Tuning into Sound Masking
When dealing with poor speech privacy and noise control, “too quiet” are not the words most occupants would choose to describe their workplace. However, a lack of background sound is often what lies at the heart of these issues. Many high-performance facilities exhibit library-like conditions that allow occupants to clearly hear conversations and noises, even those that are relatively low in volume or generated a good distance away from the listener.
A sound masking system addresses this issue by distributing an engineered background sound throughout the workplace, raising its ambient level in a controlled fashion. The principle behind this solution is simple: any conversations and noises below the new level are covered up, while the disruptive impact of those above it is lessened because the degree of change between baseline and peak volumes is smaller. Niklas Moeller, vice president of K.R. Moeller Associates Ltd., explains why post-installation volume and frequency adjustments are an essential part of the commissioning process for this technology.
How effective is sound masking at managing the acoustic environment?
When properly implemented, masking is a highly effective contributor to the overall acoustic performance of open plan areas, as well as to the speech privacy levels experienced in closed rooms. However, no sound masking system can achieve these goals from the moment it’s powered on. In order to provide the expected benefits, the sound has to be tuned to meet a particular spectrum—also called a masking “curve.” Tuning can be a time-consuming process, but it’s essential to ensure the sound actually performs its intended job.
What’s involved in the tuning process?
First, the sound masking system has to be designed so that it can, in fact, be tuned. Its exact layout will vary depending on the facility’s design, but there are basic tenets that need to be met. The most important is zone size. Each one should be no larger than three loudspeakers. Exceeding this limit quickly reduces the technician or acoustician’s ability to adjust the sound, necessitating compromises between masking performance and occupant comfort across large areas within the facility. Each small zone must also provide precise volume control and third-octave equalization over the entire masking spectrum, which is usually between 100 to 5,000 hertz (Hz), or as high as 10,000 Hz.
After installation, the technician or acoustician uses a sound level meter to measure the sound at ear height—in other words, the level at which occupants experience its effects—and adjusts the volume and frequency settings until they meet the curve within each small zone. This process is handled after the ceilings and furnishings are in place, and with mechanical systems operating at daytime levels. Because activity and conversation prevent accurate measurement, it should also be done prior to occupation of the facility or after hours.
Are there guidelines governing how the sound is measured?
A minimum guideline is to require the masking sound to be measured in each 1000 ft2 (90 m2) open area and each closed room, at a height between 4 to 4.7 ft (1.2 to 1.4 m) from the floor, and adjusted within that area as needs dictate. Masking volume is typically set to between 40 and 48 dBA, and the results should be consistent within a range of ±0.5 dBA or less. The curve should be defined in third-octave bands and range from 100 to 5,000 Hz (or as high as 10,000 Hz). ±2 dB variation in each frequency band is a reasonable expectation. After tuning, the vendor should provide a detailed report verifying the results and indicating areas where the sound is outside tolerance and why (e.g. unwanted sound from HVAC).
What happens if the tuning process isn’t handled correctly?
Compromises in design or tuning can reduce masking performance by as much as 50 percent in unpredictable locations across a space. For that reason, the specified curve should also be accompanied by a “tolerance” that limits the amount by which the sound is permitted to deviate from the goal across the space. Historically, this value was often set to ±2 dBA (i.e. plus or minus two A-weighted decibels), giving an overall range of 4 dBA, but such wide swings in volume can have a profound impact on speech intelligibility. Specifications that allow ±2 dBA or even ±3 dBA are still in circulation, but they’re largely a remnant of the capability of legacy technologies. Newer sound masking systems can achieve ±0.5 dBA, giving an overall range of just 1 dBA.
Can occupants tell the difference?
The challenge for the client is that it’s very difficult to subjectively assess masking performance, so it should be well-specified in advance. Occupants’ attention is only drawn to the sound in extreme cases – when it’s either so loud or poorly tuned that it’s causing discomfort or so low that it’s providing no effect whatsoever. In between, there’s a large range over which occupants aren’t likely to complain, but the system still isn’t doing what it should. Again, it’s strongly advisable to require a detailed report of the tuning results—one that clearly indicates the desired curve has been met throughout the workplace and identifies the few areas that might remain outside spec and why.