Calculating the benefits
For example, sound masking can be used in combination with walls (or demountable partitions) built to a suitably ceiling attenuation class- (CAC) rated suspended ceiling to provide a cost-effective and more flexible alternative to deck-to-deck construction. Budget wise, sound masking may represent one to two dollars of cost per square foot 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 can also increase the ease and cost-effectiveness of relocating them to suit future needs. However, is an equal or greater level of privacy achievable using this alternative?
The most objective method to resolve the speech privacy question is to quantify the effects of increased attenuation and sound masking on intelligibility. This exercise can be done using the Standard Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index (ASTM E1130-16) for calculating articulation index (AI), which is a metric of speech intelligibility and takes both factors into account. While this ASTM standard references open plan spaces, it is generally agreed this method can also be applied to closed spaces, with slight modification to the test equipment.
Calculation of articulation index is based on several measurements taken in the space in question, as well as a standardized normal voice level. Onsite testing determines the amount by which voice level reduces between the source room and the listener location. The difference between the voice level and the background in each of the third-octave frequency bands (200 to 5000 Hz) provides the signal-to-noise ratio in the listener location. The AI method assigns a specific weighting formula to determine an AI contribution within each frequency band, and these are summed to arrive at the AI value.
Using this method, one can quantify the impact of increasing the attenuation of the wall and masking level, allowing comparison of the two strategies. Obviously, as wall attenuation increases, for each decibel reduction there is an increase in speech privacy levels.
Mathematically, the same can be achieved by raising the background sound level by a decibel. To understand why, one need only look to the step in the above AI calculation for determining the signal-to-noise ratio. If a wall decreases the intrusion of voice into the room by a decibel, then the signal-to-noise ratio drops by a decibel. An identical drop occurs when the masking level is raised by one decibel.
Masking typically adds approximately five to 12 dBA of ambient sound to closed rooms, which is why one sometimes hears the general statement it “adds 10 STC points” to walls.
Designing, tuning, and reporting
This type of integrated acoustic design is only viable when the minimum background level is consistently delivered by the sound masking system. Therefore, it is incumbent on those responsible for acoustic planning to ensure it is properly designed and implemented.
As noted earlier, not all system architectures can provide effective and comfortable masking sound in the fragmented and individual environments presented by private offices and other closed rooms. So, what is current best practice in these areas?
First, each room should be provided with its own loudspeaker. In open plans, loudspeakers are typically located according to a standard grid at 4.5-m (15-ft) spacing. Including closed rooms in this pattern reduces localized control because one loudspeaker may span more than one room.
Second, the loudspeaker should be allocated to its own control zone. Ideally, this means it is fed by a dedicated masking sound generator and it also has dedicated volume control and third-octave equalization. Having a number of loudspeakers connected to a shared set of controls inherently limits the system’s ability to meet the specified masking curve in each closed room.
Third, each control zone must have precise output adjustments for level and equalization. Third-octave equalization over the specified range of the masking spectrum is necessary, which is typically from 100 to 5000 Hz. Precise volume control is also needed. Modern masking technologies provide fine steps (e.g. 0.5 dBA increments) for individual zones.
In terms of tuning, the masking spectrum in closed rooms should be identical to the one used in open plans. However, the overall volume level is typically several decibels lower. This provides a good degree of consistency between the open and private spaces, but addresses occupant’s expectation the ambient levels in smaller rooms are lower than in large open venues. Overall masking levels in private offices usually range from 42 to 45 dBA, but as stated above, benefits can be realized even at lower volumes.
A well-designed and professionally-tuned system is able to keep variations in masking level to ±0.5 dBA and those in frequency to ±2 dB per third octave, providing dependable coverage throughout treated areas.
The newly released ASTM E1573-18, The Measurement and Reporting of Masking Sound Levels Using A-Weighted and One-Third-Octave-Band-Sound Pressure Levels, will predominantly be used by acousticians tasked with verifying the performance of an installed and calibrated sound masking system against a specification outlining such target levels and tolerances. However, sound masking vendors should also follow this standard to ensure they provide good data for the verification process. As with ASTM E1374-18, this standard’s title has also been updated to reflect the fact sound masking is used in closed rooms as well as open plans.
Indeed, the debate over whether sound masking should be included in closed rooms should be put to rest. In almost all situations, it is better to combine a reasonable amount of isolation with a reliable ambient level, allowing organizations to save on wall construction by reducing the STC ratings of walls and/or using floor-to-ceiling rather than deck-to-deck construction. As long as the system is properly engineered for this type of environment, it is possible to provide the client with a suite of acoustic benefits that could not otherwise be achieved in private offices and closed spaces, and also prevent the noticeable voids in the background sound level, which are created when masking is only applied to open plans.