The temporal component of sound refers to the variation in the level of sound as a function of time; in other words, from one moment to the next.
Neither HVAC nor mechanical, electrical, and plumbing (MEP) systems can be relied upon to provide continuous and constant (i.e. unchanging) control—and nor should they, for reasons relating to the spectral characteristics of these noise sources. Figure 2 illustrates the issue. While the receiver experiences a moment of privacy (highlighted in blue), they are not free from distraction the remainder of the time because the signal-to-noise ratio is positive. When “C” is applied, it not only improves speech privacy, but also increases occupants’ perception of acoustical consistency by reducing the frequency and severity of the intermittent changes in sound levels (i.e. dynamic range) caused by speech and noise, over time.
The spectral component of sound is a more nuanced topic. Just as visible light comprises a range of wavelengths, sound, as one hears it, is the result of a combination of frequencies.
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 favourable 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.8 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.