2008-5435/13/52-64-70
INTERNATIONAL JOURNAL OF OCCUPATIONAL HYGIENE
Copyright © 2013 by Iranian Occupational Health Association (IOHA)
IJOH 5: 64-70, 2013
ORIGINAL ARTICLE
Mental Processing of Human Subjects with Different
Individual Characters Exposed to Low Frequency
Noise
MOHAMMAD JAVAD JAFARI, and MARZIEH KAZEMPOUR*
Department of Occupational Health Engineering, School of Health, Shahid Beheshti University of Medical Sciences,
Tehran, Iran.
Received December 22, 2012; Revised Murch 3, 2013; Accepted Murch 27, 2013
This paper is available on-line at http://ijoh.tums.ac.ir
ABSTRACT
Low Frequency Noise (LFN) is ubiquitous in both occupational and general environments. Mental
performance of subjects exposed to LFN is expected to be impaired. Individual characters seem to play a
significant role in this process. This study aimed to evaluate the effects of moderate levels of LFN (20–
200 Hz) on mental performance. Fifty-three subjects (in two groups) were exposed to low and flat
frequency character noise each at the same sound pressure levels of 45 dBA. Personality traits of extrointraversion and neuroticism were determined using the Eysenck Personality Questionnaire. The subjects
were asked to carry out two different mental tasks while being exposed to the noise. Subjective reports
were collected using questionnaires. Concentration problems, fatigue and noise annoyance during the
experiments were measured using a numeric ten-point self-rating scales. The results revealed that LFN
impaired mental performance. There was no significant effect of LFN on the accuracy of mental
processing, compared to the reference noise condition. Extrovert subjects performed significantly faster,
compared to the introverts and subjects that exposed to LFN performed slower than reference noise.
More stable personality, with extroversive tendencies is likely for better adaptation to LFN during mental
performance, compared to people with the opposite personality traits. Correlation analysis showed a
highly significant negative relation of extroversion and noise annoyance during mental processing.
Keywords: Low Frequency, Reference Noise, Mental Performance, Extrovert, Introvert, Neuroticism
INTRODUCTION
Low Frequency Noise (LFN), is usually described as
a broadband noise in which the low frequencies from 10
Hz to 200 Hz, are dominant. It can be considered as an
occupational hazardous agent, which may affect
sensitive individuals [1, 2]. In addition to the control
room operators and office workers, which are subjected
to LFN, normal occupants of today’s living
environments, may also suffer from it [3, 4]. The
significance of LFN was highlighted by WHO in its
* Corresponding author: Marzieh Kazempour, E-mail: [email protected]
Published online: April 8, 2013
community noise documents [5]. Ventilators, pumps,
compressors, diesel engines, gas turbines, power
stations and transportation vehicles could be considered
as the common sources of LFN. Both infrasonic and low
audible frequencies are included in LFN, therefore,
almost all impairments attributed to them are expected
to be included in LFN as well [6].
Since unwanted environmental stimulus taxes the
subjects mental capacity, therefore noise exposure is
expected to impair his or her mental capacity leading to
a higher risk of accident or injury. The relationship
between noise exposure and the performed task seems
to be sophisticated. The influences of different tasks and
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Mental Processing of Human Subjects with Different Individual Characters
noises on mental performance are not expected to be
similar. Some authors believe that the performance
degradation depends largely on the character of the
noise rather than its pressure level. The obligatory
access of noise to memory could be the main reason [7].
The results of previous investigations, mainly laboratory
experiments, were equivocal [2, 8]. LFN can be
perceived as annoying and adversely affecting the
performance. This is true at relatively low sound
pressure levels of about 40 dBA particularly, when
mentally demanding tasks are being carried out [2, 9].
The effect of LFN on human behavior is not well
known. LFN centered at 40 and 100 Hz (at 25 dB above
the individual’s hearing threshold) led to more errors in
a dual task condition than traffic noise with 90 dB Lin
and silence [10].
The effects were especially pronounced during the
last 10 min of the total 30-min exposure. Furthermore,
Benton and Robinson [11] found support for
performance impairment caused by a narrowband (1
Hz) LFN at the sound pressure levels of 70 and 95 dBC,
as compared to speech and white noise (20–20 kHz).
The latter exposures were matched for loudness against
the narrowband LFN. The LFN was also rated as more
annoying. In contrast, there was no difference in
subject’s performance on a complex task when working
in a LFN (one-third-octave band centered at 125 Hz,
having SPL of 90 dBA) as compared to noise with a
high-frequency character (SIL=90 dBA) or ambient
noise (55 dB SPL) [12].
Berglund believes that Since LFN includes both
infrasonic and low audible frequencies thus, it is
expected to have most of effects attributed to infrasound
[6]. The primary and the most frequent effect of LFN on
human subjects is expected to be annoyance [7]. Noise
related annoyance is a combination of physical and
psychological factors. It is difficult to accurately predict
and assess the degree of annoyance or disturbance
initiated by noise in individuals. The same noise may
result in very different responses in different peoples,
depending on their cultural factors, activity at the time
of exposure, attitude to the noise source, subject's
sensitivity to noise, controllability of the stressor, and
other individual differences [6, 13]. Annoyance caused
by noise is influenced by a number of acoustical and
individual characteristics. Individual characteristics may
include personal attitude towards the source of noise
(noise acceptance), the degree of adaptation, hearing
sensitivity and individual sensitivity. Some individuals
exposed to LFN complain from irritation, anxiety and
stress [14]. Noise annoyance and LFN sensitivity do
not have a significant correlation with age and sex [2].
Several studies have shown the differences in the degree
of annoyance caused by exposure to low and medium or
high frequency noise at the same pressure levels. The
annoyance experienced from LFN seems to be higher
than that from noise without dominant low frequency
components. Moreover, that effect is frequently present
at relatively low-pressure levels that comply with
guidelines based on ordinary environmental noise [8].
Published online: April 8, 2013
ijoh.tums.ac.ir |
65
Exposure to LFN is often accompanied by many
subjective effects such as tiredness, feelings of
irritation, unease or stress, headache, pulsating feeling
or feeling of pressure on the eardrum, nausea or
dizziness. According to different studies [6, 8, 15], some
of these symptoms, especially fatigue, lack of
concentration, headache and irritation, can reduce
working capacity.
The personality trait of intro-extroversion and
neuroticism has been denoted as the relevant individual
factors for the effects of noise on mental performance.
A theoretical basis for the relation between introversion
as well as extroversion and mental performance of
subjects exposed to noise may be found in Broadbent's
arousal theory of stress. According to this theory,
neurotic persons might show enhanced provocation in
stress [16]. It is a well-known fact that the introverts
show higher basic levels of psycho-physiological
activity [17]. According to arousal theory, this may lead
to more pronounced reactions to noise and over arousal
during mental performance in noisy locations. There are
reported evidences that support this concept [18]. Some
traits appear to have a consistent impact on performance
(depending on the anxiety-inducing nature of the testing
session). The relationship between intro-extroversion
and neuroticism is very unstable depending largely on
factors such as the arousing nature of the testing
situation and the motivation of the participant. Wolf and
Ackerman (2005) showed test measures and age
moderated the correlation between extraversion and
intelligence [18]. The relationship between personality
and work is expected to be the same. Mood, time-ofday, task complexity and the rate of task anxiety are the
factors that potentially moderate the relationship
between personality and task performance [19]. Not
many studied the relationship between mental
performance and personality traits of subjects exposed
to LFN.
There are large inter-individual differences in the
response to noise. Compared to less sensitive subjects,
subjects with a high subjective sensitivity to noise have
been found to show greater performance impairments
and rate their annoyance higher when carrying out tasks
in noisy conditions. The categorization of sensitivity to
low-frequency noise could be distinguished from the
categorization of sensitivity to noise in general [9].
Thus, subjects sensitive to low-frequency noise might
be a risk group when working during exposure to lowfrequency noise [20].
The objective of this work was to study the
influences of LFN in mental performance of individuals
considering their personality traits of intro-extroversion,
neuroticism, annoyance and fatigue.
M ATERIAL AND METHODS
Fifty-three young subjects were exposed to 45 dBA
Low Frequency and Reference Noises (RFN) while
performing mental tasks. The test sessions started at 2
pm and lasted until about 6 pm. The mental
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Jafari, and Kazempour
Fig 1. 1/3-Octave band frequency spectrum of LFN and RFN used during test sessions, measured at the position of subjects’ head
performance of the subjects was evaluated using search
and memory, proof reading and mental arithmetic tests.
Thirty volunteer male and twenty-four female
students with an average age of 23.9± 3.2 years and
normal hearing (<25 dBA) were selected after an
audiometric screening using Mevox ASB15
Audiometer. The subjects were honored a present for
their participation. The procedures were explained
implicitly prior to the test sessions to minimize
subjective influence caused by the subjects’ attitude to
noise, motivation and level of expectation. The subjects
were trained the procedures and tried a short version of
the performance tasks for about 20 min while being
exposed to the RFN at a sound pressure level of 35
dBA. The subjects were band from wearing a watch
during the test sessions to relief stress. The subjects
were advised to eat only a light meal no later than 2 hrs
before the experiment, as performance may have been
impaired immediately after the consumption of a heavy
meal [12].
Two questionnaires were applied after the last test
session to assess the sensitivity of the individuals to
Low Frequency and General noise. Weinstein noise
sensitivity evaluation questionnaire was used to assess
general sensitivity to noise [21]. The subjects were
asked to answer the questionnaire used by PawlaczykLuszczynska et al. (2005) to assess sensitivity to LFN
[22].
All items had five response alternatives ranging
from “do not agree at all” to “agree completely”, graded
from 1 to 5. However, the first and third items were
scored in negative direction before responses were
summed. Subjects were categorized as high (LFN+) or
low (LFN-) sensitive to LFN based on their
questionnaire scores. The higher scores referred to, the
higher sensitivity. Thus, subjects who obtained at least
median score (≥9 points) were classified as high
sensitive to LFN. The others were categorized as low
sensitive.
The experiments were conducted in an acoustic
chamber. The noise was transmitted through a set of
Published online: April 8, 2013
loudspeakers placed in the corners of the room and
concealed behind curtains. LFN was the simulation of
the noise in industrial control rooms (Fig. 1). The RFN
was the broadband noise of a predominantly flat
frequency quality. The pressure levels of both noises
were kept at 45 dBA. The corresponding C-weighted
sound pressure level of the RFN and the LFN were 53
dBC and 72 dBC respectively.
Prior to the exposure, the sound pressure levels of
noises were calibrated using 2238 Bruel & Kjaer (B&K)
pressure microphone, 4188 B&K artificial ear, SV01A
SVANTEK microphone preamplifier, SVAN 912E
sound and vibration analyzer and 4231 B&K sound
level calibrators.
Three performance tasks corresponding to the
relevant assignments and reflecting the relevant
demands in control rooms were selected. The search and
memory task (SAM1) were of a monotonous and routine
type character, chosen to evaluate tiredness, as well as
sustained and selective attention. The proofreading task
was a somewhat more mentally demanding verbal task,
requiring sustained attention and concentration. The
Mental Arithmetic task (MA) was used to estimate
parallel mental processing.
The low-memory load version of the search and
memory task (SAM1) was a low demanding task that
previously has been used to assess effects on sustained
attention when performing the task in different
combinations of noise, night work and meals [12]. The
task involved searching through lines of 60 singlespaced pseudo-random capital letters, searching for the
occurrence of one target letter defined at the beginning
of each line. There were zero to three targets per line
and the lines were arranged in sets of six, with four sets
on each page. The number of lines searched during a
period of 10 min and the percentage of correct and
number of erroneous target letters were recorded.
The proofreading task [23] was a moderately
demanding verbal task, requiring sustained attention and
concentration. The subject read a printed text for 25
min. The task was to mark typographical and contextual
ijoh.tums.ac.ir |
Mental Processing of Human Subjects with Different Individual Characters
67
Table 1. The results of the proofreading task (mean ± SE)
Performance
No of lines read
Typographical marks/line
Contextual marks/line
Correct marks/line
Erroneous marks/line
Total marks/line
Number of lines read
Typographical marks/line
Contextual marks/line
Correct marks/line
Erroneous marks/line
Total marks/line
Introverts
n=21
85.9±6.4
0.15±0.007
0.07±0.006
0.22±0.011
0.008±0.001
0.23±0.011
Neuroticism
n=23
85.9±6.4
0.15±0.007
0.07±0.006
0.22±0.011
0.008±0.001
0.23±0.011
LFN
Extroverts
n=32
99.9±6.4
0.19±0.12
0.08±0.007
0.28±0.017
0.007±0.0025
0.29±0.016
Stability
n=30
99.9±6.4
0.19±0.12
0.08±0.007
0.28±0.017
0.007±0.0025
0.29±0.016
errors in the text. The number of lines read, the number
of typographical errors detected, the number of
contextual errors detected and the total number of
correct marks, erroneous marks and total number of
marks were recorded.
Personality trait of intro-extroversion was estimated
with the Eysenck Personality Questionnaire [24, 25],
comprising 24 written questions, with given binary
answers of agreement, concerning the intro-extroversion
and neuroticism tendencies in behavior. Each answer
was given 0 or 1 point on the above and lying. The
answers were considered reliable if the sum of the
points on the lying scale was under eight.
Before and directly after the test session, the subjects
completed questions on the degree of different
symptoms, such as headache, tiredness, lack of
concentration, irritation, sleepiness, pressure in the
eardrum or head and nausea. The answers used Likert
scale. Furthermore, after the test session, the subjects
answered how performance had been affected by noise,
temperature or lighting, with seven response alternatives
ranging from “major improvement” to “major
impairment”. Questions were also asked on how
interesting the tasks were, with five response
alternatives ranging from “very interesting” to “very
boring” and how difficult the tasks were, with five
P-value
0.29
0.003
0.39
0.005
0.33
0.003
P-value
0.29
0.003
0.39
0.005
0.33
0.003
Introverts
n=21
84±6.3
0.16±0.007
0.07±0.006
0.23±0.009
0.005±0.0014
0.24±0.01
Neuroticism
n=23
80.3±6.08
0.16±0.007
0.07±0.006
0.23±0.009
0.005±0.0014
0.24±0.01
RFN
Extroverts
n=32
97.18±7.08
0.17±0.008
0.08±0.009
0.25±0.01
0.006±0.001
0.26±0.01
Stability
n=30
81.2±6.1
0.17±0.008
0.08±0.009
0.25±0.01
0.006±0.001
0.26±0.01
P-value
0.090
0.27
0.21
0.17
0.36
0.24
P-value
0.69
0.27
0.21
0.17
0.36
0.24
response alternatives ranging from “very difficult” to
“very easy”. One question was also posed on how much
effort the tasks required, with five response alternatives
ranging from “not at all” to “extremely”.
The annoyance was also assessed before and
immediately after completing the exposure using an 11score graphical rating scale with verbally labeled two
poles (not annoying and very annoying).
All analyses were two-tailed, and the P-values of
<0.05 were considered statistically significant. The Pvalues in the analyses of variance were based on
degrees of freedom corrected with Greenhouse–Geisser
epsilon, when appropriated. The statistical analyses
were carried out using the SPSS software (version 18.0
for Windows).
RESULTS
In proof reading task (Table 1) for the number of
typographical marks per line, there was a significant
differences between Introverts and Extroverts (P–
value=0.003) in LFN. Extroverts had also a tendency to
make more correct marks per line during the RFN
condition (P–value=0.005). For total marks per line,
there were significant differences between Introverts
and Extroverts in LFN (P–value=0.003). These
Table 2. The results of the search and memory task (SAM1) (mean ± SE)
Performance
No of lines searched
Correct marks (%)
No of lines searched
Correct marks (%)
Published online: April 8, 2013
Introverts
n=21
9.2±0.56
71.4±2.8
Neuroticism
n=23
10.7±0.57
69.9±3.5
LFN
Extroverts
n=32
11.7±0.58
72.5±3.9
Stability
n=30
11.1±0.75
75.5±2.5
P-value
0.004
0.64
P-value
0.61
0.23
Introverts
n=21
9.5±0.65
70.2±3.06
Neuroticism
n=23
10.85±0.58
70.09±3.7
RFN
Extroverts
n=32
11.8±0.59
72.4±3.3
Stability
n=30
11.2±0.79
71.6±2.8
P-value
0.016
0.82
P-value
0.69
0.73
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Jafari, and Kazempour
Fig 2. The relationship between LFN sensitivity and personality traits
Fig 3. Subjective sensitivity to noise in general and to LFN in the study group
Fig 4. Average reported perception of irritation for the RFN condition and the LFN condition
differences in RFN and in Neuroticism and stable
persons were not significant. There was also a tendency
to make more erroneous marks per line during the LFN
condition compared to the RFN condition. There was a
tendency to read a greater number of lines during the
low-frequency noise condition too.
The results of search and memory are shown in
Table 2. The analysis showed that Extroverts read
significantly more number of lines searched (P–
value=0.004) than Introverts but the differences in LFN
is more significant (P–value=0.016). The number of
lines the subjects searched and correct marks were about
equal in both noise conditions. These differences are in
the Neuroticism and Stable persons but the differences
are not significant.
The average value of annoyance rating after LFN
was about 25 and the value before LFN was 11. In spite
Published online: April 8, 2013
of this difference, RFN had fewer margins than LFN. It
was about eight. This means that annoyance from Low
Frequency Noise was more perceptible.
The results showed that more stable personality,
with extroversive tendencies is likely for better
adaptation to LFN during mental performance,
compared to people with the opposite personality traits.
As it is depicted in Fig. 2, extroversion and Stabile
persons are less sensitive to LFN.
In the study group, 29 subjects were recognized as
high sensitive to LFN and 30 subjects as high sensitive
to noise in general, but the two sensitivity distributions
were not identical. This means that higher sensitivity to
LFN was not necessarily connected with higher
sensitivity to noise in general.
Subjects exposed to LFN reported a higher value on
irritation directly after but not before the test session
Mental Processing of Human Subjects with Different Individual Characters
compared with subjects exposed to the RFN (Fig. 4). No
significant effect of noise condition was found for the
oth Noise annoyance in the LFN condition was
positively correlated (but not significant) to lack of
concentration, sleepiness and pressure in the eardrums,
while these relationships were not found for the RFN
condition. No other relationships that reached
significant level were found.
DISCUSSION
The results of present study pointed out that
Extroverts were less annoyed and had better
concentration during mental performance in noise, as
compared to introverted subjects. This is in accordance
with the findings of Belojevic et al. (2001), concerning
the poorer performance in noise among the Introverts
compared to Extroverts [26]. This finding is also
congruent with Eysenck's (1967) [24] suggestion that
Extroverts and Stable persons are able to tolerate
significantly higher levels of stimulation, such as noise,
for better performance. This assumption was supported
in an experiment, which showed that Extroverts and
stable persons regularly selected higher noise intensities
as optimal, compared to Introverts [25].
The study showed acceleration in performance of
tasks by Extroverts exposed to LFN compared to RFN.
This finding is consistent with Bengtsson (2004) [20].
Due to practice and learning, subjects working with the
search and memory task (SAM1) generally become
faster over time [12]. More lines were searched in the
RFN condition than in the LFN condition. An
explanation for this effect could be decreased attention
and/or increased tiredness, during LFN exposure,
although increased tiredness was not supported by the
subjective reports of tiredness. A second explanation
may be that the LFN impaired the ability to learn.
Support for this effect was found in a previous study
[19], in which, a larger decrease in response-time over
time was found during work with a verbal grammatical
reasoning task in the RFN, as compared to the LFN
condition.
Proof-reading tasks has been done in many studies
already and have been used to measure effects of noise
exposure, but rather inconsistent results have been
found on number of different marks made and how fast
the text was read. In this study, subjects exposed to the
RFN found more typographical marks per line. Subjects
in the LFN condition read the text somewhat faster and
seemed to make less erroneous marks per line. The
larger amount of number of lines read and lower amount
of erroneous marks made by the subjects working in the
LFN condition could indicate that they performed the
task better in this noise condition. On the other hand, as
the subjects at the same time failed to find the errors in
the text they were instructed to find, another
interpretation of the results is that the subjects treated
the text material less thoroughly when working with the
task during exposure to LFN. Strategies to cope with
extra load from e.g. noise exposure could be to work
Published online: April 8, 2013
ijoh.tums.ac.ir |
69
more rapidly and less thoroughly, to work at a slower
working rate or to add extra effort to complete the task
correctly [20]. Another strategy could be the reverse,
where a person continues to work as before even though
the working condition has changed.
Concerning the speed of performance, there were no
significant differences between extro-intraversion and
neuroticism in the speed of performance in RFN
condition. Extroverts and Stable persons performed
significantly faster in RFN, compared to LFN
conditions. It means Extroverts and stable have more
number of completely correct in RFN condition. This
finding is congruent with Belojevic et al. (2001) [26].
Eysenk suggested that those classified as Introverts have
a lower optimum arousal threshold and therefore do not
need much stimulation before passing their optimum
functioning level. Those who are Extroverts and Stable
have higher optimum arousal thresholds and therefore
tend to seek arousal or stimulating situations [24]. To
improve the comparative and predictive validity of this
experimental study, we used the concept of work
efficiency in noise, which is the ratio between objective
results to subjective cost of adapting to noise during
performance. Lower cost leads to a larger spare mental
capacity. This could be very important in real
occupational settings, where in addition to much longer
exposure to noise than in a laboratory experiment,
workers must cope with other stressful factors. A
significant correlations exists between sensitivity to
LFN and deep mental process (r=0.33, P=0.01) [277].
In numerous experiments Introverts and Neuroticism
persons have showed higher sensitivity to noise during
mental performance compared to Extroverts and Stable
persons, while Extroverts and Stables often cope with a
boring task even by requesting short periods of noise
during performance. Correlation analyses have regularly
revealed a highly significant negative relation between
extroversion and noise annoyance during mental
processing. People with high noise sensitivity may be
prevented from achieving the same work results as other
people in noisy environment, thus leading to
psychosomatic, neurotic or other difficulties. Positive
relation between noise annoyance and subjective noise
sensitivity might be very strong. Our results have
shown, after matching with the results of other relevant
studies, that more stable personality, with extroversive
tendencies and with a relatively lower subjective noise
sensitivity measured with standard questionnaires, may
be expected to better adaptation to noise during mental
performance, compared to people with opposite
personality traits.
CONCLUSION
In this experimental study, Extroverts showed better
mental performance in noise than Introverts. Extroverts
performed faster, while Introverts had more pronounced
subjective effects of annoyance, poor concentration and
fatigue during mental performance in noise compared to
quiet conditions.
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It is suggested that introversion may be regarded as a
risk factor for work in LFN environment where mental
performance is dominant. This might decrease
employees' annoyance reactions to LFN, preserve and
improve their productivity and psychosomatic health.
ACKNOWLEDGEMENT
This paper is extracted from Kazempour thesis
supervised by Dr Jafari. The authors wish to appreciate
Shahid Beheshti University of Medical Sciences for
their financial support. The authors announce no
conflict of interests.
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