Sound Masking

Sound Masking is a very cost effective solution to the problem of improving speech privacy, so often found in today's modern and fast moving office environment. Often installed as retrofit but best installed during office fit out, Sound Masking has gained the reputation amongst many international companies as the way to improve the office environment for employees. Sound Masking is also known as sound conditioning, or white noise systems.

In a conference hall clarity of speech is essential, and every word should be heard. In today's office the objective is almost the opposite and, with only some speech needing to be heard, privacy of speech becomes a key aim.

Why Speech Privacy Is Important

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.

Voice clarity in other people's conversation will lead to a loss of concentration, and eventually to a noticeable reduction in work performance. A survey of office environments carried out in the USA concluded that “acoustics is important to employees, but few offices have good acoustics”.

Achieving Speech Privacy

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

• * Reducing speech level so it is close to background noise levels.
• * Increasing the ambient noise level so it exceeds the speech level.
• * A combination of the two, i.e. increasing the ambient level slightly and reducing the resultant effect of speech to achieve speech and ambient level parity.

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 together in zones allows the background sound thus generated to be adjusted to suit the individual requirements of 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.