August 28, 2020
by Viken Koukounian, PhD, P.Eng., and Niklas Moeller
Background sound is an intrinsic part of our daily experiences, yet its fundamental role within the built environment can often be overlooked. This omission is most apparent in noise mitigation projects where the building professionals exclusively rely on the absorptive materials and partition assemblies and, hence, performance indicators such as the noise reduction coefficient (NRC), sound transmission class (STC), and the noise isolation class (NIC)—none of which provide insight into occupant experience of sound in that space.
In other words, this singular focus on objective physical acoustics fails to consider the human (psychological and physiological) component—specifically, that occupants can perceive a space with a higher controlled level and spectrum of background sound as ‘quiet.’ As a result, organizations forgo opportunities to significantly improve acoustical privacy and satisfaction and realize labor and material savings engendered by a more holistic approach to design.
Revisiting the acoustical lexicon and refining use of terms such as ‘noise,’ ‘sound,’ ‘silence,’ and ‘quiet’ allows for a more nuanced discussion of occupants’ needs and expectations, and fosters opportunities to improve the design of the built environment through a proportional exchange between the measure of attenuation and the control of background sound.
The study of acoustics dates back thousands of years. Given its roots are deeply entangled with those of mathematics and physics, it is unsurprising that the typical approach to acoustic investigation is quantitative. Consideration of ‘soft’ parameters (i.e. subjective and descriptive) is relatively scarce until the last century.
Interest in evaluating human response to acoustics gained momentum in the 1900s, with the rise of architectural acoustics, also known as building or room acoustics. Notably, contributions from Bell Telephone Laboratories, Bolt Beranek & Newman Inc., and others have formed the foundation for psychoacoustics, a branch of psychology focusing on the perception of sound and its physiological effects. Research examined occupants’ assessment of the intruding noise (e.g. annoyance, distraction, inadequate acoustical privacy) in their environment.
Motivated by the need to develop an objective approach to effective architectural acoustical design, renowned acoustician William Cavanaugh et al. published Speech Privacy in Buildings (1962), asserting neither acoustical privacy nor acoustical satisfaction could be guaranteed by any single design parameter. This work was instrumental in what later became to be understood as the ‘ABCs’ of architectural acoustical design:
Although the authors of Speech Privacy in Buildings appreciated the importance of background sound, they tended to use the words ‘noise’ and ‘sound’ interchangeably—a practice deeply rooted in historical habits, and continues today.
Initially, acousticians were primarily interested in evaluating the conditions needed to clearly hear sounds. They determined the critical factor was the level of the desired sound—called the ‘signal’—relative to the background sound present in the listener’s location. In most cases, the background sound used during testing was broadband and did not contain information (i.e. noticeable patterns, such as running speech, nature sounds, traffic noise). However, it was termed ‘noise’ because it could potentially interfere with the intelligibility of the desired sound. The ratio of the desired sound to background sound was termed the signal-to-noise ratio.
In the above case, the ‘signal’ is the sound one wants to hear because it conveys useful information, while the ‘noise’ is an unwanted input challenging one’s ability to clearly hear the desired sound. As acousticians developed an understanding of background sound as a fundamental component of speech privacy, the methodology—and the terminology—remained the same. Hence, the word ‘noise’ continued to be used to describe the background—or, in this case, the masking—sound, despite the fact it was now being viewed in a positive light.
Understandably, the term ‘noise’ can cause confusion when the ‘signal’ is the unwanted sound and the ‘noise’ is actually the desired background sound. Meanwhile, the general public tends to use ‘noise’ as a non-technical descriptive word, typically when relating negative acoustical experiences—ones that are uncomfortable, annoying, disturbing, or even painful.
To highlight the difference in the way in which ‘noise’ is used, consider an individual working in an office, who complains about ‘noise’ to a colleague. There are several sources of sound within the space (exterior traffic, fan, and a radio playing music), but the source bothering the individual is a heated debate taking place in the meeting room adjacent to their workspace. In this scenario, the terms are defined in this way:
Technical use of the word ‘noise’ requires a ‘signal.’ In this scenario, noise accounts for the combination of sounds (i.e. it considers everything that is not the signal), while the signal only considers the source of the sound that is of interest.
The human factor
What turns a ‘sound’ into a ‘noise’ in the common vernacular? Humans demonstrate remarkable tolerance to sound and are only susceptible to its disruptions when they become aware of it—typically when the level of sound is too high, its qualities are unbalanced (e.g. it is too ‘hissy’ or ‘rumbly’), or it presents with temporal instability of its dynamic range (i.e. the change and/or rate of change in sound level over time).
A person’s assessment of sound generally depends on personal preferences and expectations for the occupied environment, as well as the activity in which they are engaged. For instance, consider a conversation at ‘normal level’ in two environments: a library and a busy restaurant. In the former, the nearby occupants engrossed in a task requiring concentration are likely to find the conversation too loud, annoying, and disruptive. In the latter, the level of conversation may not be sufficiently loud to allow for clear communication. Expectations are based on an understanding of the purpose of the space and the task at hand.
One’s description of sound tends to focus on two main properties: level (often referred to as volume) and spectral distribution. A ‘hissy’ or ‘screechy’ sound is one that has a lot of high frequency information (e.g. a young child screaming). A ‘bassy’ or ‘rumbly’ sound is one with a lot of low frequency information (e.g. a lion roaring). A space without a balanced sound spectrum can sound worse than one with a higher sound level, but with a balanced spectrum.
The human experience is also determined by the space’s background sound level, which is considered to be the collection of all (ambient) sounds within it. Often, a room is too ‘silent’ (i.e. its ambient level is too low) and a source of sound becomes uncomfortable (e.g. a clock ticking, cars driving by, people talking, lights humming). In these cases, the ‘signal’ is disturbing because its level is higher than the background sound. The disruptive impact of these annoying noises can be lessened by reducing the signal-to-noise ratio, which is achieved by raising the background sound level. In some cases, it is possible to raise the background sound level sufficiently to completely cover up these unwanted sounds.
The need for control
Given both the scientific and human factors, one can readily see there are advantages to controlling background sound, rather than accepting large variations in its level and spectra.
The consequences of neglecting this principal parameter of architectural acoustical design is an environment that is perceived to be ‘noisy,’ as presented in Figure 1. The alternative—to add sound to reduce the perception of a noisy environment—might seem counterintuitive, but consider Figure 2. By precisely controlling the spectrum and level of sound (in this case, to a target overall sound pressure level of 47 dBA), one can make the space sound more comfortable.
The difference between one’s experience in a space with a very low ambient level and one with a higher ambient level suggests just as there is a need to differentiate between ‘sound’ and ‘noise,’ there is value in distinguishing between a ‘silent’ space and a ‘quiet’ one. Whereas ‘silent’ infers the absence of sound, a ‘quiet’ space can be characterized by a constant ambient sound that is comfortable and not readily noticeable by its occupants. Spaces such as these are perceived to be less ‘noisy’ and more comfortable—or ‘quiet.’
Evolution of sound masking
Advanced technologies—called sound masking systems—have been developed to control the level and properties of ambient sound within commercial spaces. These systems consist of a series of loudspeakers installed in a grid-like pattern in an open ceiling or above the ceiling treatment. The loudspeakers’ output is controlled using additional equipment. The first instances of their installation followed the publication of Cavanaugh’s book. However, at the time, there were several obstacles to the adoption of sound masking as an effective acoustical design strategy.
Firstly, these early systems were considered failures due to technological limitations and a lack of understanding and application of acoustical theory, which affected both their design and commissioning (e.g. large zones, limited control over volume and frequency settings). Ultimately, they failed to deliver a consistent, comfortable sound.
Secondly, their deployment coincided with the sudden awareness and aggressive regulation of ‘noise exposure.’ Although earlier efforts were made in other countries, the most comprehensive attempt to combat noise nuisance came in the form of the United States of America’s Noise Control Act of 1972. Many other governments and organizations have since used this document as the basis for their own regulations regarding occupational health and safety, environmental noise, transportation noise, and built environments (e.g. HVAC, building services), the primary focus of which is to limit the sudden or prolonged exposure to high ‘noise’ levels that would cause hearing loss (in the current discussion of appropriate noise exposure limits for activity-based spaces [e.g. for learning, recovery, sleep]—such as those set by the World Health Organization [WHO]—the focus continues to be on sound level, with little consideration of the other factors playing into human evaluations of acoustics. There is little appreciation of psychoacoustical parameters, which would consider the existing level and spectrum of background sound in review of the intruding sound from the noise source. By way of example, lacking understanding of the existing level and spectrum of sound in space, it is impossible to conclude sleep disruption can occur as a result of intruding traffic noise at any defined value [e.g. 30 dBA]).
However, research shows the definition of ‘noise’ should also include ‘unwanted sound’ (e.g. that which interferes with one’s ability to concentrate on the task at hand or get a good night’s rest). These noises do not meet the same criteria as those damaging physical structures, but their impact is nonetheless undesirable. Background sound can have a positive and mitigating effect here and, hence, the need to make the distinction. Rather than trying to create a silent, library-like space in which there is little to no sound at all (i.e. a ‘silent’ space), the goal is to create a ‘quiet’ space—one in which there is little to no unwanted sound.
Lastly, developing the masking spectrum was an iterative process spanning several decades. With the development of methodology to assess the acoustical privacy of spaces—namely, the articulation index (AI)—discussion turned toward specification of reasonable targets for acoustical privacy and renewed interest in determining what type of background sound would work best and how it could be delivered. After all, if the intention is to improve privacy, one not only needs to control the level of background sound, but also ensure the sound has specific qualities. In the 2000s, the National Research Council Canada (NRC) refined the spectrum, based on tests measuring both comfort and effectiveness, resulting in the cost-effective open-plan environment (COPE) masking spectrum (Figure 3).
It is important to note delivery of effective ‘masking’ is not a product of the sound generating and control equipment (i.e. the electrical signal), but rather the ability of the sound masking system to adapt the generated sound that is actually delivered to the space and which is dependent on the space’s architecture—its layout, furnishings, and finishings. To achieve the desired effect, the sound produced within the space must be adjusted to a specific spectrum through a post-installation process known as tuning.
Ensuring effective performance also requires verification. ASTM E1573-18, Standard Test Method for Evaluating Masking Sound in Open Offices Using A-Weighted and One-Third Octave Band Sound Pressure Levels, offers guidance and instruction on the measurement procedure to evaluate the performance of a commissioned masking system. Measurements are performed in every 93 m2 (1000 sf) of open space and a representative number of closed rooms to review effectiveness of the tuning process against performance targets and tolerances, and to provide an indication of the spatial uniformity of the masking sound.
In the last decade or so, great advancements have been made with regards to masking technology’s ability to accurately and consistently achieve a comfortable and effective masking sound across treated spaces. When designed with small zones no larger than 21 to 63 m2 (225 to 625 sf) offering fine volume (i.e. 0.5 dBA) and frequency (i.e. 1/3 octave) control, a networked-decentralized architecture can provide consistency in the overall masking volume not exceeding ±0.5 dBA, as well as highly consistent masking spectrums, yielding much better tuning results than possible with previous architectures. Some systems can also be automatically tuned using software, which first measures the sound within a zone and then rapidly adjusts the volume and frequency settings to achieve the specified curve.
Standardizing sound masking
The importance of managing background sound levels using sound masking technology is now recognized in many standards, guidelines, and building codes, including:
However, many have yet to capitalize on the ways in which masking systems can be used as a tool in architectural design. For that reason, the strategies employed by two of these documents are worth further discussion.
Ensuring minimum background sound level
In closed rooms, speech privacy depends on the background sound at the listener’s position being higher than the residual voice level penetrating the wall (for more information, read Niklas Moeller’s “Mind the Gap: Using Sound Masking in Closed Spaces” in the October 2012 issue of Construction Canada). This point is highlighted in ASTM E2638, Standard Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room. Background noise is presumed to be due to building systems (i.e. HVAC) and is, therefore, highly variable. In the absence of continuous masking sound, the measurement—and, hence, any conclusion based on it—is valid only at the time it is done (in any case, the dBA levels produced by traditional HVAC varies and this equipment cannot generate a spectrum conducive to speech privacy).
To promote a more well-rounded design approach, AS/NZS 2107:2016 specifies criteria acknowledging several benefits of minimum background sound levels, including the ‘insurance policy’ it provides against loss of acoustic isolation and speech privacy. The document introduces guidance with regards to adequate level and spectrum for the built environment. While this standard specifically excludes setting performance guidelines for masking sound, it promotes sound masking systems as a possible solution building professionals may consider to ensure acoustical privacy and satisfaction.
Using a point-for-point exchange
An even more beneficial approach was proposed by Cavanaugh, when he said, “an increase in the background sound level has the same effect on intelligibility as an increase in the transmission loss,” (referenced from Speech Privacy in Buildings ). It is on this basis that Sound & Vibration 2.0: Design Guidelines for Health Care Facilities—the companion document to the FGI’s 2018 Guidelines for Design and Construction of Hospitals and 2018 Guidelines for Design and Construction of Outpatient Facilities—allows for a point-for-point exchange in kind between the measure of isolation—the sound transmission class (STC)—and the background level (dBA) (for more information, read Niklas Moeller’s “Placing Sound Masking on the Front Line of Acoustic Design” in the July 2017 issue of Construction Canada).
THE LOMBARD EFFECT AND MASKING SOUND
|Subconsciously raising one’s voice level in order to be more clearly heard within a noisy environment is known as the ‘Lombard effect’—or, colloquially, the ‘cocktail-party effect.’ Sometimes, people express concern it will be triggered by the increased ambient level provided by a sound masking system.
In brief, this concern is unwarranted. For reference, masking is usually set to 45 to 48 dBA in open areas and 40 to 45 dBA in closed rooms. Research shows the Lombard Effect begins when “disturbing noises exceed 45 dBA”—a value near the typical limit of masking. The literature also shows the impact of raising background noise levels to 48 dBA is negligible—prompting less than 1 dBA increase in speech levels.*
Moreover, there is a distinct difference between ‘disturbing noises’ and masking sound. The latter is designed to be as comfortable as possible. In fact, when the masking system is properly engineered, installed, and tuned, the sound is unobtrusive to occupants.
In closed offices, there is even less concern due to the lower masking levels that is typically specified for these environments. Not only would the person speaking not feel ‘triggered’ to raise their voice, but the intelligibility of speech at normal levels would be unaffected. Rather, masking would perform its intended function: bolstering speech privacy (transmission of sound out of the room) and the perception of privacy (intrusion of noise into the room).
* Read H. Lazarus’ article, “Prediction of Verbal Communication in Noise–A Review: Part 1,” from the 19th volume of Applied Acoustics (1986) and J. H. Rindel and C. L. Christensen’s article, “Dynamic sound source for simulating the Lombard effect in room acoustic modeling software,” from the proceedings of InterNoise 2012 in New York.
Historically, background sound levels were (and continue to be) non-uniform in level and spectra, and highly variable over time and throughout the built environment. As a result, partition walls were often overbuilt in an effort to reduce the transmission of sound from source to receiver. Rather than employing effective controls for background sound, designers and engineers heavily overcompensated by using additional materials to provide greater isolation and absorption. This hyper-focused approach on objective components of acoustics consistently failed to appreciate the importance of human factors—namely, that a space can be perceived as quiet.
It is in the consideration of the pseudo-subjective evaluation of acoustical privacy—estimating acoustical privacy of a space using a combination of objective metrics (i.e. measure of isolation and background sound)—that we have the opportunities to realize cost savings at the design stage of a project.
Consider the following simplified scenario, which is intended to quickly illustrate the approach:
A mere five STC points may not seem significant; however, the cost savings in terms of materials, labor, and time can be. Also, if one reduces the STC rating of the partition by five points and raises the controlled background sound levels by 10 dBA to a level of 40 dBA, acoustical privacy is more assured and the psychoacoustics of the space are improved.
A more assertive effort could pursue the reduction of the STC rating of the partitions from STC-45, which relies on 30 dBA of background sound, to an STC-35 with 40 dBA of background sound (this considers field-tested STC values reflecting composite acoustic performance of all sound transmission paths). Providing the background sound is precisely generated and consistently delivered by the masking system, this enhanced design process affords the opportunity to explore new options including:
As mentioned earlier, using masking to control background sound levels also acts as an ‘insurance policy.’ Should the space underperform—which can occur for any number of reasons—there still remains the opportunity to raise the background sound level upward to 45 dBA for enclosed spaces and 48 dBA for open areas to improve the psychoacoustical measures
of the space.
Although sound is ubiquitous—a constant and inescapable experience—its positive role within the built environment is underappreciated, leading to ongoing debate about the control (or lack thereof) of background sound levels within various types of facilities. Reviewing the technical and popular use of various words allows one to gain appreciation of how they can lead to misunderstandings of what it takes to achieve an effective acoustic environment and, more specifically, the role played by background sound set to a controlled level and spectrum. The distinction between ‘noise’ and ‘sound’ is expansive and the implications are significant in terms of subjective and objective attributes of the built environment. It is by refining the definitions of those terms, as well as that of ‘silence’ and ‘quiet,’ that appreciable opportunities can be fostered to improve the design of the built environment and promote occupant well-being.
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