Understanding Acoustical Privacy

Photo courtesy of Vincent Lions Photography

Typing the word “privacy” into a search engine yields a lengthy steam of entries describing the many ways in which it can be violated, including reports of hackers acquiring credit card information, various groups mining social networking sites, and voice-activated electronics with the ability to eavesdrop on their owners.

Our preoccupation with the vulnerabilities exposed by the internet and electronic products is understandable given their relatively rapid spread into almost every aspect of our lives. But we should not forget that privacy can still be violated in traditional ways—whether by deliberate eavesdropping or simply being in audible range of a conversation. While privacy legislation tends to focus on securing access to information stored on computers and within filing cabinets, attention also needs to be paid to our built environment.

Many immediately equate acoustical privacy with speech privacy, but there is more to this concept than the ability to clearly hear what another person is saying. For example, even if the conversation taking place in the room next to you is unintelligible, you may still be able to identify the speaker’s tone and determine whether they’re happy, sad, or angry. This type of information can be considered private under certain circumstances, such as when overheard outside of a human resource manager’s office.

How much we understand of a conversation also depends on whether or not we can see the speaker. This effect—known as visual cues—has been quantified by various studies. Generally speaking, if you can only understand 20 percent of someone’s conversation when you’re not looking at them, the ability to see their lips increases that amount to nearly 55 percent. If you start at 50 percent, visual cues increase it to almost 90. In other words, there’s also a visual component to acoustical privacy, which is important to bear in mind when designing a space.

A lack of acoustical privacy carries real risk, particularly in facilities where there is a perceived need for it—or an expectation on the part of its users. Examples that readily spring to mind are hospitals, banks, law offices, and military facilities. However, other types of spaces—such as commercial offices—have privacy needs as well. The degree required usually depends on the activities the space hosts.

It is easy to understand the need for acoustical privacy—or even acoustical security—from a speaker’s perspective, particularly in environments where occupants are discussing medical or financial information, but a lack of acoustical privacy can have impacts beyond divulging sensitive information to unintended parties. This fact becomes clear when we look at the conversation from the viewpoint of the (often involuntary) listeners, rather than that of the individuals talking.

When a noise or voice enters “our space,” some degree of annoyance is typical, but it can also make us feel as though our privacy is being invaded or our sense of physical separation from others violated. Perhaps the most relatable examples of this sensation are when the guest in a neighboring hotel room turns up their television’s volume or the patient at the other end of a waiting area starts speaking on their cellphone. If we can inadvertently hear a conversation, we also become self-conscious about our own level of privacy. In some contexts, it creates a sense of unease, which in turn impacts our ability to freely communicate. For instance, if we visit a medical clinic and hear what is happening in the adjacent exam room, we are less inclined to disclose information to the doctor, knowing that we too can be overheard.

The degree of acoustical privacy afforded by the built environment can even impact an organization’s brand image. We want to be in control of our personal information when meeting with a financial or legal advisor, for example, and a positive acoustical experience can reinforce our confidence in their firm. This level of protection is also indispensable for staff to effectively negotiate. In some countries, protecting verbal communication within particular types of facilities is actually mandated by law. The U.S. Health Insurance Portability and Accountability Act (HIPAA) is a good example.

Acoustical privacy is also vital to employees’ overall satisfaction with their workplace. A decade-long survey run by the Center for the Built Environment (CBE) found that lack of speech privacy is the number one complaint in offices. Participants expressed irritation at being able to overhear in-person and telephone communications, as well as concern for their own level of privacy.

The topic of workplace satisfaction also emphasizes the need to consider those occupying spaces other than closed rooms. Although some might dismiss the importance of acoustical privacy when designing open-plan space, studies show that it has a significant impact on productivity. For instance, research conducted by the Finnish Institute of Occupational health shows that unwilling listeners demonstrate a 5 to 10 percent decline in performance when undertaking tasks such as reading, writing, and other forms of creative work. Simply hearing that someone is speaking can disturb concentration, but this problem is amplified when you can clearly understand what they are saying. Essentially, if you can follow a conversation, it is much harder to “tune out.”

Although an organization might not consider privacy a goal within open plan, it is impossible to justify increasing distractions. Occupants working in an acoustically comfortable environment have an easier time concentrating on their tasks, and also suffer less stress and fatigue. An organization may decide it is more motivated by the need for a high-performance workplace than acoustical privacy, but taking the steps required to lower speech intelligibility allows them to reap both rewards. The only difference is how you see the benefit: from the perspective of the group listing rather than the person talking.

Understanding Sound Masking

Photo courtesy of KR Moeller Associates Ltd.

Sound masking systems are a common component of today’s interiors, from their original use in commercial offices to relatively newer applications such as patient rooms in hospitals and guest rooms in hotels. This technology uses loudspeakers to distribute an engineered background sound throughout a facility, raising its ambient level in a controlled fashion. The new level obscures noises that are lower in volume and reduces the disruptive impact of those that are higher by minimizing the degree of change perceived by listeners. Similarly, conversations are either entirely covered up or their intelligibility is reduced, improving speech privacy and decreasing the number of disruptions to occupants’ concentration.

Most people have experienced this type of effect—for example, 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 they are saying because the running water has raised the ambient level in their area. In fact, everyday examples are virtually endless: the drone of an airplane engine, the murmur of a crowd in a busy restaurant, or even the rustling of leaves in the wind. They all have the potential to mask sounds the listener would otherwise hear.

Of course, when introducing a sound to a workplace or a health-care facility, it is vital to ensure that it is as comfortable and unobtrusive as possible. Otherwise, it risks becoming a source of irritation and rather than helping to solve an acoustic problem, it becomes one itself—as was the case with the original masking systems developed in the late 1960s which used white noise generators.

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 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.

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 picked up electromagnetic noise instead. It is unsurprising that these early masking systems were typically turned down or off soon after they were installed.

“Pink noise” is another term often inaccurately 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 comparisons 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 colors (e.g., brown, blue, or purple).

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. The curve includes a wide range of randomly generated frequencies; however, it is narrower than the full audible range—typically from at least 100 to 5,000 Hz, and 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.

It is important to understand that the curve defines what the sound masking system’s measured output should be within the space. Regardless of how the system is designed, its out-of-the-box settings, or the orientation of its loudspeakers (i.e., upward- or downward-facing, sometimes called “direct-field”), the sound is influenced as it interacts with various interior elements within the facility, such as the layout and furnishings. If the sound is to meet the specified curve, the system’s volume and frequency settings have to be adjusted in small, localized zones. 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. Because 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 desired curve at each tuning location.

Most people compare the sound of a professionally tuned masking system to that of softly blowing air. However, there is much more significance to the tuning process than simply providing a pleasant auditory experience. One must also ensure that the sound performs its intended job.

The effectiveness of the masking sound is directly related to the sound masking system’s ability to closely match the specified curve. Some degree of variation is expected; it is impossible to achieve perfection in every tuning location. However, because variations in the masking sound can profoundly impact performance, the specification should not only provide a target curve, but also a “tolerance” that indicates by how much the sound is allowed to deviate from that curve across the space. Achieving consistency is also important for comfort; a uniform sound fades into the background more easily and occupants come to consider it a natural part of their space.

Historically, tolerance was often set to ±2 dBA (i.e., plus or minus two A-weighted decibels), giving an overall range of 4 dBA. However, such wide swings in overall volume across the space can allow occupants to understand up to 43 percent more of a conversation in some areas than they can in others. Advances in masking technology now permit tolerance to be set as low as ±0.5 dBA (i.e., a range of 1 dBA), yielding far more consistent results and providing dependable masking coverage throughout the installation.