Page 36 - January 2006

How does indoor noise affect occupant perception and performance?
Many sources list the range of indoor noise criteria rat- ings that are appropriate for some space usage; for example, an auditorium should maintain a lower noise criteria than the lobby that serves it. But questions have arisen about how well these criteria systems are related to actual occupant per- ception of the noise. Building owners are also very interested in whether or not meeting a particular noise criteria may be linked to worker performance. Unfortunately, the range of research in this field is not yet broad enough to provide definitive answers to these questions.
Recent studies linking subjective perception of ambi-
ent noise with measured sound spectra have demonstrated
conflicting results. In these studies, people were asked to
rate their general perception of the background noise, and
their responses were then related to background noise
measurements and criteria systems. Tang13-15 and Ayr16-17
consistently found LAeq to be highly correlated with subjec-
tive auditory sensation in office surveys. Persson-Waye and
Rylander, on the other hand, found that LAeq was not a good
predictor of annoyance to long-term noise exposure in res-
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affect productivity,19 but the pool of research examining how typical office background noise affects worker performance is more limited. Some researchers have focused on the
20-24
idences.
Much work has been done on how high levels of noise
effects of low frequency noise on task performance. Among the main conclusions from these studies are that productivity can be affected by background noise, the impact can potentially change over time, and that the fre- quency character should be considered. While previous work has been instrumental in proving that a relationship exists between indoor noise and productivity, few studies have tried to determine whether or not the commonly used indoor noise criteria can represent this relationship for a variety of background noise conditions.
Research at the University of Nebraska “Indoor Environment Lab”
We are conducting an extensive study at the University of Nebraska to draw more connections between indoor noise criteria and the effects of HVAC noise on
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occupant perception and productivity. Human test sub-
jects have been exposed to a wide range of background noise conditions that are typical of existing indoor back- ground noise from HVAC systems. Noise conditions at var- ious levels and spectral quality are used, as well as some with tones and time-varying fluctuations. The test subjects complete performance tests and perception questionnaires under each noise condition, and their scores are then relat- ed to criteria ratings of the noise, including NC, NCB, RC, RC Mark II and LAeq. Our analysis seeks to determine how well the indoor noise criteria systems match human percep- tion and productivity and also to find links between the lat- ter two.
An “Indoor Environment Lab” was specially constructed at the University of Nebraska to conduct this type of research. As shown in Fig. 1, this lab resembles a typical office with
Fig. 1. View of a work station in the University of Nebraska Indoor Environment Lab.
carpeting, gypsum board wall construction, and acoustical ceiling tiles. The lab is designed to be acoustically isolated from adjacent spaces, and thus has very low naturally occur- ring background noise levels. The room is both thermally and acoustically controlled. For our research on indoor noise criteria, the temperature in the room is kept at a relatively constant level, so that the only factor changing is the noise. In the future, the lab may be used for studies on the interaction between the lighting, thermal and acoustical environments.
The variety of background noise conditions are present- ed over two loudspeakers that are disguised so that neither is easily identifiable. An overhead panel loudspeaker resembles a ceiling tile and a subwoofer in the corner is fashioned to look like an end-table. The overhead loudspeaker primarily supplies mid- to high-frequency energy, while the subwoofer provides the low-frequency components.
So far, the effects of 18 different noise conditions have been examined over several phases of testing. The noises mimic ventilation sound that might be encountered in real- world buildings, and span a range of sound levels (from rough- ly 37 to 57 dBA), spectral qualities (neutral, rumbly, roaring
Fig. 2. Two of the tonal noise exposures used in the study. The dashed line shows the levels of the background noise measured from the real-life space. Noise condi- tions A and B are the laboratory-reproduced spectra with two different tonal prominence ratios.
34 Acoustics Today, January 2006