Rethinking acoustics: Understanding silence and quiet in the built environment

Figure 3: The graph illustrates the National Research Council Canada’s (NRC’s) cost-effective open-plan environment (COPE) masking spectrum. The slope is approximated by 4.90 dB/oct. between 125 and 5000 Hz. While the low frequencies do not contribute as greatly to speech privacy as the frequencies in the middle to high range, they are necessary for comfort. Without those, occupants would perceive the sound as ‘hissy.’
Figure 3: The graph illustrates the National Research Council Canada’s (NRC’s) cost-effective open-plan environment (COPE) masking spectrum. The slope is approximated by 4.90 dB/oct. between 125 and 5000 Hz. While the low frequencies do not contribute as greatly to speech privacy as the frequencies in the middle to high range, they are necessary for comfort. Without those, occupants would perceive the sound as ‘hissy.’

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:

  • Standards Australia/New Zealand (AS/NZS) 2107:2016, Acoustics, Recommended design sound levels and reverberation times for building interiors;
  • Canadian Standards Association (CSA) Z412-17, Office Ergonomics, an application standard for workplace ergonomics;
  • Facilities Guidelines Institute (FGI) documents such as Sound & Vibration (2010), FGI Guidelines for Design and Construction of Hospitals (2018), and FGI Guidelines for Design and Construction of Outpatient Facilities (2018);
  • General Services Administration’s (GSA’s) PBS-P100, Facilities Standards for the Public Buildings Service (2017);
  • Green Building Initiative’s (GBI’s) Green Globes for New Construction 2019;
  • The U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) v4.1 Building Design & Construction (BD+C) and Interior Design & Construction (ID+C);
  • International WELL Building Institute’s (IWBI’s) 2014 WELLv1 and 2018 WELLv2;
  • ASTM 1374-18e1, Standard Guide for Office Acoustics, and Applicable ASTM Standards;
  • American National Standards Institute/Acoustical Society of America (ANSI/ASA) S12.70-2016, Criteria for Evaluating Speech Privacy in Healthcare Facilities; and
  • The International Organization for Standardization (ISO) 3382-3, Acoustics – Measurement of room acoustic parameters – Part 3: open plan offices, and ISO 22955, Acoustics – Acoustic quality of open office spaces.

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 [1962]). 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).

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.

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