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.