by Jennifer Wilson | June 2, 2015 2:59 pm
By Niklas Moeller
Many people believe white noise, pink noise, and sound masking are synonymous, and tend to use these terms interchangeably. However, there are distinct differences between each of them.
White noise is a random ‘broadband’ sound—meaning it includes a wide range of frequencies—that typically spans the audible range of 20 to 20,000 hertz (Hz). Graphical representations of this type of noise vary depending on the horizontal axis. If it shows individual frequencies, volume is constant; however, if the scale is in octaves, each octave’s volume increases by three decibels (dB). This is because each octave contains double the number of frequencies than the one before it, and as a general rule, the combined volume of any two sounds of equal volume is three dB higher. Thus, a graph depicting white noise shows either flat or increasing volume.
Of course, how we experience this type of sound is more important than how it is visually represented. Most people describe white noise as ‘static’ with an uncomfortable, hissing quality. Those of us old enough to remember analog televisions compare it to the ‘snow’ broadcast when the antenna lost the transmission signal and picked up electromagnetic noise instead.
Pink noise is another term often substituted for sound masking. It is also a random ‘broadband’ sound, but instead of being equal in volume at each frequency, volume decreases at a rate of three dB per octave as frequency increases. However, because these decreases are offset by the increases created by the doubling of frequencies in each octave, pink noise is constant in volume per octave. Subjectively speaking, this sound is less hissy than white noise. On the other hand, the relatively louder low frequencies give it a rumbling quality, prompting comparison to the sound of a waterfall.
Given these descriptions, it is understandable why modern sound masking systems do not emit white or pink noise, or in fact any of the other colours (e.g. brown, blue or purple). When introducing a sound to a workplace in order to cover conversations and noise—in other words, to improve occupants’ concentration, productivity, and overall workplace satisfaction—it is also vital to ensure it is as comfortable and unobtrusive as possible.
A sound masking spectrum—often called a curve—is engineered to balance effective acoustic control and comfort. It is usually provided by an acoustician or an independent party such as the National Research Council (NRC), rather than by the masking vendor. Though a sound masking spectrum also includes a wide range of randomly generated frequencies, it is narrower than the full audible range—typically 100 to 5,000 Hz, but sometimes as high as 10,000 Hz. Further, the volume of masking frequencies is not equal, nor do they decrease at a constant rate as frequency increases. Most people compare the sound to softly blowing air.
It is important to understand the specified curve defines what the sound masking system’s measured output should be within the space into which the sound is broadcast. Regardless of the system’s ‘out of the box’ settings, how small its zones are, or the orientation of the loudspeakers (i.e. upward- or downward-facing, sometimes called ‘direct-field’), the sound changes as it interacts with elements of the workplace interior, such as the layout, furnishings, and other variables. In order for the sound to actually meet the desired curve, the system’s volume and frequency settings have to be adjusted. In other words, it must be tuned for the particular environment in which it is installed.
This process is handled by a qualified technician after the ceilings and all furnishings are in place, and with mechanical systems operating at normal daytime levels. Since conversations and activities can prevent accurate measurement, it is done prior to occupation or after hours. Basically, the technician uses a sound level meter to measure the masking sound at ear height. They analyze the results and adjust the system’s volume and equalizer controls accordingly. They repeat this process as often as needed until they meet the curve, within the specified tolerances, at each tuning location.
The purpose of tuning is to ensure the system’s benefits are consistently experienced by all occupants across the facility. Deviations from the curve have a profound impact on masking performance. For instance, even if the curve’s shape is maintained, each decibel drop in overall volume results in a 10 per cent drop in speech privacy. Consistency is also important for comfort—the sound fades into the background and occupants come to consider it a natural part of their space.
So, why is white noise often associated with sound masking? It is because the original masking systems developed in the late 1960s and 70s actually used white noise generators. The problem is while white noise is an effective masker, it is also irritating. Due to the poor quality of the sound, these systems were usually turned down or off soon after installation.
In that sense, when people talk about white (or pink) noise systems, they are inadvertently referring to older masking technologies. These systems typically featured centralized architecture, meaning their electronics were housed in a central location within the facility and the sound output from that equipment was broadcast over zones consisting of dozens or even hundreds of loudspeakers. They offered little to no local control over the masking volume and equalization, so these systems were unable to consistently deliver the specified curve across a facility. Centralized designs typically allow up to four A-weighted decibels (dBA) of variation across a space, causing as much as a 43 per cent drop in privacy.
Modern sound masking systems are capable of producing far more consistent—and, hence, reliable—performance, with overall volume variations of just ±0.5 dBA. They use small zones of one to three loudspeakers and offer precise control over volume and individual one-third octave frequencies within each. Local adjustment of each zone is facilitated by networked control over settings. Clients can be assured of their sound masking system’s performance by asking the acoustician or vendor to provide detailed reporting of tuning results.
To be clear, most people do not think of the technical implications when they say ‘white noise’ or ‘pink noise’ and really mean ‘sound masking.’ However, it is important to differentiate between these terms because the negative associations with white noise persist, despite sound masking technology coming such a long way since those very early days.
For more on sound masking, read this author’s article on acoustic privacy in the June 2015 issue of Construction Canada.
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network sound masking system. He also writes an acoustics blog at soundmaskingblog.com.
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