In-Plenum Sound Masking

Notes on In-Plenum Sound Masking:

what is sound masking?

what is sound masking?

In-Plenum Sound Masking is installed above your drop ceiling in the “Plenum” area. This means that the sound masking reflects up towards the deck and filters through the drop-ceiling area.

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 typically 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 cause 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” or transmission loss (their degree of sound penetration directly to the space below). Some common lightweight office ceilings tiles, particularly those 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 act to “compartmentalize” the masking sound and 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 the intended 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, when used in conjunction with treated open-air returns, have been shown to provide uniformity within many target sound masked spaces. Uniformity can be achieved 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.

When In-Plenum Systems Fall Short
In comparison, competing plenum-based (indirect) sound masking systems use speakers above the ceiling to project sound upwards.
Obstructions in the plenum (above the ceiling), like beams and HVAC components, affect how and where the sound filters back into the

workspace. The result is an inconsistent and unpredictable sound field that requires tedious tuning to achieve even coverage

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