Speech Privacy
Speech privacy is achieved when speech can not be understood. This effectively means that in a meeting room, someone listening from outside would know there was a conversation but would not be able to understand what was being communicated. In an open plan area, speech privacy is less practical, however reducing the distance that a conversation can be understood is a practical goal. With sound masking the radius in which speech can be understood is greatly reduced by raising the background noise level therefore reducing the speech to noise ratio.
An open plan office can suffer from two speech problems. First, other people’s conversations can be a disturbing and irritating distraction; and secondly, confidential conversations can be almost impossible to conduct. Similar problems also exist in cellular offices, particularly where acoustic partitions, false ceilings and access floors are installed. Sound flanking paths are abundant in such situations, even when people remember to close doors. Apart from noise breakthrough via partitions, flanking over, under and around them, other problem areas include light fixtures, air conditioning systems and services trunking.
Attention to office acoustics at the building planning stage will obviously produce the best results, and it is at this stage that the acoustic specification for each area should be determined. The acoustician’s objective must be to create an environment which will allow communication within individual work areas, while minimising distractions and providing speech privacy in adjacent work areas for open plan areas and to provide speech privacy for cellular offices but still retaining a flexible partitioning system because of the rate of churn in today’s business environment.
Speech privacy is usually defined as the inability to understand conversations, although some words may still be audible. Two parameters need to be considered: speech noise levels, and ambient or background noise levels. The balance between the two will determine the degree of speech intelligibility. Whenever speech levels exceed ambient noise levels there will be some degree of communication. Where good communication is needed, for example in a telephone system or in a lecture hall, the speech to noise ratio is increased by amplifying the speech and reducing the background noise.
Communication occurs whenever the speech level exceeds the background noise level, and conversely, privacy of speech is achieved when the speech level and background levels are equal. Speech intelligibility ranges from 0% (speech privacy) to 100% (perfect communication). For privacy of speech in office situations the speech intelligibility target is usually about 5%. This allows some words to be heard, but complete sentences cannot be understood; thus people’s conversations become less distracting. The 5% level can be achieved in three ways:
The direct speech path might be blocked by a screen, which could be a purpose designed acoustic screen with a solid core and acoustically absorbent surfaces, or it could be office furniture, e.g. lateral filing etc. Workstation orientation is also important, and benefits will be attained by arranging the layout so that staff do not face each other. The larger the screen or sound barrier, the more effective it becomes in blocking the direct speech path; however, the open plan concept usually restricts the number and size of screens which are acceptable. With cellular offices the screen is already there in the form of the walls.
The reflected speech paths can be minimised in a number of ways. Soft furnishings will help significantly, but purpose designed acoustic wall panels will be much more effective. Lightweight panels can be fixed to reflective walls or columns, and their fabric facings matched to the office decor scheme, enhancing the visual aspect of the office in addition to improving its acoustic characteristics. Doors should have acoustic seals fitted to all four sides. Ceilings also play a major part in reflecting sound, and ceiling systems with higher absorption coefficients should be installed. It should be noted, however, that some perforated metal ceilings can be highly reflective acoustically at certain angles, and need careful consideration. Carpets can also make a significant contribution.
When increasing the background noise level, a system is needed which is capable of producing predictable and adjustable results, i.e. a background sound level with precisely controlled frequency spectrum and amplitude. Sound Masking systems were designed and developed specifically for this function.
The basic principle of sound masking is very simple, with sound being generated electronically and introduced into the open plan area through a number of loudspeakers. The low level background sound which is thus produced is a combination of frequencies carefully selected to mask speech and other noises, typically from office machines.
The loudspeaker network is usually installed within the false ceiling void with the speaker cones facing upwards (i.e. towards the solid structural ceiling slab above), in order to provide a diffused sound field in the office area below. Grouping the loudspeakers in zones allows individual adjustments to the speaker background sound, to suit different areas of the office.
Speaker density is very important, with too low a density of speakers leading to the directional effect of the sound source becoming apparent. If people are able to identify speaker positions, either visibly or audibly, the system will not be as effective as it can be in providing speech privacy without being obtrusive. This is why we endeavour to place the speakers out of sight in the void, rather than in the ceiling tiles facing down. The systems flexibility for speakers to go under floor, or be surface mounted always enables us to give our clients the best sound masking solution available.
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