Understanding Sound Masking: Enhancing Speech Privacy and Acoustic Comfort

In modern office environments and other shared spaces, managing noise levels can be challenging. Sound masking is a powerful tool for creating comfortable and productive environments by reducing noise distractions and enhancing speech privacy. Or, more simply put, sound masking is noise used to cover other sounds and make them harder to distinguish. When background sounds are harder to distinguish, they're easier to tune out.

It may seem counter intuitive to fight noise with noise, but by adding sound masking you're filling in the sound spectrum which makes speech unintelligible. When speech is unintelligible (hard to understand) it's easier to ignore, often you don't notice it at all. Similar to the hum of your computer, your brain tunes it out. But when speech is intelligible (easy to understand), it can be distracting and make it difficult for people to focus.

Sound masking is often used in corporate and health care settings in places like patient exam rooms, waiting for areas, conference rooms, and office spaces. Sound masking can improve your open office environment, but that isn't the only application. If you want to help people feel more comfortable and focused, and ensure that private conversations remain private, then you could benefit from sound masking.

What is Sound Masking?

Sound masking is the inclusion of generated sound (commonly, though inaccurately, referred to as "white noise" or "pink noise") into an environment to mask unwanted sound. Sound masking is a technique that adds unobtrusive background noise to an environment and specifically aims to cover up and mask human speech. It relies on auditory masking.

Sound masking is not a form of active noise control (noise cancellation technique); however, it can reduce or eliminate the perception of sound. Sound masking seeks to reduce the intelligibility of sound from a source by reducing the signal-to-noise ratio. Sound masking essentially reduces the intelligibility of speech, adding privacy and making conversations across the room less noticeable and bothersome. Sound masking can make buildings, offices, waiting rooms, and other places more comfortable and private.

Sound masking means controlling background sounds in a developed environment. It is significant and prioritizes modifying the background sound (in contrast to background noise); however, there is substantial evidence produced and published by Banneker (BBN) and Kavanaugh indicating that acoustical satisfaction within a space cannot be guaranteed without consideration of the three principal parameters of architectural acoustical design, formalized and established in the early 1900s by Sabine.

A sound masking system can be used to reduce the impression of intruding sound (reducing annoyance, distraction) and improve acoustic privacy (including speech privacy). Suppose you enter a restaurant with a friend, mid-conversation. As you enter, you are greeted by the background noise of other patrons’ conversations. You and your friend begin to speak louder so you can hear one another. You’ve just experienced masking, a very familiar yet fascinating phenomenon that many of us encounter every day without even noticing.

Masking is “the process by which the threshold of hearing of one sound is raised by the presence of another.'' In this restaurant experience, your friend’s voice functions-to you-as the masked sound (known in lab-based research as the test stimulus) and the background noise of the restaurant patrons functions as the masker; this background noise raises your masked detection threshold-the minimum audible intensity of the masked sound (your friend’s voice) in the presence of a masker (the background din of the restaurant) making it harder to detect.

Masking can be partial, when the masked sound is still audible (but softer), or total, when the masked sound can’t be heard at all. Additionally, there are several contexts in which masking can occur, including simultaneous masking (with two concurrent sounds), forward masking (when the stimulus occurs after the masker), backward masking (yes, when the stimulus precedes the masker!), central masking (when the masker is presented in one ear and the stimulus in the other) , energetic masking (peripheral masking, due to interferences) and informational masking (higher-level masking which is not energetic).

Simultaneous masking, as the restaurant experience above describes, is by far the most well-understood type. It is a direct consequence of competition on the auditory nerve, occurring when the masker activates the receptors of the inner ear that would have otherwise been activated by the masked sound, meaning the masked sound does not fully reach higher levels of the auditory system. The activation of these receptors strongly depends on critical bands, a very important concept in psychoacoustics.

Sound propagates into the ear canal (outer ear) before transmitting the vibration to the eardrum and ossicles of the middle ear (malleus, incus and stapes).

Critical Bands and Masking

Critical bands act as a series of band-pass filters characterizing the behavior of our basilar membrane. A band-pass filter lets through only a part of a sound’s spectrum. Critical bands thus split the spectrum of an incoming sound into many frequency bands. The width of a critical band increases as the center frequency increases, but always covers the same distance on the basilar membrane.

Simply put, critical bands determine when masking will occur, and when it won’t occur. For example, one sound (“sound A”) can mask another (“sound B”) if the distance between the two sounds’ frequencies is less than a critical band’s bandwidth. By contrast, sound A will not mask sound B if the distance between the two sounds’ frequencies is greater than a critical band’s bandwidth.

By measuring the masked detection thresholds of a tone with variable frequency, we can obtain a masked audiogram (or masking pattern), a curve often used to measure the masking effect generated by a fixed masker as a function of stimulus frequency. Masked audiograms reflect the physical oscillation pattern of the basilar membrane provoked by the masker. In the case of a pure tone masker, their asymmetrical shape is affected by the level of stimulation. At very low levels, the shape is symmetrical.

As the level increases, the slope of the pattern of the low-frequency side remains constant but the slope of the high-frequency side is progressively more shallow because the excitation pattern extends toward the high frequencies more (this is often called “upward spread of masking”). In other words, the critical bands involved by two sine waves will-depending on their energy levels-overlap more when the masked tone is above the masker, making the masking effect more intense.

Sound Masking vs. White Noise vs. Pink Noise

Most often when sound masking systems are discussed, the term “white noise” is used. But in the world of audio and acoustics, there are two common definitions of white noise:

• It can refer to a very specific type of noise that is described in mathematical terms.
• The term also is a layman’s way of referring to any constant background sound such as running water, music, an HVAC system, etc. In this regard, virtually anything can be considered “white noise” so long as it is relatively constant.

A subset of white noise, pink noise is used quite often in the audio world for tuning and testing loudspeakers and other audio equipment. While sound masking is often called white noise or pink noise, the sound masking spectrum is more sophisticated and tailored for covering up speech while producing pleasant noises for occupants. But a sound masking solution shouldn’t be confused with products that produce either white or pink noise.

In fact, at its core, a properly installed and effectively commissioned sound masking solution should produce a sound that’s neither white nor pink. Electronic sound masking systems are often called “white noise systems” or “pink noise systems” but neither term is technically correct. White noise (for audio use) is defined as “equal sound energy at each frequency over the usable audio spectrum.” Pink noise is defined as “equal sound energy per octave over the usable audio spectrum.”

A sound masking system doesn’t use white or pink noise but instead incorporates a very specific filtered noise (described by the NC-40 Contour) that has been proven scientifically to produce the maximum speech-masking effect. Properly engineered sound masking output should be thought of as a filter in front of a light; a filter that was developed for the very specific purpose of ensuring speech privacy. It considers how we as humans receive sound with our ears and use our brains to interpret those sounds as either human voices or other noises occurring within a given indoor environment.

The takeaway is that sound masking output is a very specific type of sound that acoustical engineers developed for the purpose of speech privacy. As shown on the above graph, sound masking differs from both white and pink noise.

Sound masking output falls within the speech spectrum.

What does sound masking sound like? It isn't loud static, rain forest sounds, waves crashing, or music, it's more like ambiance that fits into the background. It sounds like gentle airflow, and if deployed correctly, is barely noticeable.

Unlike white noise, sound masking is specifically engineered to match the frequencies of human speech and to sound comfortable, even pleasant, to the human ear. Sound masking specifically blends into the background and is less noticeable, unlike white noise, which includes all frequencies at equal intensity. White noise contains all frequencies at equal intensity, making it more noticeable and potentially irritating.

Sound masking covers a sound. Noise cancellation physically flattens a sound wave. An example of noise cancellation would be earplugs or headphones. The problem with sound cancellation is that there is no organization-wide solution on the market today... unless you plan on buying everyone earplugs! White noise is on a different frequency and would be incredibly unpleasant to listen to when played at the necessary level. Imagine what it would be like listening to radio static over the intercom all day. Although white noise is similar to sound masking, it is not a solution for your speech privacy needs. White noise may help you fall asleep at night, but it can't be deployed in a corporate setting.

Sound Masking vs Noise Cancelling

Benefits of Sound Masking

By reducing noise distractions and enhancing speech privacy, sound masking technology creates a more consistent and comfortable acoustic environment. Sound masking enables greater privacy and can prevent the spillover of confidential information, which is important for industries such as health care, government, law, and finance. By protecting customer and consumer information, employee information, and ensuring privacy during meetings and conversations, sound masking has become an important tool for industries that deal with sensitive information and must conform with regulations.

Sound masking can also boost productivity. The actual noise emitted by sound masking systems registers at the same frequency as typical human speech, allowing speech and many other sounds to be blocked or lessened. The actual noise emitted is also designed to be pleasant but passive, meaning that occupants will typically “forget” that they’re hearing any noise after a short while. By reducing distractions and noises, sound masking is a solution for employees who can’t focus in a noisy environment or workplace with chatty coworkers. Even the occasional distant conversation or ringing telephone can interrupt concentration. Here again, sound masking can help to reduce audible interruptions.

With the era of open offices in full swing, people are more distracted than ever. A study by Ipsos found that workers lose up to 86 minutes per day due to noise distractions, be it loud coworkers, printers, ringtones, etc. The Center for the Build Environment in San Francisco surveyed more than 25,000 worked in 2,000 buildings and found that acoustic privacy was the number one complaint among workers. These results mirror those found in dozens of other studies, time and time again people complain about acoustic privacy.

The good news is that more speech privacy alleviates these problems. When people feel like they have more privacy, they'll speak more freely and feel better while doing it. Speech privacy is also an important consideration for employee satisfaction. A workplace that lacks privacy can lead to lower employee satisfaction and negatively affect job performance.

In recent years a number of psychological studies have been undertaken in office environments to gauge the need and effect of Sound Masking. In one study it was found that there is a modest stress increase and diminished motivation caused by typical office noises, including speech. Typically an office without Sound Masking will have an ambient sound level of under 40 decibels. An Ambient level is defined by the noise which occurs when nobody is present in the room. Conversational speech levels tend to be near 65 decibels causing conversations to be understood, and distracting to others, from up to 15 metres away.

Deploying Sound Masking Systems

Sound masking systems use speakers (emitters) to distribute a carefully engineered sound throughout a space. Sound masking is typically deployed via speakers installed in or above the ceiling. The speakers connect via a cable that further connects to a control mechanism within a server closet or similar space. Sound masking can be deployed directly into spaces for a consistent ambient sound through direct speakers. Some spaces incorporate speakers above ceiling tiles that point upward and reflect sound downward, creating a more indirect sound masking effect.

Open ceilings also permit the installation of sound masking speakers. Still, the best sound masking system will depend on your ceiling architecture and height, music and paging needs, and building layout. While sound masking systems often operate uniformly, some systems enable volume control and different settings for individual speakers and rooms.

Implementing Sound Masking in Your Space Haverford’s team can install IP controllable sound masking systems that permit users to still play music and send paging notifications while making an area both more private and productive. Haverford is also qualified to conduct site surveys for installed sound masking systems. We can recommend needed upgrades or look at the logistics behind installing a new sound masking system that stands alone or integrates with existing speakers and audio needs.

Types of Sound Masking Systems

• Plenum Sound Masking Systems: Employ a network of loudspeakers located completely within the plenum. Plenum-based speakers typically range 4-10 inches (10-25 cm) in diameter and generally face upwards, towards the upper deck. This is done to reflect sound from the speakers to broaden, as much as possible, the footprint from the speaker in the work area.
• Direct Field Sound Masking Systems: Have been in use since the late 1990s. The name takes after the mechanics of sound transmission which considers the "direct sound path" from the loudspeaker emitted towards the recipients (listeners) underneath. When installed in dropped ceilings, direct field systems use speakers that are mounted facing down. When a ceiling tile is not available, they are mounted facing down on any available structure, sending the masking noise directly into the intended space.

Uniformity can be achieved by adjusting the acoustic output of individual or a small groups of speakers. Adjustments routinely include changes in the output volume and output spectra of individual speakers. Only the most sophisticated sound masking systems can control the background sound level and spectra of masking sound accurately and precisely throughout a space, made possible only with the smallest zones (spatial limits around a speaker) and sophisticated electronics and software.

Applications of Sound Masking

Sound masking systems are often relied upon as a basis of design with Sound Transmission Class (STC, as supported by ASTM E336) or Noise Isolation Class (NIC, as supported by ASTM E336) to ensure an appropriate level of privacy between contiguous rooms. Various organizations (ASTM, ASA/ANSI, GBI, LEED, ASHRAE, WELL, etc.) define unique categories for labeling acoustical zones with purpose and/or function.

Here are some common applications of sound masking:

• Open office plans - open offices can be either too quiet (where someone dropping a pen in the next cubicle is distracting) - or too noisy (where the conversations of others in the office make it impossible to concentrate).
• Private offices - private offices and other enclosed spaces often appear to provide privacy but do not. Many times, walls are lightweight and do not extend to the ceiling deck, but only to the ceiling tile. In these cases, sound can easily travel through partitions or over the walls.
• Public spaces - sound masking is useful for reception areas, pharmacies, waiting rooms, and financial institutions. Sound masking is provided in the area where conversations should not be heard - not necessarily in the area where the conversation is taking place. For instance, a psychiatrist would not want those in the waiting room to overhear a private conversation with a patient, so sound masking is provided in the waiting area, but not in the psychiatrist's office.

Several cases exist where sound masking has been successfully installed for exterior applications, the most common target of concern being roadway noise. In one example application, a large artificial waterfall was constructed as part of the garden exterior of an urban hotel in Santa Rosa, California.

Diagram of how far sound can travel with and without sound masking.

Key Considerations for Effective Sound Masking

The key to a successful sound masking system lies in the planning. Designers must account for every wall, cubicle, light fixture, building material, etc. If a sound is localized, meaning you can tell where it's coming from, then it will just be another distraction. With effective sound masking, you should be able to walk around without hearing any gaps or differences in the sound. It should be a continuous and seamless sound level wherever you go.

Avoid placing too many emitters in one area, as this can lead to an overly dampened sound environment. Failing to identify and cover key reflection points can result in suboptimal sound quality. As with any commercial-grade sound masking system, an in-plenum sound masking system requires proper layout design, commissioning, and verification of the performance. Disregarding the importance of any of these stages in implementation will result in a sound masking system that does not perform according to the specifications of an acoustician.