Noise Health Home 

[Download PDF]
Year : 2008  |  Volume : 10  |  Issue : 40  |  Page : 90--98

The relationship between noise frequency components and physical, physiological and psychological effects of industrial workers

KV Mahendra Prashanth1, V Sridhar2,  
1 Department of Electronics Engineering, VKIT, Visvesvaraya Technological University, Bangalore - 560 074, Karnataka, India
2 Department of Electronics Engineering, PESCE, Visvesvaraya Technological University, Mandya - 571 401, Karnataka, India

Correspondence Address:
K V Mahendra Prashanth
Department of Electronics Engineering, Vivekananda Institute of Technology, Gudimavu, Kengeri Hobli, Bangalore - 560 074, Karnataka


A corollary to industrialization and urbanization is a significant increase in noise levels. In many industrial settings, the noise levels are such that they are potential health hazards. There are many studies which suggest that prolonged exposures to high noise levels have a negative impact on various aspects of human physiology. However, not much work has been conducted in studying the effects of various noise frequencies in the industrial environment. This paper has made an attempt to identify various noise frequency components to which the workers of six major industries in Mysore (Karnataka State, India) are being exposed, and their effects on the physical, physiological, and psychological systems of the working community with respect to their noisy industrial environment. The study results showed that the sampled industrial workers were repeatedly being exposed to noise of dominant low- and mid-octave band center frequencies. It is found that symptoms such as 'eye ball pressure,' 'awakening from sleep,' 'pains in neck,' 'frequent ear vibration,' 'chronic fatigue,' 'repeated headache,' 'backache,' and 'repeated ear pulsation' are observed to be highly associated with low- and mid-octave band center frequency noise exposure among the sampled workers. Furthermore, among the major psychological symptoms identified to be associated with octave band center frequencies, it is evident that 'irritability' is highly associated with low- and mid-octave band noise frequency characteristics.

How to cite this article:
Mahendra Prashanth K V, Sridhar V. The relationship between noise frequency components and physical, physiological and psychological effects of industrial workers.Noise Health 2008;10:90-98

How to cite this URL:
Mahendra Prashanth K V, Sridhar V. The relationship between noise frequency components and physical, physiological and psychological effects of industrial workers. Noise Health [serial online] 2008 [cited 2021 Apr 14 ];10:90-98
Available from:

Full Text


Industrial noise is one of the major noise pollutants that leads to various health hazards. Continuous exposure to noise is reported to have many adverse effects, both physiological and psychological, and it is a very serious problem among people working in an industrial environment. [1] A study by Mokhthar et al ., [2] on the effects of noise observed that the physiology, hearing capability, auditory communication, and sleep of industrial workers were significantly affected by noise levels. Noise levels and their frequencies are observed to be varied in various industries involving multiple operations of machinery. This affects the workers who work very close to the machinery. Repeated exposure to noise levels between 80 and 90 dB (A) involves some degree of risk. [3] Several studies were conducted to investigate the effects of industrial noise. Their results [4],[5],[6],[7] reveal that industrial noise adversely affects workers who suffer from various noise-related health problems. The three general physiological responses to sound are: voluntary musculature mediated by the somatic nervous system, smooth muscles and glands mediated by the visceral nervous system, and the neuroendocrine system. [8] Health effects of noise depend on various factors such as the level of noise, frequency, impulsiveness, intermittency, duration of noise influence etc. Furthermore, sex, health status, occupation, personality, sensitivity, adaptability, and other factors are also closely related. [9] There is ample evidence that occupational noise exposure is also linked with cardiovascular diseases. A study [10] reveals that systolic and diastolic blood pressures were significantly higher in 44 male industrial workers with a noise-induced auditory impairment for SPL of 65 dB at 3000, 4000, or 6000 Hz, than in 74 males of the same age with normal hearing. Another investigation reports that although repeated exposure to noise leads to elevated blood pressure and increased incidence of cardiovascular disease, the contribution of other factors such as age, alcohol consumption, smoking, etc., also plays a prominent role. [11] An analysis of the relationship of the potential workplace, stress of job strain, noise, and shift work, showed their association with cardiovascular disease. [12] Furthermore, it substantiates that high job strain and high noise level exposure cause a shift in cardiovascular regulation towards sympathetic dominance. A temporary rise in systolic blood pressure was observed by another study [13] on exposure to prolonged light floor impact sounds of 60 db and 80 dB(A). The review [14] reveals that exposure to loud and habituated noise above 90 dB (A) leads to sympathetic activation and an increased release of noradrenaline from the sympathetic synapses. It can also observed from the studies [15] that not only the loudness of noise, but also the vibration induced by the noise, contributes to the subjective unpleasantness of pressure exposed to complex low-frequency noise (31.5 and 50 Hz, having 90 dB SPL). Most of the aforementioned studies have dealt with adverse health effects due to noise based on its sound pressure level, but not with respect to its frequency spectrum. Not many studies have been undertaken to elucidate the influence of various noise frequency components on human physiological and psychological parameters. One study [16] observed that there has been too little research on the roles of different frequency spectra of noise in the production of effects on humans. Being the first of its kind in India (to the best of the author's knowledge), this study has made an attempt to identify major noise frequencies to which industrial workers are being exposed in six industries. Additionally, it investigates the probable association between noise frequency components and affected physical, physiological, and psychological symptoms among industrial workers exposed to noise. The Chi-square test was used to test the independence and association between various symptoms and octave band center frequencies.

 Materials and Methods

Subjects and survey

The preliminary approach dealing with the problem was to study the adverse health effects of various noise frequency components among workers in six industries who were constantly being exposed to noise. The study involved noise measurement and cross-sectional questionnaire investigation. A total of 93 subjects were finally selected and interviewed. These workers were chosen based on their work being attached to machinery of various kinds, generating noise, not fully automated, and their reported facts on health effects associated to continuous exposure to noise. The questionnaire for the survey comprised of 32 questions, covering all possible effects of noise on workers, in particular, the physical, physiological, and psychological effects of noise on industrial workers. The questionnaire was distributed to the workers and sufficient time was given to answer them followed by a personal interview in the local language, Kannada. The purpose of the study was explained to the subjects who were assured of confidentiality. They were invited to provide actual facts about their health problems, if any; responses were analyzed. The preliminary survey, interviews, and noise measurements were carried out in six major industries in and around Mysore (Karnataka State, India). These industries were mainly polymer, automotive part manufacturing, dairy product processing, and plywood. After a preliminary walk-through survey studying the work environment, important sections among these industries were selected to identify maximal exposure levels of noise.


A detailed questionnaire was prepared in consultation with a general physician, a medical consultant for industries, industrial workers, and referring survey reports. [17],[18] The first 18 questions were set to find an overview of the occupational environment and the workers' profiles. The respondents were asked to answer the questionnaire during which assistance was provided, wherever needed. A list of possible symptoms was mentioned (both related and unrelated to noise) in the questionnaire and they were asked to state whether they suffered from any of them, and helped to identify relevant symptoms. The association of major psychological symptoms was also assessed. Also, subjects were requested to report any other effects of noise that they had come across. Questions on subjective sensitivity to noise, physical tiredness, general health problems, and work efficiency related to noise, were also included. Subjects were asked to report any possibility of occurrence of cardiovascular disease linked to noise exposure. Feelings of annoyance were predicted based on the average score on an annoyance scale. To determine whether they had a hearing problem, subjects were asked whether they faced any hearing difficulty and had undergone audiometric testing. Also, they were asked whether they had taken any measures to protect themselves from continuous noise exposure.

Noise measurements

Noise measurement was made using Type 1 model 2236 precision integrating sound level meter (Bruel and Kjaer, Denmark) to assess the noise level and frequency characteristics. The instrument was calibrated on a daily basis using a pistaphone supplied by the manufacturer of SLM before the noise level measurements. The sound pressure levels were being recorded in octave band center frequencies in various preidentified sections in six industries. These measurements were made by selecting 1/1 octave band and C-weighting network. The instrument contained eight standardized octave band filters centered at octave band frequencies between 31.5 and 8000 Hz. Noise parameters were measured at a distance of 1.5-2 m from the sources of noise, placing the sound level meter and microphone at a height of approximately 1.5 m, closer to the identified worker's hearing position during work.

Statistical analysis

The Chi-square test has been applied to find out the test of independence and association between various symptoms and octave band center frequencies using statistical software SPSS for windows (version 16.0)

 Results and Discussion

This study assessed noise frequency components and sound pressure levels and their impact on the physical, physiological, and psychological aspects among workers exposed to noise in six industries. The detailed questionnaire used in this extensive study facilitates the extraction of required information such as the respondent's age, the nature of work, work period in the present organization, number of hours of daily exposure, etc. It was observed that the age distribution of the workers was: 16-25 years (4%), 26-35 years (40%), 36-45 years (37%), 46-55 years (16%), and one respondent was older than 56 years. It was observed that impairment of hearing was also related to age. A report [19] reveals that there is an age limit of 55 years for the onset of a detectable age-induced hearing loss. As most of the sampled subjects were younger than 50 years of age, age factor was observed to be insignificant on hearing problem. Almost all the sampled workers worked in association with machinery, which produces noise. These workers are exposed to noise for seven hours daily, six days a week. Around 67% of the workers have been working for more than ten years, 13% for 8-10 years, 5% for 4-6 years, 11% for 6-8 years, and 4% for up to 4 years. Questions such as sources of the noise heard, type of location and noise, when the noise is heard the most, revealed the characteristics of the noise, its origin, and nature. It was observed that most of the sources of noise were from rotating machines and industrial activities. Common noises identified were continuous, impulsive, hammering, cracker sounds, welding, and intermittent, etc. Noise characteristics were found to be different among surveyed sections in different industries, making it difficult to investigate its influence on effects. But according to the observations of Atherly and Martin, [20] for a noise level [21] indicate that the effects of noise on higher nervous activity differ not only as a result of the degree of its physical energy, but also according to the conditions of its generation. Also, on-off effects of brain activity are caused by regular or irregular intermittent generation of noise, which can be estimated with the alpha-waves of EEG.

In this study, the maximal SPL level observed was 108.1 dB(c) at 31.5 Hz OBF (Octave band center frequency). Although the duration of exposure is more or less the same among the sampled workers, noise characteristics were found to be different. Consideration of work stations, each work station consisting of a group of workers employed in similar physical work and environmental conditions, may perhaps give better results in evaluating noise effects. However, this issue is out of the scope of the present study. When enquired about their working in shifts, it was observed that all workers worked a full shift (seven hours) daily, alternating between morning and afternoon shifts. Out of 93 respondents, 84 were observed to be sensitive to noise. Out of the options mentioned to determine the source of noise: road traffic, neighbors, industry, exhaust fans, and vibration from motor vehicles encountered at home, in industry, and in the environment, all respondents identified industrial noise as the major one. Furthermore, they also reported to have repetitively come across industrial noise among the other noise sources. The response rates among subjects who were affected by industrial noise, are shown in [Table 1]. Except in the plywood industry, where the response rate was 60%, the response rates of the remaining industrial workers were > 80%. These results reveals the severe noise effect, the attitude towards noise, individual sensitivity, and beliefs among the sampled industrial workers who have been continuously exposed to noise for many years. [Table 2] shows the sampled working population observed to be annoyed due to continuous industrial noise exposure. Annoyance levels have been predicted based on the average scores on the annoyance scale bearing levels viz ., not at all, not very, rather, and very. It can be observed [Table 2] that the majority (83%) of the workers were found to be very aggravated by the noise in their working atmosphere, which is similar to the findings reported in a previous study. [22]

When asked about any hearing problems and whether they had undergone any audiometric testing, around 36% of the workers were found to be suffering from hearing problems. It was noticed that 125, 250, and 500 Hz OBFs (88-707 Hz) were the noise frequency components that prominently affected hearing among these subjects. This is supported by the results from the study [23] stated that about 16% workers were of the opinion that they had a slight hearing problem, after being exposed to noise levels > 85 dB(A) with dominant noise frequency characteristics at 40-160 Hz . Asking the workers whether they had taken any steps themselves to avoid noise revealed that only 33% of the respondents used earmuffs whereas 67% of the respondents expressed the inconvenience-irritability, physical discomfort, interference in interpersonal communication, and a reduced perception of the trouble associated with the machinery through abnormal sound. The possibility of the occurrence of cardiovascular disease linked to noise exposure revealed that 26% of the respondents answered, "yes". Out of this 26%, the majority of the workers (around 29%) who had cardiovascular problems, were found to be associated with a noise frequency of 500 Hz OBFs (354-707 Hz). They mentioned problems such as chest trembling, a burning sensation in the heart, high blood pressure, breathing problems, chest pain, frequent cough, etc. A past study [24] found that communal and industrial noise are possible contributing factors in the development of arterial hypertension, myocardial infarction, coronary risk factors, as well as vasospastic and atherosclerotic changes on arterial blood vessels of the lower extremities. Similar findings show that the correlation between noise and high blood pressure has been interpreted as evidence that noise is a cardiovascular risk factor. [25] The Inquiry about how often do the subjects come across health problems revealed that 40% of respondents have expressed never, 26% a few times a week, 20% a few times a month and 10% once in a year. When subjects were asked whether noise interference affected their work efficiency, around 36% of the respondents answered, "yes". Other factors that were cited as affecting work efficiency (besides noise) were age, work load, continuous online work, excess of heat in the working environment, improper food intake, weakness, etc.

Unusual physical tiredness was also assessed and around 40% of the respondents related tiredness to the noise. Apart from noise exposure, other reasons associated with tiredness were high temperature environments, especially near boilers and hot pressers, standing for seven hours a day, food style, age, etc. Heat, stress, and heavy workload have been found to be potential factors of increasing temporary threshold shift (TTS) in noise exposure. [26] Therefore, it can be concluded that the combined effects of workload and heat stress may enhance the effects of noise. Noise measurements were made by Selecting 1/1 octave band and C weighting network, because the dominant noise sources were observed to be in the low- and mid-frequency range. It has been observed that dB(A) does not give a true picture of the annoyance potential for low-frequency noise. [27] Another study supports the finding that dB(A) weighting is a poor predictor of annoyance due to low-frequency noise. [28] As the objective of the present study was the identification of noise frequency components and its associated symptoms, selection of weighting C network was not an issue and thus, out of the scope of this study.

[Table 3] shows the measured dominant noise frequency characteristics having maximal SPL in various sections of the six industries. (Dominant noise frequency is the frequency corresponding to the maximal sound pressure level.) A frequency band is said to be an octave in width when its upper band-edge frequency (f2) is twice the lower band-edge frequency (f1): f2 = 2 f1.Each octave band is named for the center frequency of the band, calculated as follows: fc = (f1f2) 1/2 , where fc = center frequency. Measured octave band levels of noise in various sections of different industries are depicted in [Figure 1],[Figure 2],[Figure 3],[Figure 4],[Figure 5],[Figure 6], showing maximum sound pressure levels, centered at octave band frequencies between 31.5 and 8000 Hz.

Statistical results

A significant association was observed between noise (in octave band center frequencies) and various physical, physiological, and psychological symptoms among industrial workers exposed to noise [Table 4]. These were identified by the workers from a list of possible symptoms (both related and unrelated to noise-related problems) mentioned in the questionnaire:

From [Table 4], it is evident that 'eyeball pressure' and 'backache' were observed to be highly associated (X2 = 85.75, P = 0.000) at 31.5 Hz OBF. However, symptoms such as 'chronic fatigue' and 'frustration' have no significance at 31.5 Hz OBF as the obtained chi-square values were found to be nonsignificant. The symptom of 'awakening from sleep' is observed to be highly associated with whereas symptoms such as 'pain in the neck,' 'frequent ear vibration,' 'chronic fatigue' are nearly associated to 63 Hz OBF. On the other hand, 'repeated ear pulsations,' 'breathing problem,' 'chest pain,' 'frustration,' and 'repeated headache' have not been associated with this frequency band. 'Eyeball pressure' symptom is significantly identified at 125 Hz OBF. However, symptoms of 'chronic fatigue' and 'frustration' were not significant at this octave frequency band. 'Eyeball pressure' is significantly associated whereas 'chronic fatigue,' 'nervousness,' and 'frequent awakening' are not associated with 250 and 500 Hz octave band center frequencies. 'Frequent ear vibration' is observed to be significant whereas 'eyeball pressure,' 'repeated headache,' and 'backache' are nearly significant in cases of noise exposure of 1 kHz OBF. Symptoms such as 'headache,' 'chronic fatigue,' 'nervousness,' 'frustration,' 'sleep disturbance,' and 'frequent awakening' were nonsignificantly associated with this frequency band. Symptoms such as 'headache' and 'repeated ear pulsation' are observed to be significantly associated with a noise exposure of 2 kHz OBF. However, 'pains in the neck,' 'chronic fatigue,' 'breathing problem,' 'nervousness,' 'frustration,' 'repeated headache,' 'backache,' and 'frequent awakening' have no significance for this octave band center frequency.

Accordingly, symptoms such as 'eyeball pressure,' 'backache,' 'awakening from sleep,' 'pain in neck,' 'frequent ear vibration,' and 'chronic fatigue' are associated with low OBFs of noise frequency characteristics ranging between 22 Hz and 176 Hz. The maximal SPLs at these OBFs in various sections of six industries were in the range of 76.6-108 dB(C) [Table 3]. Furthermore, in the mid-range OBFs (noise frequency characteristics ranging between 177 and 2828 Hz) the maximal SPLs were 81.2-101.9 dB(C). The symptoms observed to be associated with these noise frequency characteristics were headache, repeated ear pulsation, repeated headache, in addition to eyeball pressure, frequent ear vibration, and backache, which were also noticed with low OBFs. Some symptoms unrelated to noise were also identified by the subjects: pain in the neck, backache, eyeball pressure, breathing problem, frequent ear vibration etc. In most cases, neck pain may be a result of faulty posture, poor ergonomics at work, or fatigue of the muscles from repetitive activities using improper body mechanics. The muscles at the back of the neck get fatigued and the neck begins to hurt. Furthermore, in a noisy environment, an individual may adopt an abnormal posture to minimize noise-related discomfort, resulting in the straining of the back in such a way that the bones, ligaments, nerves, and muscles of the spine are compressed or stretched, which could finally result in backache. Eyeball pressure plays a role in noise-induced hearing loss because the visual pathways, vestibular, and auditory pathways have various interconnections in the central nervous system.

Fluid in the anterior chamber behind the cornea nourishes the eye. Glaucoma or eye pressure occurs when this fluid drains too slowly and as the fluid builds up, the pressure inside the eye rises. Exposure to dust, chemical fumes, and smoke in an industrial working environment might also be causes. One possible symptom connected to breathing problems might be pulmonary fibrosis-a hardening of the lungs caused by fiber-like scarring-is a condition that progressively impairs one's ability to breathe. Ear vibration is closely linked to other symptoms such as tinnitus, ringing, roaring, buzzing, or clicking sounds in the ears. This symptom is associated with hearing problem and occurs in an individual exposed to continuous loud noise, which can damage the delicate cells in the inner ear and lead to noise-induced hearing loss.

These symptoms that were unrelated to noise exposure, were also similar to the findings identified by the workers exposed to low frequency noise in an epidemiological survey of an affected and a control group as reported in a review study. [17] The association of identified important psychological symptoms with octave band noise frequencies among industrial workers has been depicted in [Table 5]. It can be observed that 'irritability' was found to be highly significant (?2 = 9.480, P = 0.009) at 31.5 and 63 Hz OBFs (?2 = 23.138, P = 0.000) and also significant (?2 = 6.000, P = 0.05) at 2 kHz octave band center frequencies. The present study has a few limitations: i) although the purpose of the study and assurance of confidentiality were explained to the industrial workers, severe health-related information might not have been revealed due to the fear of losing their jobs, ii) precise identification and analysis of the noise frequency spectrum can be further improved using sophisticated SLM by selecting narrower frequency bands, iii) detailed information about the socioeconomic status, habits, previous work history (although around 80% workers have been working from the past 8-10 years and more) was unavailable. Despite these limitations, this pilot study has laid down the framework for further investigations which can overcome the limitations of the present study.


This study has helped in identifying major noise frequency components and SPLs in six major industrial sectors. Low and medium octave band center frequency characteristics were found to be dominant. Furthermore, data suggest that the identified physical, physiological, and psychological symptoms are associated mostly with these frequency components. The attempt to determine the association between noise frequency components to which the subjects were exposed and various symptoms, succeeded to some extent. Considerations of additive stressors as mentioned in the limitations would further support the study to assess their influences. Individual noise exposure assessment along with additional medical monitoring and testing procedures would enhance and substantiate the findings of this study. This study demonstrates that noise frequency components are also one of the important parameters to be considered in evaluating health effects. This study provides a promising ground for future follow-up evaluation of the association of noise frequency components with medical symptoms.


The authors would like to thank Dr. H R Rajmohan, Deputy Director, Dr. Ravichandran Technical officer and staff, Regional Occupational Health Centre (south), Bangalore, Karnataka, India and industrial workers who participated in the study, for their support and assistance during noise measurement. We also thank Dr. Lancy D'souza, department of Psychology, University of Mysore, India, for the supporting statistical analysis in the study.


1Kryter KD, editor. Physiological, psychological and social effects of noise. NASA Reference Publication; 1984. p. 179-93.
2Mokhthar M, Kamaruddin S, Khan ZA, Mallick Z. A study on the effects of noise on industrial workers in Malaysia. Jurnal Teknologi 2007;46:17-30.
3Tempest W. Noise in industry. In: Tempest W, editor. The Noise Handbook, London: Academic Press; 1985. p. 179-93.
4Ising H, Kruppa B. Health effects caused by noise: Evidence in the literature from the past 25 years. Noise Health 2004;6:5-13.
5Stansfeld SA, Matheson MP. Noise pollution: Non-auditory effects on health. Br Med Bull 2003;68:243-57.
6Eleftheriou PC. Industrial noise and its effects on human hearing. Appl Acoust 2002;63:35-42.
7Samuel M, Yitzhak F. The interactive effect of chronic exposure to noise and job complexity on changes in blood pressure and job satisfaction: A longitudinal study of industrial employees. J Occup Health Psychol 2001;6:182-95.
8Miller JD, editor. General physiological effects of noise. In: Carlerette EC, Friedman MP, editors. Handbook of Perception, New York: Academic Press; 1978. p. 677-84.
9Osada Y. An overview of health effects of noise. J Sound Vibration 1988;127:407-10.
10Jonsson A, Hansson L. Prolonged exposure to a stressful stimulus (noise) as a cause of raised blood-pressure in man. Lancet 1977;1:86-7.
11Rylander R. Physiological aspects of noise-induced stress and annoyance. J Sound Vibration 2004;277:471-8.
12Van Amelsvoort LG, Scouten EG, Maan AC, Swenne CA, Kok FJ. Occupational determinants of heart rate variability. Int Arch Occup Environ Health 2000;73:255-62.
13Sueyoshi S, Miyazaki Y, Morikawa T. Physiological and psychological responses to prolonged light floor-impact sounds generated by a tapping machine in a wood house. J Wood Sci 2004;50:424-97.
14Ising H, Braun C. Acute and chronic endocrine effects of noise: Review of the research conducted at the institute for water, soil and air hygiene, Noise and 2000;2:7-24.
15Leventhal HG, editor. Low frequency noise and vibration and its control, Proc. 11th Int. Conf., Journal of Low Frequency Noise, Vibration and Active Control vol. 234. 2004. p. 199-205.
16Berglund B, Hassmen P, Soames Job RF. Sources and effects of low frequency noise. J Acoust Soc Am 1996;99:2985-3002.
17Leventhal G, Pelmear P, Benton S. A review of published research on low frequency noise effects: A report. Published by the Department for Environment, Food and Rural Affairs; May 2003.
18Moorhouse A, Waddington D, Adams M. Proposed criteria for the assessment of low frequency noise disturbance. Prepared for Department for Environment, Food and Rural Affairs; February 2005.
19Celik O. Yalcin S, Ozturk A. Hearing parameters in noise exposed industrial workers. Auris Nasus Larynx 1998;25:369-75.
20Ahmed HO, Dennis JH, Badran O, Ismail M, Ballal SG, Ashoor A, et al . Occupational noise exposure and hearing loss of workers in two plants in eastern Saudi Arabia. Ann Occup Hug 2001;45:371-80.
21Saito K. Effects of noise on higher nervous activity. J Sound Vibration 1988;127:419-24.
22Bedi R. Evaluation of occupational environment in two textile plants in northern India with specific reference to noise. Ind Health 2006;44:112-6.
23Prasanna Kumar GV, Dewangan KN. Noise exposure in oil mills. Indian J Occup Environ Med 2008;12:23-8.
24Jovanovic J, Popovic V, Milosevic Z, Javanovic M. Cumulative effects of communal and industrial noise on cardiovascular system. The Scientific Journal Facta Universitatis 1997;4:57-61.
25Singh AP, Rai RM, Bhatia MR, Nayar NS. Effects of chronic and acute exposure to noise on physiological functions in man. Int Arch Occup Environ Health 1982;50:169-74.
26Chen CJ, Dai YT, Sun YM, Lin YC, Juang YJ. Evaluation of auditory fatigue in combined noise, heat and workload exposure. Ind Health 2007;45:527-34.
27Persson K, Bjorkman M. Annoyance due to low frequency noise and the use of the dB (A) scale. J Sound Vibration 1988;127:491-7.
28Persson K, Rylander R. The prevalence of annoyance and effects after long- term exposure to low frequency noise. J Sound Vibration 2001;240:483-97.