by Niklas Moeller
From their early uses in commercial offices to relatively newer applications such as patient rooms in hospitals, sound masking systems are becoming a more common component of interior design.
This technology distributes an engineered background sound throughout a facility, raising its ambient level in a controlled fashion. The principle is simple—any noises below the new engineered level are covered up, while the impact of those still above is lessened because the degree of change between baseline and peak volumes is smaller. Similarly, conversations are either entirely masked or their intelligibility is reduced, improving speech privacy and decreasing various disruptions to occupants’ concentration.
Most people have experienced this effect when washing dishes at their kitchen sink while trying to talk to someone in the next room. They can tell the other person is speaking, but it is difficult to understand exactly what is being said because the running water has raised the ambient level in their area.
When introducing a sound to a workplace, it is vital to ensure it is as comfortable and unobtrusive as possible. Otherwise, it risks becoming a source of irritation rather than a way to help solve an acoustic problem, as was the case with the original masking systems developed in the late 1960s, which used white noise generators.
White and pink noise
Though the term ‘white noise’ still tends to be used interchangeably with ‘sound masking,’ it is a very different type of sound than what is produced by modern masking technologies.
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) because it contains double the number of frequencies than the one before it. 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.
Most people describe white noise as ‘static’ with an uncomfortable, hissing quality. Those old enough to remember analog televisions compare it to the ‘snow’ broadcast when the antenna lost the transmission signal and instead picked up electromagnetic noise. Unsurprisingly, these early masking systems were typically turned down or shut off soon after they were installed.
‘Pink noise’ is another term often inaccurately substituted for ‘sound masking.’ This 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 masking systems do not emit white or pink noise, or in fact any of the other colors (e.g. brown, blue, or violet).
A sound masking spectrum
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 masking curve includes a wide range of randomly generated frequencies, it is narrower than the full audible range—typically from at least 100 to 5000 Hz, though sometimes as high as 10,000 Hz. Further, the volume of masking frequencies is not equal, and does not decrease at a constant rate as frequency increases.
It is important to understand the curve defines what the system’s measured output should be within the space. Regardless of how the system is designed, its ‘out-of-the-box’ settings, the size of its zones, or the orientation of its loudspeakers (i.e. upward- or downward-facing, sometimes called ‘direct-field’), the sound is influenced as it interacts with elements of the workplace interior, such as the layout, furnishings, and other variables. Therefore, in order for the sound to actually meet the desired masking 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.
Tuning 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. 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. This process is repeated as often as needed until they meet the curve at each tuning location.