the unfortunate choice between ventilation and noise exclusion. Thoughtful planning can provide the answer in such cases.
If the problem is foreseen at the stage of planning, sealed noise-excluding windows and air conditioning may be provided in offices on the sides of building exposed to high levels of ambient noise.
In a semi-reverberant office, where sound levels only fall off at 3 dB (A) per doubling of distance, speech sound levels originating at one work station are partially attenuated with distance; but they are still effectively higher than the background noise level, and hence no acoustic privacy is obtained.
In a “dead” environment, on the other hand, where noise levels fall off by 5 dB (A) per doubling of distance, the speech levels are masked by the pre-existing background noise levels, thereby ensuring a degree of privacy for the speaker.
The background noise level in an office has to be specified with great care. If it is too low, the masking of speech will not be sufficient. On the other hand, if the background noise level is too high, it will give rise to complaints from the office workers. Not only must the level of the masking background noise be correctly determined, but also the spectrum of it must be carefully defined.
For example, the hump (i.e., a maximum) at the middle frequencies in the spectrum of background noise helps to mask the information-carrying components of human speech. The noise spectrum must also be acceptable subjectively, without identifiable “rumble” or “hiss” components.
It has been shown that most people in open offices are not aware of a broad-band, steady-state sound at levels below 35 dB (A), and will accept noise levels up to about 45 dB (A).
Constant-level noise above this value is generally unacceptable in small offices. A constant-level, speech-masking sound may be generated by an air conditioning or ventilating system. It is, however, extremely difficult to ensure the correct shape of its frequency spectrum and even the distribution of the regenerated sound throughout the offices.
4. Noise Level at Open-Plan Offices:
Much research has been carried out on the noise climate in open-plan offices. It has been shown by such research that in open-plan offices, the occupational noise level should ideally vary by about 15-18 dB (A) around a mean value in the range of 50-55 dB(A). In order to ensure this level of occupational noise, it is normally necessary to have 70-100 persons present in the office.
The background noise levels in open-plan offices are essentially constant. They usually vary only between a general lower limit of 50 dB (A) and an upper limit of 70 dB (A). This level of background noise has been judged to be entirely satisfactory by the occupants of open-plan offices.
Although the peak values of background noise in smaller offices (with occupancy of less than 30 persons) are essentially the same (around 70 dB (A)), and so are the average values, lulls in activity in smaller offices may allow the background noise level to fall to 45 dB (A) for short periods, leading to a total variation of 25 dB (A) around the mean value. This variation is known as the dynamic range of the background noise level.
This background noise level in smaller offices, with the average value and dynamic range has been judged to be only partially satisfactory. This is due to the fact that there are periods when the masking of speech is insufficient and, moreover, the dynamic range of 25 dB (A) is subjectively too high.
This underlines the fact that people are likely to complain not only about unduly high noise levels, but also of unduly high fluctuations in the noise level. This situation may be corrected to some extent by using a partial speech masking system, which introduces a barely perceptible, constant-level background noise.
This reduces the dynamic range of the noise and provides supplementary speech masking during lulls in office activities.
5. Reverberation Noise Time:
Offices may generally be classified as “quiet” or “noise-producing”, as shown in Table 2. A quiet room is basically the one requiring a low level of background noise for normal work to be carried out without distraction, annoyance or interference. Such a room, however, may also be a noise-producing room if, for example, it has one or more typewriters in use.
A noise-producing room may require a treatment to reduce the levels of reflected sound within the room (i.e., a sound- absorption treatment) in order to maintain a satisfactory background noise level. Thus the reverberation time is also a significant parameter for an office.
The recommended reverberation times for various types of offices and halls of commercial establishments are shown in Table 3. The reverberation times quoted in this table are for empty rooms.
6. Articulation Index of Office Noise:
In addition to the average value of background noise level in an office and its dynamic range, a further consideration may affect the structural requirements of the office. This is the degree of privacy afforded by executive offices and the like, or the intrusion of conversation from one office into the adjoining one.
This, subjectively is the difference in the levels between intruding speech and the background noise level, since this governs the degree of intelligibility of the intruding sound. The intruding speech only becomes objectionable when it is intelligible, and the level at which this happens depends primarily on the level of background noise.
This concept is closely related to the “Articulation Index”, which is derived by dividing the frequency range of intruding speech into bands and comparing the differences between the levels of intruding speech and the background noise in each band These are then multiplied by weighting factors (to give the bands equal importance with respect to intelligibility) and then divided by 600.
The resulting Articulation Index (AI) will he between 0.0 and 1.0; the lower the value of AI, the lower the speech intelligibility. An AI not greater than 0.03 will generally provide adequate privacy to executive offices and conference rooms. Speech intelligibility generally remains low up to AI values of about 0.1. Above about 0.5, speech intelligibility is generally high. .
The Articulation Index can, therefore, be a significant parameter in the design of office structures. The design requirements of sound insulation in partitions, doors and ceilings are largely governed by this index.
The effect of AI is, however modified by the background noise. In the presence of background noise, a high value of AI may afford satisfactory privacy. The Articulation Index is also affected by other factors such as the nature of the sound field, noise levels, the distance between the speaker and listener and the attenuation produced by screens.
7. Office Noise Control:
In a room having reflecting surfaces on the floor, ceiling and walls, a “reverberant” sound field is created, which will have a more or less constant sound level at all points, irrespective of their distance from the source. The offices with plastered walls and ceilings and uncarpeted floors, therefore, sound noisy and acoustically unsatisfactory.
A “semi-reverberant” sound field is created if the acoustically hard surfaces are replaced by absorbents. Such a field gives an improved rate of fall-off in the noise level with distance. It is obviously desirable in open-plan offices that the theoretical maximum rate of noise fall-off with distance (6 dB (A) per distance doubling) is approached, and a target value of at least 5 dB (A) per distance doubling should be required.
This target can only be achieved if all the available surfaces are treated with highly efficient sound absorbents.
The sound absorbing properties of the floor covering should approach the maximum feasible performance. A good-quality carpet, laid over a felt or foam underlay, should ensure that a target absorption coefficient value of 0.50 is obtained over the frequency range 500 to 2,000 Hz.
The use of a carpet also helps to control the surface noise (the noise of footfalls and furniture movements), and the transmission of impact sound to the rooms below.
The ceiling ordinarily constitutes the principal path of sound reflection in a room. The ceiling in the office should, therefore, be surfaced with a highly efficient sound-absorbing material, especially if it is a plain, unbroken surface.
Further improvement in ceiling performance can be achieved if it is broken up by baffles. The ceiling should have an average sound absorption coefficient of at least 0.80 over the frequency range 500 to 2,000 Hz.
In most of the large open-plan offices, the wall area is small in comparison with the floor and ceiling area and, consequently, does not have a measurable effect on the overall sound field in the office. Wherever possible, however, the office walls should be treated with acoustically absorbing materials.
This is particularly required if a noise source, e.g., apiece of office machinery, is located close to a wall; otherwise there would be a tendency for the acoustically “hard” surface of the wall to reinforce the reflected sound. Reflections of sound off windows and glazed walls, on the other hand, can be controlled by the use of drapes and blinds.
In small offices, where the walls are less than 15 m apart and, therefore, comprise a larger proportion of the total surface area of the room, it becomes essential to apply some kind of sound-absorbent surface treatment.
Such a treatment may take the form of:
(a) Sprayed acoustic plasters;
(b) Acoustic tiles or battens; or
(c) Slabs of Rockwool or glassfibre.
These may be faced by slotted wood or plastic covers. Under no circumstances should two acoustically reflective surfaces face each other; otherwise an objectionable “flutter” echo will occur between them.
In the case of individual offices, the treatment for noise reduction involves specific attention to sound insulation of boundary surfaces, the elimination of flanking transmissions and structure-borne sound, and attenuation of the sound generated by the ventilating system.
The typical average noise level in an office will tend to increase with the number of people working in the office. Noise control at the source may be desirable, or even necessary, in an office. Various possibilities for controlling the noise at source in an office are mentioned in Table 4.