Effects on Humans
The office workers involved in the study mentioned above2 rated the ventilation noise as “somewhat disturbing” to “quite disturbing” at the two workplaces where the level of exposure was about 40 dB(A). At the two workplaces
Where the mean level was 5 dB lower, the mean rating lay between “not at all disturbing” and “somewhat disturbing.” The difference in level resulted in a clearly perceivable lowering of the average disturbance and inconvenience levels. The fact that a 5-dB reduction in the ventilation noise level can result in such a pronounced reduction of the perceived inconvenience can be explained by the circumstance that a change in level in the low-frequency range has a significantly greater effect on the loudness than would be the case in a high-frequency sound range. Measures to achieve a reduction in ventilation noise of the order of 5 dB can thus result in measurable gains in the form of lower inconvenience reactions.
The range of answers to the question, “How does ventilation noise affect your ability to perform your tasks?” reveals that about one in every five office workers on average felt that ventilation noise made their work more difficult. A significantly greater number assessed the higher level at 40 dB( A) as an aggravating factor in the performance of their tasks at the office. About 20% considered that the higher level made their work “somewhat" or “much” more difficult. About 10% made a similar assessment at the noise level of 35 dB(A).
S.4.5.2 Influence Due to Spectral Distribution
Systematic studies have been carried out for the purpose of studying how low-frequency tones, broad-band components, and/or time fluctuations in ventilation noise interfere with disturbance reactions.4 In one of these studies, the respondents were exposed to ventilation noise that is representative of the noise encountered in office premises. The respondents were asked to use a rotating potentiometer to “set” the “most acceptable noise level” and the “least acceptable noise level” for each noise, taking account of comfort, disturbance, and performance, while performing their work at the same time. The noise level was maintained at a constant level of 40 dB(A).
When setting the most acceptable ventilation noise level consisting of a single tone, all the respondents selected a lower tone frequency for both settings than when they set the least acceptable level. The average frequencies set for the most acceptable and the least acceptable noise levels were 58 and 380 Hz respectively. The disturbance experienced and the discomfort experienced were significantly higher, and the performance was significantly lower, during exposure to the least acceptable noise. A higher level of exertion was also experienced when exposed to the most acceptable noise.
The results clearly indicated that the ventilation noise was perceived as most acceptable when the tone was situated in the lower part of the frequency range. The experience of disturbance and the associated effects occur at exposure levels above the auditory perception threshold. Above this level, the risk of these effects increases as the perceived loudness increases, provided that the other conditions remain constant. Since the loudness can be predicted relatively accurately by means of technical measurements, any differences in the degree of disturbance can also be predicted by reference to these measurements, provided that they are dependent on differences in the loudness.
At the same dB(A) level, the noise with the stronger low-frequency feature was thus experienced as being significantly less disturbing than the more high-frequency noise. This result suggests that the A-weighting overestimates the contribution made by low-frequency tones to the disturbance experienced. This could be taken to mean that the general applicability of the dB(A) level is extremely limited at rimes when the goal is to carry out evaluations of the anticipated disturbance effects of ventilation noise con taining tones.
An investigation designed as a rone experiment was also carried out on a broad-band ventilation noise. The average mid-range frequency tor the broad band indicated by each respondent when the most acceptable and least acceptable noise levels were set reflects the situation applicable to tone exposures. The average set mid-range frequencies for the broad-band components were 129 and 456 Hz, respectively. The most acceptable noise level had a lower frequency than the least acceptable noise level for all the respondents. The estimate of the mean values in all the inconvenience variables was signilkant! higher for the least acceptable noise level than for the most acceptable noiv level in all variables.
The results revealed by these investigations on the whole indicated that the measures taken to counter ventilation noise in order to reduce the effects on disturbance, performance, and exertion should be directed at higher frequency components within the low-frequency range. A greater general lowering of the dB(A) level based on measures to counter the low-frequency parts of the ventilation noise may involve a smaller limitation of the inconvenience effects than a smaller comprehensive lowering of the dB(A) level based on a measure to counter the higher frequencies of the ventilation noise.
The influence of the period of exposure on the disturbance experienced due to ventilation noise has been studied both in authentic exposure situations in offices and in laboratory experiments. The link between the estimated disturbance experienced due to ventilation noise and the period of exposure, i. e., the time during which the office personnel stated that they could hear the ventilation, was tested on quite a large group of respondents.2 The whole test group was divided into two groups, based on estimated values above or below 50 mm on a 100 mm estimation scale. The group with a lower average disturbance experience exhibited significantly lower experience periods (231 min) than the group with a higher average disturbance experience (3,90 min). The link between the estimated disturbance experience and the time for which the ventilation noise could be heard points to a positive linear correlation, according to which the disturbance experience increases in line with the increase in the period for which it is experienced. T his can be interpreted as indicating that exposed persons become habituated to or adapt to the low-frequency ventilation noise only to a very small extent, or not at all. It is felt that similar conclusions can be drawn from a recent, more systematic field study into the significance of the period of exposure prior to the experience of disturbance.5 The phenomenon of habituation and adaptation is believed to be consistently stronger for high-frequency noises than for low-frequency noises.
Laboratory experiments strengthen the picture and the conclusions formed from the field studies. Both the estimated disturbance experience and the degree of exertion exhibit gradually higher values over time during a studied exposure period of 60 minutes.6 The investigation included exposure to ventilation noises with different characteristics at levels ranging from 35-40 dB(A). The requirement to correlate the disturbance experience to the period for which a ventilation noise was experienced gave rise to the idea of possibly masking the experience of an unfavorable ventilation noise. Pure-tone (100 Hz), broad-band, and masked ventilation noise were compared in a laboratory experiment with regard to the effects on performance, alertness, and experience of disturbance.7 When a masking “pink noise” was added to the pure-tone ventilation noise, there was a tendency for performance to improve and for alertness to increase, although at the same time people were more disturbed by the noise. All the effects were weak, however, and in most cases they were not statistically confirmed. The opportunities for improving the acoustic climate in an environment with ventilation noise via masking effects are thus regarded as limited. Efforts should rather be targeted at a more general reduction of the level of those parts of the ventilation noise that contribute to the overall experience of loudness.
Laboratory studies have demonstrated clearly that the experience of disturbance, the degree of exertion, and the performance are consistently affected more negatively when exposed to intermittent noises than when exposed to continuous noises at the same equivalent level. s Studies into ventilation noise in authentic environments are still very limited, however. Systematic evaluations of the links between inconvenience symptoms and fluctuations have, as expected, indicated an increased risk of symptoms with an increased breadth of fluctuation in the level.8 The effect of an increased breadth of fluctuation in the level was also higher at higher dB(A) levels. Fluctuations in level in the vicinity of the threshold of auditory perception were correlated to lower disturbance reactions. More rapid fluctuations (2 Hz) were also experienced as more disturbing than slower fluctuations (0.5 and 1 Hz).8 Comparisons also indicate that fluctuating tones and fluctuating higher noise frequencies are experienced as more disturbing than corresponding broad-band noise and lower noise frequencies.8 The situation relating to the effect of fluctuations on inconvenience reactions thus reflects, as anticipated, an increased risk of influence with greater psycho-physical potential for experiencing a ventilation noise. There are strong indications that the particularly disturbing effects of ventilation noise can be explained in many cases by the pronounced fluctuations which often characterize experiences of this type of noise. Fluctuations in ventilation noise can be the reason for marked increases in the experiences of inconvenience. The importance of countering the fluctuating characteristics of the ventilation noise in various ways should be emphasized.
The most usual effect of exposure to ventilation noise, as previously mentioned, consists of annoyance and disturbance of various kinds. Such effects may occur as a result of the relatively low levels of exposure occurring in offices, schools, etc. In industrial environments, workshops, warehouses, etc., however, the levels from a fan system may sometimes even reach the level of risk of hearing damage or of speech-masking. The risk of hearing damage and the speech-masking effect arise at levels around 70 dB( A). Pronounced or well-defined health effects expressed as a function ot long time or repeated exposure to ventilation noise have not been demonstrated. However, the possibility that repeated exposure to ventilation noise may cause increased stress and in this way may have an effect on health cannot be ruled out. An increased risk of stress-related complaints may occur, not least because human ability to acclimatize to low-frequencv noise seems very limited.5
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