Singular—or discrete—frequency values are called ‘tones,’ and the human ear can hear between approximately 20 and 20,000 hertz (Hz). To simplify reporting data for the nearly 19,980 individual frequencies, it is common practice to divide this range into sections called “fractional octave bands.” The customary fractions are full octave bands (also referred to as ‘1/1’) and one-third octave bands (or ‘1/3’). Between 20 and 20,000 Hz, there are 29 one-third octave bands. The combination of all audible frequencies of a sound sum to its overall level.
It is possible for two sounds equal in overall level to be perceptibly different. Borrowing descriptors from the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), one can generally state a sound that has too much low-frequency content is ‘too rumbly,’ while a sound that has too much high-frequency content is ‘too hissy,’ and sound that has too much mid-frequency content has a strong ‘hum’ or ‘buzzing’ quality.
If empowered with the ability to adjust the frequency content for a fixed level of sound (e.g. 45 dBA), there exists a favorable combination of frequencies that is ‘most comfortable’ or balanced. This ‘shape of sound’ is documented in literature by Beranek (and BBN) and Warnock—and, more recently and precisely, by the National Research Council of Canada (NRC)—and forms the basis for the synthesis of masking sound (although they are still often referred to as ‘white noise’ systems, modern sound-masking technologies synthesize the spectrum and level of the sound that actually exists within the space. For more information about how masking sound is different from white and pink noise, see Niklas Moeller’s ‘Tuning into sound-masking technology’ at www.constructionspecifier.com/tuning-into-sound-masking-technology). When professionally tuned to meet this ‘shape’ (typically called a ‘spectrum’ or ‘curve’) for the majority of the audible frequency range (100 to 10,000 Hz), background sound resides in the ‘Goldilocks zone.’ Occupants’ perception of the final product may be described as ‘quiet’—free from rumble, hiss or buzz, and absent of hum or buzzing. Further, the overall level is neither too high to disturb occupant comfort, nor too low to compromise acoustical privacy.
The spatial component of sound is no less complex. It refers to the variability of the level—also, inherently, that of the spectra—of sound, in space. These variations are a function of many parameters, including not only the source and location from where the sound originates (e.g. building systems, occupants, appliances, and even oneself), but also the space’s architecture (i.e. size, shape, geometry) and fit out (i.e. finishings, fixtures, furnishings).
As sound from a source is generated, it propagates with its level decaying as a function of distance, and by the number of times it is reflected (loses energy) from other surfaces or at room boundaries. While its energy continually dissipates, its eventual inaudibility is not because its level is attenuated below one’s auditory threshold, but because it drops below the background sound in one’s environment—the background sound that actually exists. This phenomenon is known as the ‘masking effect,’ where the background sound covers the propagating noise. Figures 3 and 4 provide simplified modelling of this effect. Not only does masking sound reduce the distance over which a noise can be heard (sometimes referred to as the ‘radius of distraction’), it creates a more consistent—and equitable—acoustical experience for occupants, both in their individual work areas and as they move throughout the space.
Control versus cover
While many still associate the ‘C’ in the ‘ABC Rule’ with ‘cover,’ ‘control’ is a more accurate term, for several reasons.
Use of the word ‘cover’ can unintentionally reinforce the view this crucial element of architectural acoustics simply involves placing any sound overtop of others—like a blanket—strengthening the historical misperception that only level matters; in other words, that a sound only needs to be ‘louder’ than other sounds to provide the masking effect and, hence, meet the requirements of ‘C.’ This misperception opens the door to commoditization of sound-masking systems—the notion the effect will simply be provided by the product, rather than in tandem with a service that ensures the sound actually meets the specified masking spectrum.
The study of architectural acoustics demonstrates the physics of the behavior of sound within the built environment is exceedingly complex—and this is true for any sound, even those introduced via a sound-masking system. Regardless of the sophistication of the technology, the system’s layout or loudspeaker orientation (e.g. upward-facing within the plenum or downward-facing using cut-throughs), the masking effect can only be achieved through skilled field commissioning—or ‘tuning’—which adapts the sound actually produced in the room/space by accounting for its architecture and fit out. Small zones (i.e. no larger than one to three loudspeakers in size) offering fine volume (in 0.5 dBA steps) and frequency (1/3-octave) adjustment capabilities provide the technician with frequent and precise control points across the environment, helping to consistently achieve the masking effect throughout the space and, hence, a better outcome for the occupants.
Post-installation tuning and performance verification are crucial to ensuring the sound-masking system is, in fact, effectively controlling the spectrum and level of the sound that actually exists within the built environment—and, therefore, dependably providing the masking effect throughout the space. It is only under these assured conditions—temporally, spectrally, and spatially consistent acoustics—that occupants can appreciate acoustical privacy.
In 1962, William Cavanaugh et al., authors of Speech Privacy in Buildings, affirmed acoustical satisfaction could not be assured by any single parameter, forming the foundation for the ‘ABC Rule’ of architectural acoustics. However, until rather recently, building codes, standards, and certification programs largely focused on ‘A’ and ‘B,’ while ‘C’ often succumbed to a historical preoccupation with limiting the ‘loudness’ of sound and corresponding belief that the goal is to make spaces as silent as possible. That said, architectural acoustics are amid a paradigm shift.
In the pursuit to better understand how one can be psychologically and physiologically supported by the spaces they inhabit, the important role played by ‘C’ becomes apparent. Sound will always remain within the built environment, and the impact of such low-level background sound—that which actually exists in the space—cannot be separated from acoustical satisfaction and its equitable delivery. Therefore, controlling it is as important as controlling the ‘signals.’
As Greenhouse states, the built environment “impacts us whether designed well or poorly, so why not design well?” If one is to reliably design buildings to function acoustically for their users (e.g. provide adequate speech privacy, freedom from distraction, reduced annoyance, a good night’s sleep, and so on), one needs to establish a known level of spectrally neutral (or balanced) background sound, rather than leaving it—and the end result—in question.
Viken Koukounian, PhD, P.Eng., is an acoustical engineer at K.R. Moeller Associates Ltd. He is an active and participating member of many international standardization organizations, such as the Acoustical Society of America (ASA), the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), the American Society of Testing and Materials (ASTM), the Green Building Initiative (GBI), and the International WELL Building Institute (IWBI), the Standards Council of Canada (SCC), and also represents Canada at International Organization of Standardization (ISO) meetings. He completed his doctorate at Queen’s University (Kingston, Ontario, Canada) with foci in experimental and computational acoustics and vibration. Koukounian can be reached via email at email@example.com.
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network and MODIO Guestroom Acoustic Control. He has more than 25 years’ experience in the sound-masking industry. Moeller can be reached via email at firstname.lastname@example.org.