Sound masking

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Sound masking is the addition of natural or artificial sound of a different frequency (more commonly though less-accurately known as "white noise" or "pink noise") into an environment to "mask" or cover-up unwanted sound by using Auditory masking. This is in contrast to the technique of Active noise control. Sound masking reduces or eliminates awareness of pre-existing sounds in a given area and can make a work environment more comfortable, while creating speech privacy so workers can be more productive. Sound masking can also be used in the out-of-doors to restore a more natural ambient environment.

Sound masking can be explained by analogy with light. Imagine a dark room where someone is turning a flashlight on and off. The light is very obvious and distracting. Now imagine that the room lights are turned on. The flashlight is still being turned on and off, but is no longer noticeable because it has been "masked". Sound masking is a similar process of covering a distracting sound with a more soothing or less intrusive sound.

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[edit] Building interiors

Sound masking can be used anywhere to ensure speech privacy or reduce distractions. Sound masking is typically used in selected workspaces but it can also be helpful in residential environments. The most common sound masking installations are:

  • Open Office Plans - open offices can be either too quiet (where someone dropping a pen in the next cubicle is distracting) - or too noisy (where the conversations of others in the office make it impossible to concentrate). Open offices can benefit from sound masking because the added sound covers existing sounds in the area - making workers less distracted and more productive.
  • Private Offices - private offices and other enclosed spaces often appear to provide privacy but actually do not. Many times, walls are lightweight and do not extend to the ceiling deck - only to the ceiling tile. In these cases, sound can easily travel through partitions or over the walls. Sound masking can be provided in adjacent private offices, or in hallways outside of private offices, to ensure that confidential conversations remain confidential.
  • Public Spaces - Sound masking is useful for reception areas, pharmacies, waiting rooms, and financial institutions. Sound masking is provided in the area where conversations should not be heard - not necessarily in the area where the conversation is taking place. For instance, a psychiatrist does not want those in the waiting room to overhear private conversation with a patient so sound masking is provided in the waiting area: not in the psychiatrist's office.

Sound masking may also be used to hide other unwanted noise, such as the intermittent sounds from machinery. In an office this could be sound of elevators and compressors. Sound masking may render conversations unintelligible by nearby listeners and may this help compliance with HIPAA and GLBA regulations.

[edit] Exteriors

A number of cases exist where sound masking has been successfully installed for exterior applications, the most common target of concern being roadway noise. In one example application a large artificial waterfall was constructed as part of the garden exterior of an urban hotel in Santa Rosa, California. The waterfall cascades down an extensive wall approximately four meters in height and functions both for sound masking and as a physical barrier to road noise.

[edit] Sound Masking Systems

[edit] In-plenum

The plenum is the space between a “dropped” ceiling and the upper deck for the floor. In-Plenum sound masking systems, which employ a network of loud speakers located completely within the plenum, were the first such systems developed - they have been in use since the 1960’s. Plenum-based speakers range from 4” to 10” inches in diameter and generally face upwards, towards the upper deck. This is done to reflect sound from the speakers to broaden, as much as possible, the footprint from the speaker in the work area.

The actual pattern of the received acoustic energy in the workspace from speakers in the plenum is complicated by a number of factors, all of which can cause some spatial variability in the sound masking field in the workspace. First, because loud speakers actually radiate in all directions, some energy at low frequencies is radiated downwards. Thus some sound variability occurs directly below the speakers. Second, dropped ceilings have a wide range of acoustical "transparency" (allowing sound to penetrate). Some common office ceilings tiles, made of fiberglass, have a high degree of transparency, which increases sound variability below the speakers. Third, the plenum is acoustically complicated by the presence of, HVAC ducts, large beams and other structural members which cause scattering and reflections. This scattered sound can also cause spatial variability. Fourth, when less transparent acoustical tiles e.g. mineral fiber tiles, are used, a reverberant acoustic build-up occurs in the plenum that can cause significant "overflow" from a treated space, e.g. an open plan office, into spaces where sound masking may not be needed or wanted, e.g. private offices or conference rooms. Finally, when the plenum is used as a vehicle for return-air for the HVAC system, the ceiling necessarily has vents or open-air returns. If these returns are untreated, they will act as direct transmitters of the acoustic field from the plenum to the office area and create additional variability. Treating the open-air returns is straightforward, but does add cost to the installation.

Properly tuned and adjusted, plenum-based systems with multiple noise sources, have been shown to provide excellent uniformity within many target sound masked spaces. Uniformity is achieved by adjusting the acoustic output of individual or small groups of speakers. Adjustments routinely include changes in output volume and output spectra of individual speakers. To provide this adjustment capability, additional system electronics for individual speakers or for small groups of speakers are required.

[edit] Direct Field

Direct Field sound masking systems have been in use since the late 1970s. The masking acoustics is called “direct field” because the sound from any specific masking speaker travels directly to a listener without interacting with any other reflecting or transmitting feature. Initially used as an accessory for open office cubicles, direct field systems have been fully integrated into at least one open office system and have been designed to be installed both in dropped ceilings and in offices without any absorptive ceiling systems. When installed in dropped ceilings, direct field systems use speakers that are mounted facing down, When a ceiling tile is not available, they are mounted facing down on any available structure, sending the masking noise directly into the intended space. Direct field masking requires speakers that are directional, meaning that they transmit energy directly. The use of direct-field speakers, spaced appropriately for the work area (typically on a grid equal to the ceiling height), provides sound masking that is somewhat uniform.

Using direct field speakers barely eliminates the few issues of spatial uniformity and “overflow” due to plenum conditions and open air returns. Also, because the sound from direct field speakers does not have to interact with the ceiling tile, the amount of sound energy (volume) required to produce a sound masked space is reduced. Most direct-field speakers are small and use little speaker housings with small magnets, consequently only able to produce a narrow-band sound as low frequencies are not able to be created within these small structures. Subsequently, these same small direct-field speakers may create speech-privacy but without comfort.

Moreover, direct field speaker locations must be coordinated with other ceiling features such as lights, HVAC components and sprinkler systems in addition to walls and partitions. Direct field speakers (downward facing) located in acoustical ceiling systems can become a distraction of their own. Their locations are obvious, especially while walking through a treated area. This effect can reduce the acceptability by the end user. In addition, speakers must be relocated in the ceiling with each space and partition change throughout the life of the space.

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