1、OR-05-6-3 Adverse Effects of Moderate Levels of Low Frequency Noise in the Occupational Environment K. Persson Waye, PhD ABSTRACT This paperprovides an overview of adverse effects due to moderate levels of low frequency noise in the occupational environment. The paper reports factors that influence
2、the response to low9equency noise and gives suggestions for an improved method of assessment of low frequency noise. Present studies indicate that low frequency noise can lead to adverse reactions during work starting at 40 dBA. Low frequency noise has been found to cause annoyance and rela- tionshi
3、ps between annoyance and subjective reports of lack of concentration, sleepiness, and tiredness. The A-weighted soundpressure level is apoor measure of adverse reactions to low frequency noise. Data indicate that the spectral balance between high and low frequencies and the presence of modu- lations
4、 or ltvelfluctuations are of large relevance for adverse efects. Compared to other noise sources, howevel; data from low frequency noise are limited, andfurther studies are clearly needed. INTRODUCTION The overall effects of occupational exposure to noise on humans are influenced by a number of non-
5、physical parame- ters as well as the physical character of the sound (Kjellberg and Landstrm 1994). Non-physical parameters that are reported to affect humans include type of work and work demands, habituation, and various individual factors. Among the number of physical parameters that should be me
6、ntioned are the strength or the sound pressure level (SPL), the frequency characteristics of the noise, and various time factors. The most commonly studied physical factor is the equivalent A-weighted sound pressure level. While occupa- tional noise induced hearing damage over the years has been wel
7、l studied, the knowledge of overall noise-induced effects on humans is still limited, and for most effects there are no established dose/response relationships. Even less knowledge exists of adverse effects of a specific type of occupational noise exposure, namely, noises with a large content of fre
8、quen- cies in the region of 20 to 200 Hz, low frequency noise. The lack of data for these noises is accentuated by the fact that the A-weighted SPL has been found to underestimate the adverse reactions to low frequency noise (Kjellberg et al. 1984; Kjell- berg and Goldstein 1985; Persson and Bjrkman
9、 1988; Pers- son Waye 1995a; Bengtsson 2003). The majority of studies describing occupational noises hence do not give sufficient information for evaluation of the presence of low-frequency noise and its effects. This review focuses on studies of relevance for occupa- tional low frequency noise and
10、particularly adverse effects of moderate A-weighted SPL in the range of 40 to 60 dB. DEFINITION Low frequency sound lacks an internationally established definition but usually indicates the frequency range of 20 to 200 Hz (Persson Waye 1995a; SOSFS1996:7/E; Mdler and Seijer Pedersen 2004). Other def
11、initions include frequencies up to 500 Hz (Castel Branco et al. 1999), or 250 Hz (Berglund et al. 1996). Although the upper limit for infrasound is 20 Hz, at sufficiently high sound pressure levels, certain noises contain in practice both perceptible infrasound and low frequencies. The division betw
12、een infrasound and low frequency sound should therefore be seen as merely conven- tional. For both infrasounds and low frequency sounds, their relationship to the perception threshold is of relevance as a first estimate of risk assessment. For the low frequency range, Kerstin Persson Waye is associa
13、te professor in the Department of Environmental Medicine, Gothenburg University, Gothenburg, Sweden. 672 02005 ASHRAE. Sound Pressure Level -Weighting Factors 20 I I i1 10 O -10 I J -20 m 4 -30 3 40 -50 -60 -70 %O Figure 1 A-, C-, and D-weightings. comparisons are made to the standardized normal hea
14、ring threshold (IS0 226 2003), while for infrasounds there exists no standardized normal hearing threshold and assessments have to be made to approximations of present studies in the field (see Maller and Seijer Pedersen 2004, for a review). With respect to effects on humans, many studies have shown
15、 that adverse reactions appear when the noise consists of perceivable sound pressure levels in the low frequencies that are considerably higher relative to the sound pressure levels over about 200 Hz. Thus, in terms of effects, a low frequency noise can be defined as a noise with dominant frequencie
16、s in the region of 20 to 200 Hz and is thus used in this document. SOURCES OF LOW FREQUENCY NOISE IN OC C U PATI ON AL E NVI RON M E NTS Low frequency noise is emitted from a multitude of sources in the workplace such as large ventilation systems, climate systems, diesel motors (heavy vehicles, dies
17、el loco- motives, work machines, generators), aircrafts (propeller planes, helicopters, jets), compressors (refrigeration compres- sors, pressurized air drills), and turbines. Compared to high frequencies, low frequencies propagate for long distances. Low frequencies will also pass with little atten
18、uation through walls and windows. At long distances from the source, or indoors, the noise spectrum will be selectively attenuated, resulting in a spectrum dominated by low frequencies. Airborne noise of a low frequency character may also occur as a result of vibrations in the ground or in building
19、structures. Low frequency noise is generated also when explosives are detonated and in the use of heavy artillery. Because of the relatively lower attenuation of low frequencies by building constructions and hearing protection equipment, an attenuated noise will be dominated to a higher degree by lo
20、w frequencies. Examples of situations in which the resulting noise can contain a large portion of low frequencies are interior control rooms, steering compartments and cockpits, and when tradi- tional hearing protection equipment is used. ADVERSE EFFECTS ON HUMANS IN OCCUPATIONAL ENVIRONMENTS Experi
21、ence of Relief When the Noise Ceases Compared with high frequency sounds, low frequency noise does not usually pose an immediate distraction. A common reaction to low frequency noise, and especially steady-state noise such as ventilation noise, is a feeling of relief when the noise ceases, even when
22、 the exposed persons have not been aware that the noise was present. Landstrm et al. (1991a) reported that an average of 65% of 155 office employees experienced a subjective feeling of relief, while 16% were not aware of any difference, when the ventilation system was turned off at night. To investi
23、gate the significance of this experience for work performance, Kjellberg and Wide (1 988) designed an experiment where subjects were not aware that the ventilation noise level of 5 1 dBA, 57 dBD was part of the experiment (see Figure 1 for explanation of the A- weighed SPL and the D-weighted SPL). T
24、wo groups were compared, one that worked in a quiet situation for 20 minutes before the noise was tumed on (Q-N) and one that first worked with noise for 20 minutes and continued working for a further five minutes in quiet conditions (N-Q). It was found that response times were longer in the N-Q gro
25、up than in the Q- N group, which may be an indication that the noise made learning more difficult. The number of mistakes was higher in the N-Q group and the number of mistakes decreased signif- icantly when the exposure stopped. The small increase in the response time in the study subjects in this
26、group could not explain the decrease in the number of mistakes. Annoyance and Subjective Discomfort Noise-induced annoyance is the most common and most researched adverse effect of noise on people. This is also the case for low frequency noise. Annoyance has been defined as “a feeling of displeasure
27、 evoked by a noise” and “any feeling of resentment, displeasure, discomfort and irritation occurring when a noise intrudes into someones thoughts and moods or interferes with activity” (Fields et al. 2001). Whether a person becomes annoyed when exposed to noise, depends on the noise (e.g., how inten
28、se it is, how it varies with time, and what frequencies it contains), but also many other parameters have an impact on the annoyance. These can be divided into three groups: individual factors, such as hearing impairment, noise sensitivity, attitude to the noise source, physioIogica1 and psychologic
29、al state; situational factors, such as activities performed or intended to be performed; and noise source- related factors, such as controllability of the noise source, information content, and permanence. Annoyance is measured using questionnaires or interviews and the rating is usually done on a v
30、erbal or numeric scale with endpoints “not at all annoying” to “very” or “extremely annoying.” ASH RAE Transactions: Symposia 673 dB SPL -1 I 113 octave band (ti - d SPL “I I I -1 Bo I -II Figure 2 Third octave bandsoundpressure levels of the two exposure sounds used in the experiments. Light bars s
31、how the flat frequency Ventilation noise (Rea and dark bars show the low .frequency ventilation noise (LFN). Top graph shows the sound at 40 dBA, und bottom graph shows the sound at 45 dBA. The majority of case studies on low frequency noise carried out in general environments (indoor and outdoor re
32、si- dential areas), and work environments report that annoyance occurs even though the A-weighted SPLs were within perrnit- ted limits for ordinary noise in the different countries in which the studies were carried out (e.g., Tempest 1973; Vasudevan and Gordon 1977; Challis and Challis 1978; Leventh
33、all 1980; Fuchs 1990, Cocchi et al. 1992). The most commonlyreported symptoms were headache or a feeling of pressure in the head, unusual fatigue, concentration difficulties, irritation, vibra- tions in the body, and a feeling of pressure on the eardrum (e.g., Scott 1978; Leventhall 1980; Lundin and
34、 Ahman 1998). Although these reports have been made on the basis of case studies and may thus have a number of sources of error, the agreement between them in terms of symptoms and sound descriptions is good. There are fewer published studies of annoyance reactions due to low frequency noise in the
35、occupational environment. Investigations of annoyance and discomfort among 155 employees in four ofice environments with low frequency Ventilation noise showed that the average estimate of distur- bance was “hardly at all” to “quite annoying” in two ofices with an average level of 33.5 to 35.3 dBA a
36、nd “somewhat” to “quite annoying” in ofices where the average levels were 38.5 to 39.2 dBA (Landstrm et al. 1991a). The difference in reported average annoyance between the ofices with lower and higher dBA levels was statistically significant. Of the 155 employees, 14% of the employees reported that
37、 they experi- enced concentration problems daily or one to three times a week and 19% reported that the ventilation noise had a nega- tive effect on work ability. In another study of control room workers reporting complaints due to the noise at the work- place, the most common complaints were proble
38、ms with concentration, drowsiness, and headaches (Pawlaczyk-Lusz- cynska et al. 2002). Measurements showed that the noises in the control rooms comprised a large amount of low frequen- cies, with A-weighted sound pressure levels ranging from 48 to 60 dB and C-weighted sound pressure levels ranging f
39、rom 59 to 79 dB. In a summary of her experimental studies, Bengts- son (2003) states that annoyance due to low frequency venti- lation noises were significantly related to reports of lack of concentration, sleepiness, tiredness, pressure over the eardrums, pressure over the head, and dizziness. Anno
40、yance afer exposure to a flat frequency ventilation noise was only significantly related to nausea. The estimation of annoyance found in Landstrm et al. (1 99 la) agrees well with the results of two experimental stud- ies with low frequency ventilation noise and a flat frequency ventilation noise (r
41、eference noise) at a level of 40 dBA. Their C-weighted SPL were 69 dB and 50 dB, respectively (top graph of Figure 2) (Persson Waye et al. 1997; Persson Waye et al. 2001). In the first experiment, 14 persons worked on different performance tests for a 60-minute exposure period. The low frequency noi
42、se was on average judged to be “somewhat annoying” and to make performance “somewhat worse” while the corresponding judgments for the reference noise were “not at all” to “somewhat” annoying, and the effect on performance was judged to be “none” or “somewhat poorer.” In the next experiment, 32 perso
43、ns worked on different tests for two hours at the same noise exposure as used in the first experiment. The low frequency noise was judged on average to be “somewhat” to “quite” annoying, while the reference noise was judged to be “somewhat annoying.” Both estimated annoyance and negative effect on p
44、erformance were significantly higher in the case of the low frequency noise exposure. In a third experiment, 14 subjects were exposed to the same exposure noises and worked with similarly mentally demanding tasks but now at an A-weighted SPL of 45 dB (bottom graph of 674 ASHRAE Transactions: Symposi
45、a 5 4.5 r 4 - 9 3.5 - I 3 3- 40 60 40 120 45 Sound level (dBA) 6o ExDonire time (mird Figure 3 Annoyance during work in diferent noise conditions related to noise exposure level und exposure time. (O = not ut all annoyed, 4 = extremely annoyed). Figure 2). The corresponding C-weighted SPL were 53 dB
46、 and 72 dB. A summary of the results from these three studies is shown in Figure 3. Figure 3 shows that the low frequency noise was always judged somewhat more annoying compared to the reference noise. The lack of significance can probably be accounted for by the small number of subjects in the firs
47、t and third study. The figure also indicates that annoyance increases most impor- tantly by noise level and, secondly, probably also by exposure time, as the first and second study are similar apart from the exposure time, which in the second study is twice as long as for study one. Further support
48、for the findings in Figure 3 were given in a later study including 38 subjects who worked at a low workload (routine type tasks and no stress) at an A- weighted SPL of 45 dB for about four hours (Bengtsson et al. 2004b). The exposure noises were the same as in the previ- ously described studies (Fig
49、ure 2, bottom graph).The average values of annoyance were 3.2 for the reference noise and 3.4 for the low frequency noise. The annoyance ratings were thus comparable to the third study. One could have expected that annoyance would have been higher with regard to the long exposure time, but the character of the tasks and task demands were probably an important factor for the annoyance ratings. In accordance with that hypothesis, a few previous studies have found higher ratings of noise annoyance under perfor- mance of more demanding tasks (Kjellberg and Landstrm 1994). An increa
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