Mind The Gap: Using sound-masking in closed spaces

Photo © Zahid Ghafoor. Photo courtesy K.R. Moeller Associates Ltd.
Photo © Zahid Ghafoor. Photo courtesy K.R. Moeller Associates Ltd.

By Niklas Moeller, MBA
Closed offices and meeting rooms are built with the intention of providing occupants with both visual and acoustic privacy. While the first goal can easily be achieved, the second often proves elusive because of the many ways in which sound can transfer from one space to another.

In an attempt to create sufficient speech privacy, walls with high sound transmission class (STC) ratings may be specified. However, these ratings are lab-tested and frequently overstate real-world performance by a minimum of five to 10 points. Site-tested field STC or noise isolation class (NIC) ratings are better gauges, but unfortunately only testable after the fact.

A common tactic used to improve speech privacy in a closed space is to construct full-height walls that extend from the concrete floor all the way to the deck above (i.e. deck-to-deck or slab-to-slab construction). The aim is to completely seal the room. While this approach increases effectiveness, it also raises costs and reduces flexibility. Vigilance must be maintained during design, construction, maintenance, and renovation to ensure penetrations in the wall’s structure are controlled. Even minor ones can substantially reduce acoustic performance.

These challenges raise the question as to whether there are alternate and preferable methods of achieving high levels of speech privacy in closed spaces—such as adding sound-masking technology to closed rooms with walls built only to the suspended ceiling.

Cracks in the armour
Each crack in a wall’s armour facilitates the transmission of sound to and from neighbouring spaces. For example, wall performance is very sensitive to gaps along the perimeter, such as those occurring along window mullions or the floor. If light can pass through, so can sound—often well enough to substantially reduce the wall’s impact. The wall’s sound-isolating performance is weakened by other ‘imperfections,’ such as HVAC elements that pass between closed spaces, and even back-to-back electrical switches and outlets. Interior windows may also contribute to sound transfer.

Any penetrations, including outlets and controls, can impact sound attenuation. Photo © iStockphoto/Banks Photos
Any penetrations, including outlets and controls, can impact sound attenuation.
Photo © iStockphoto/Banks Photos

In the case of full-height walls, the seal between the top of the wall and the deck must be maintained, which can be quite difficult if the surface is irregular (e.g. a corrugated steel deck). The sound isolation performance of the barrier above the ceiling may also be compromised by penetrations. Openings can exist from the first day of construction or be introduced during servicing or upgrades. Any gaps due to building structure, pipes, conduit, cables, and raceways must be carefully managed to ensure wall integrity. This level of care can be challenging to sustain throughout the space’s life.

Moreover, a closed space only offers acoustic isolation when the door is closed. Once open, the barrier provided by the wall is compromised. For example, an STC 40-rated wall with an open door that represents 10 per cent of the wall’s area reduces its effective STC to 10. The same is true for STC 45 and 50 walls. If the door is 20 per cent of the wall area—the case for a standard 0.9-m (3-ft) door in a 3 x 3-m (10 x 10-ft) wall—then the effective STC is only 7.

To avoid making the door the weak link, even when it is closed, the fenestration must at least match the wall’s STC rating. Any improper seals present will provide a convenient route for sound to escape (or enter) the room.

Cost and flexibility
Full-height walls also present financial challenges. Compared to a wall built from the floor to the suspended ceiling, the additional costs of materials and labour are obvious. However, there are other ways deck-to-deck construction can substantially add to the initial budget. Each time a wall is built above the suspended ceiling, the ceiling grid must be restarted—a time-consuming process. The separated plenum space requires separate return air ducts and may necessitate additional HVAC control zones. Return ducts must be treated to prevent sound transfer along their length from one location to another.

It is also more difficult and costly to renovate, because moving such a wall requires changes to the ceiling grid, tiles, and HVAC returns.

Constructing and moving floor-to-ceiling walls is a much simpler and less costly exercise. Modular wall systems permit even more rapid relocation. However, both open up a further pathway for sound transmission. Typically, the acoustical tile has a lower attenuation rating than the wall. Sounds pass through it, reflecting from the deck above and down into the neighbouring space. In this case, a tile with a ceiling attenuation class (CAC) of 35 to 40 is recommended. It lessens the flanking of sound through the ceiling and plenum, but the room is still subject to the aforementioned acoustic leakages.

Sealing a wall to the deck is challenging given both obstructions and the prospect of an uneven deck surface. Photo © iStockphoto/Pgiam
Sealing a wall to the deck is challenging given both obstructions and the prospect of an uneven deck surface.
Photo © iStockphoto/Pgiam

The speech privacy equation
Whether built from floor to ceiling or deck to deck, walls only address part of the speech privacy equation. A person’s ability to clearly understand a conversation depends on two variables: the volume of the speaker’s voice and the volume of the background sound level in the space. The relationship between the two is called the signal-to-noise ratio (SNR).

Traditional closed-room construction attempts to provide privacy by simply reducing the signal. Even if a deck-to-deck wall is well-designed and constructed (i.e. all penetrations are addressed), it still may not provide the sought-after speech privacy level. If the background sound level in the adjoining space is lower than the sound passing through the wall (as is often the case), noises and conversations will still be heard and be potentially intelligible.

Sound-masking technology is available to provide an effective background level throughout the space. This type of system consists of a series of loudspeakers that distribute an engineered sound. Though most often compared to softly blowing air, this sound is designed to mask the frequencies in human speech. It also covers up incidental noises that would otherwise impact comfort and concentration.

Calculating the benefits
Sound-masking technology can be used in combination with walls built to the suspended ceiling or demountable partitions to provide a cost-effective and more flexible alternative to deck-to-deck construction.

Budget-wise, sound-masking may represent $11 to $22/m2 ($1 to 2/sf) of space, but it offsets much more than that in terms of construction above the ceiling. The ability to provide private rooms with walls to the ceiling also increases the ease and cost-effectiveness of relocating them to suit future needs. However, is an equal or greater privacy level achievable using this alternative?

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