Page 35 - January 2006

FINDING CONNECTIONS AMONG INDOOR NOISE CRITERIA, HUMAN PERCEPTION, AND WORKER PERFORMANCE
Lily M. Wang and Erica E. Bowden
Architectural Engineering Program, University of Nebraska–Lincoln Omaha, Nebraska 68182-0681
Clickity-clickity-clickity-CLACKity
... clickity-clickity-clickity-
CLACKity ... what is creating that
background noise in your office, in your
home, or in the movie theater? It may be
hard to pinpoint, but certainly it can cause
you different kinds of distress. It may
make you annoyed, disturb the flow of
conversation, interrupt your sleep, cause
fatigue, and even impact how productive
you are. Some of the most pervasive
sources of background noise are the mod-
ern heating, ventilating, and air-condi-
tioning (HVAC) systems that keep us at comfortable tem- peratures indoors. These systems range broadly from the window unit in your apartment to a chiller supplying chilled water to cool a gymnasium in the summer.
Today’s buildings contain an assortment of HVAC equipment, each contributing its unique flavor to the indoor auditory environment, or soundscape. Add to this the fact that other characteristics of a space influence the background noise, such as the amount of absorption in the room, the geometrical configuration, and the acoustical iso- lation properties of the building materials—the result is that no two rooms have exactly the same background spectrum. So with all of this variety, how can we characterize back- ground noise in the modern built environment? Furthermore, how can we rate acceptability of these back- ground soundscapes?
Introduction to indoor noise criteria systems
Several acousticians have considered these questions over the past 50 years, resulting in empirically developed indoor noise criteria systems. These systems are commonly used today by building engineers and designers to quantify the background noise in rooms, and are also incorporated into manufacturers’ data, design guides, and standards. Some of the more popular criteria include Noise Criteria1 (NC), Balanced Noise Criteria2 (NCB), Room Criteria3 (RC), Room Criteria Mark II4 (RC Mark II), and A-weighted Equivalent Sound Pressure Level5 (LAeq). Each of these criteria provides a single number rating that describes the overall level, or loud- ness, of the sound. This rating is generally found by compar- ing the measured background noise levels in a space against a set of sound level versus frequency curves. NCB, RC, and RC Mark II also include spectral quality descriptors; terms such as rumbly, roaring, and hissy are used to describe back- ground noise with excessive low-, mid-, and high-frequency energy, respectively. If the low-frequency energy is very high, the space may also be described as causing noise-induced vibration.
“We consistently find that performance significantly decreases as participants perceive the noise
to be louder or more spectrally imbalanced...”
Despite the popularity of these criteria systems, there is still dissatisfaction in the architectural acoustics community con- cerning their appropriate use in describing the wide variety of background noise situ- ations we encounter. Previous studies have shown that there can be significant differ- ences among indoor noise criteria ratings
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Which method should you trust then? One might turn to industry experts to determine which criteria system is best to use. However, from a review of sev- eral sources, you would quickly see that there is no consensus that a particular criterion generally performs better than the
others:
• ANSI Standard S12.2-1995 recommends the use of
NCB and RC, although this standard is currently under
For example, one may characterize a particular room as sounding hissy, while another says that the same room sounds not hissy, but rumbly.
of the same spectrum.
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• ANSI Standard S12.60-2002 on classroom acoustics
revision and may replace these with other criteria.
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Air-Conditioning Engineers (ASHRAE) HVAC Applications Handbook recommends use of RC Mark II in general, but provides guidelines on how other crite-
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NC ratings.
• Noise Rating (NR) is commonly used in Europe, but
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because they may not characterize all of the qualities of back- ground noise that affect occupants. Besides overall loudness, background noise may have different effects based on (a) its distribution across frequency (spectral quality), (b) whether or not the noise contains tones, and (c) whether or not the noise changes over time. The majority of the criteria systems account for loudness and spectral quality, but few specifical- ly account for tonality or time-varying fluctuations.
Can we just ignore these other two factors? Many acoustical consultants believe we cannot, since tones and time-varying fluctuations are both frequently produced by modern HVAC systems. Motors, fans, and compressors are just a few examples of equipment that can produce audible discrete tones. Noise that fluctuates over time can occur rap- idly, as in a rattling air diffuser, or more slowly, such as with the cycling of a variable air volume (VAV) system that adjusts to changing occupancy.
sets background noise criteria in LAeq.
• The American Society of Heating, Refrigerating, and
ria may be used for design versus diagnosis.
• Manufacturers’ data on diffuser noise usually provide
not in the United States.
• Preferences among acoustical consultants vary greatly! Additional concerns exist over these criteria systems
Noise and Worker Performance 33