Hearing protectors: State of art and emerging technologies

Transcription

Buenos Aires – 5 to 9 September, 2016
st
Acoustics for the 21 Century…
PROCEEDINGS of the 22nd International Congress on Acoustics
Plenary Lectures: Paper ICA2016-895
Plenary presentation
Hearing protectors: State of art and emerging
technologies
Samir Gerges
Federal University of Santa Catarina, Mechanical Engineering, Florianopolis, SC, Brazil,
Federal Institute of Santa Catarina, Mechatronic, Florianopolis, SC, Brazil
[email protected]
Abstract
In many industrial and military situations, it is not practical or economical to reduce ambient
noise to levels that present neither a hazard to hearing nor annoyance. In these situations,
personal hearing protection devices are capable of reducing the noise by up to around 35 dB.
Although the use of a hearing protector is recommended as a temporary solution until action is
taken to control the noise, in practice, it ends up as a permanent solution in most cases.
Therefore, hearing protectors must be both efficient in terms of noise attenuation and
comfortable to wear. Comfort in this case is related to the acceptance of the user to wear the
hearing protector consistently and correctly at all times. The purpose of this paper is to review
the stat of art for the need to develop methods to address three important topics not sufficiently
treated in the published literatures: Detection of outliers and their effect on the noise attenuation
measurements, uncertainty in the measurement results of Hearing Protectors Noise Attenuation
and quantification of comfort by measurement the contact pressure between the users and
earmuff.
Keywords: hearing protectors, measurement uncertainty noise attenuation
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22 International Congress on Acoustics, ICA 2016
st
Hearing protectors: State of art and emerging
technologies
1 Introduction
When workers are exposed to sound levels in excess of occupational levels or action limits, the
first action is to reduce the noise at the source and or pathway (engineering controls). When
sound levels cannot be reduced to less than 85 dBA for an 8-hour Time Weighted Average
(TWA) or through source and or pathway control(s), hearing protection should be used to
protect workers from occupation-related hearing loss. Choosing the right kind of hearing
protective device and correct use are essential to protection from hazardous noise exposures.
Hearing loss is a function of exposure time, the average sound level, and the peak level of very
loud sounds. Exposure to excessive noise from industrial machinery, heavy construction
equipment and vehicles, power tools, aircraft, gunfire, motorcycle and auto race tracks, dental
drills, sporting events, fireworks, rock concerts, marching bands, and music from a player's own
instrument or nearby instruments can cause hearing loss depending on the level and duration of
the noise. Some persons are more susceptible to hearing loss from high-level sound than
others.
Some workers obviously need high-attenuation earplugs. Shipbuilders, flight crew who stand
behind jet aircraft on the flight deck, and army tank operators usually fall in this category. Such
individuals can't get enough attenuation for proper protection even with plugs and earmuffs
combined. But, many industrial workers can be adequately protected with as little as 10 dB of
attenuation: the majority of eight-hour equivalent noise exposures fall between 85 and 95 dB.
Therefore, hearing protectors must be both efficient in terms of noise attenuation and
comfortable. The purpose of this paper is to review the stat of art for the need to develop
methods to address three important topics not sufficiently treated in the published literatures:
Detection of outliers and their effect on the noise attenuation measurements, uncertainty in the
measurement results of Hearing Protectors Noise Attenuation and quantification of comfort by
measurement the contact pressure between the users and earmuff cushion.
2 Detection and contribution of outliers for subjective
measurements of noise attenuation of hearing protectors
by REAT method
Measuring the noise attenuation of hearing protector devices (HPDs) using the REAT “Real-ear
Attenuation at Threshold” method [1,2] is based on subjective measurements, where each
subject determines their open (without HPD) and closed (with HPD) threshold levels. The
subjective determination of the threshold levels shows a high variation between subjects even
when they are qualified and familiarized with the method used to determine these threshold
levels, as required by the relevant standard. Some subjects pay greater attention and can
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determine their threshold with more accuracy than others. Some subjects simply do not pay
attention and answer randomly depending on their mood and mental condition on that day. This
section shows a methodology to observe the statistical distribution and quantify the contribution
of each subject to the final single number Noise Reduction Ratio NRRsf. Eliminating a few
subjects (the outliers) increases the NRRsf and reduces the variability of the measurements
(from around ± 4 to ±1). The results for the measurement of 20 different brands of pre-molded
earplugs are reported as a case study.
This section describes how to identify these outlier subjects [3], that is, those with very different
results compared with most of the subjects, and investigates the effect of eliminating them on
the final NRRsf value. In a real situation in the field, most HPD users receive training on each
type of device and they are aware of the risk of permanent hearing loss if the HPD is not
properly fitted and used throughout all work shifts. Therefore, the presence of these outliers can
inhibit an evaluation of the real situation and it may be useful to consider their elimination from
the final results in order to obtain a truly representative sample.
The Real-Ear Attenuation at Threshold (REAT) method is the gold-standard method, most
commonly used and accepted worldwide for the measurement of hearing protector noise
attenuation. This is a subjective measurement where the subjects determine their own threshold
levels (with and without an HPD). The accuracy of this measurement is strongly dependent on
the subject’s perception of the sound level at the ear and each subject has to concentrate to
determine their own threshold level. Considering that the subjects are paid, earning between 10
to 50 USD for each test, there is no guarantee that the subject has properly determined their
threshold level. Some subjects pay greater attention than others and some may have work
and/or educational experience which allows them to provide better results. Therefore for each
hearing protector brand measurement, especially for plug-type devices (which are more difficult
to fit than earmuffs) there are sometimes a few subjects (generally not more than five out of
twenty) who show low accuracy in determining their threshold levels and this can result in large
variations in the NRRsf value. In this paper, the results obtained for a pre-molded earplug
brands, using the subject fitting method (B) of ANSI S12.6-2008, based on the evaluations of 20
subjects, are reported and analysed.
Figure 1 shows the results for the bootstrap statistical analysis, considering 10 subjects, with
100,000 repetitions. The statistical distribution for the NRRsf shows a complex distribution, with
four peaks for a range of NRRsf values of 11 to 20. In this case NRRsf=15 with a standard
deviation of ±3 dB.
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Figure 1: The statistical distribution of NRRsf and contribution of subject 3 and 14
The Crystal Ball software was used to evaluate the sensitivity of the result with respect to each
subject. Figure 1 shows that Subject 3 contributes 71.7 % to the NRRsf value and Subject 14
contributes 14%.
On removing Subjects 3 and 14 and recalculating the statistical distribution, a new distribution,
which is very close to Gaussian, is obtained, as shown in figure 2, and the NRRsf value
increased from 15 to 19 dB, while the standard deviation decreased from ±3 to ±1 dB.
It shows clearly that by observing the statistical distribution, calculated for each group of 10
subjects and repeated 100,000 times, it is possible to detect the extent to which the results
deviate from a Gaussian distribution. Crystal Ball software was then used to identify the
contribution of each subject to the NRRsf. With the removal of only two subjects, the NRRsf
increased from 15 to 19 dB and the standard deviation decreased from 3 to 1 dB.
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Figure 2: After removing the outliers (subject 3 and 14), showing that the NRRsf increased from 15
to 19 and the standard deviation decreased from 3 to 1 dB
3 Uncertainty of hearing protectors noise attenuation
measurements by REAT method
All measurement results have an associated with uncertainty value. In general terms, this is
known as measurement uncertainty, and is attributed to factors that influence the final results of
the measurements. Measurement uncertainties can come from the measuring instrument, from
the item being measured, from the environment, from the operator, from subjects used and from
other sources. The errors of measurement can be expressed by the measurements uncertainty
value. This value can be used to quantify the confidence limits of the measured results and
allows comparison of measurements carried out by same or different laboratories and for same
or different products. The uncertainty calculated here is based on GUM1 and other publications
[4].
Uncertainty calculation for Hearing Protector Device (HPD) noise attenuation measurement can
be carried out in different situations such as:
a) One specified brand of HPD measured in one laboratory: which is the most important case
that should be reported by that laboratory at the final results report. Usually a hearing
protector’s manufacturer or user asks a laboratory to make measurements for noise
attenuation of a certain model of HPD. Also this can be extended to measurements of
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several times the same HPD, in the same laboratory, using different subjects, which is very
common case of periodic measurements of the same HPD in the same laboratory;
b) Specified type of HPD: in this case the laboratory measured the noise attenuation of
different model of HPD (for example a number of different model of Earmuff HPD) and
should combine the results to give the uncertainty of all these models measured. Figure 1
show different types and models of HPD;
c) Inter-laboratories measurements: for example inter-laboratories round measurements of
noise attenuation of a specific model of HPD. In this case a specific model of HPD goes
around different laboratories, where the noise attenuation is carried out using the same
standard.
Most of publications and also at the standards like ISO 4869-1 [3] or ISO 4869-5 [4] and ANSI
S12.6 [6] do not consider these different cases separately. The first case is the most important
one since we are usually interested to know for a specific model measured in a specific
laboratory, what is the uncertainty of the noise attenuation measurement of this HPD.
This paper shows detailed calculation of uncertainty of measurements results of noise
attenuation of one specific model of HPD measured in one laboratory and extend that to
repeated periodic measurement of the same HPD in the same laboratory several time. This
paper is refinement of our previous paper published [5], where calculation is presented in more
details with justifications[6].
The sources of uncertainty are test subject response for determining threshold of hearing,
measurements parameters, hearing protector type, equipment used and test acoustics room. A
typical especial case for Earplug show that [6] an error in the noise attenuation results can be
up to 5 dB.
The largest contribution of uncertainty in hearing protector noise attenuation measurements is
the subject response variation and standard deviation between subjects. Therefore, for each
measurement of a HPD in one laboratory, the uncertainty calculation should be carried out. If
the whole measurements are repeated for the same HPD, even for the same subjects,
calculation should be carry out again because of the different in the subject responses used.
This calculation should be presented in the final report of measurements.
The subject response variation distribution need to be study more since it is not clear if it have a
rectangular distribution. The subject response variation distribution may have a bimodal
distribution since all the peak are near each other and all the valleys are near each other in the
trace of the subject. Therefore, is important to determine what kind of distribution this type of
uncertainty has.
4 Hearing protector comfort
Hearing protector comfort can be measured subjectively by conducting perception experiments
on a number of users via questionnaires. Subjective perception is influenced by many
physiological and psychological factors. Based on a literature review summarized in [7] it
appears that most studies on comfort have been focused on the total force of the head band or
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the average pressure (dividing total force by contact area) and subjective evaluations based on
the responses of a group of users who subjectively evaluate the comfort. However, a large
number of the studies published on hearing protectors show that there is often a lack of
correlation between comfort and total headband force or average pressure. Some published
results, as previously discussed by the author, even indicate, unexpectedly, that a strong
headband force is more comfortable than a weak headband force.
Although comfort may initially appear to be a secondary requirement, it is important to note that
an uncomfortable hearing protector device (HPD) may become intolerable after prolonged wear
time and is typically removed or refitted for comfort, and not for best attenuation, leaving gaps
which allow noise leakage. Pressure exerted by an HPD on the skin and underlying tissue and
bone is probably one of the most common direct causes of discomfort. If the contact pressure is
strong and continues for a relatively long period of time, the pain may become unbearable. Two
factors are involved in this scenario: the total force of the hearing protector against the skin and
the distribution of the contact pressure. The pressure exerted by earmuffs varies proportionally
with the force applied by their means of support. When the total force is well distributed over a
large area the resulting contact pressure is lower than when it is concentrated at a few smaller
contact points and the protector is more comfortable. In order to ensure a large area of contact
with the skin, earmuff cushions should not only be of a size and shape compatible with the ear
and head anatomy, but they should also be made of a compliant material.
Besides the most important parameter of contact pressure distribution, other parameter so
lesser importance can affect the comfort of earmuffs including [7]:
a) Total force of the headband: This is recommended to be below 14 Newton;
b)
Earmuff weight: The weights of 69 earmuffs were between 140 and 380 g, with an average
value of 220 g and standard deviation of 57 g. Earmuffs with less than 245 g are acceptable
and comfort is weakly related to earmuff weight;
c) Contact area: For the same headband force, the larger the contact area the lower the
pressure value and the better the comfort will be, but a large contact area can also result in
leakage. Consequently, earmuffs should cover the smallest possible area of the head
surface, while still accommodating the pinna, which conflicts with the need for a
homogeneous pressure distribution;
d) Noise attenuation: This is high for a strong headband force, which may reduce the level of
comfort;
e) Temperature and humidity: Ambient temperature and humidity can affect both the acoustic
performance and the comfort of an earmuff. In some cases, a moderate softening of the
material at body temperature may improve the conformability.
This paper represents a continuation of a study [1] published by the same author, which
concentrated on the contact pressure distribution, with two new contributions in relation to the
measurement technique and the calculation methodology. Firstly, the new measurement system
used in this paper is more robust and has permanent sensors fixed on a dummy head and a flat
surface at the same time (in the previous paper the sensors gave false signals due to the
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curves on the dummy head). Secondly, the comfort indices calculated are modified to cover a
larger range with better resolution. A subjective evaluation is also carried out to verify the
measurement results. This approach is under discussion at the ISO, ANSI and ABNT (Brazilian
Standards Organization) working group on hearing protectors in relation to the preparation of a
guide for earmuff comfort evaluation.
The contact pressure between two surfaces can be measured by capacitive tactile sensors
which store an electrical charge proportional to the contact pressure value. The sensors are
made of conductive cloth (conformable), Kapton (industrial), Lycra (stretchable) or a
combination of conductive cloth and Kapton (hybrid). Resistive tactile sensing is the primary
competing technology, where the resistance of a conductive material, such as an elastomer,
foam, or conductive ink, is used to detect and measure the pressure. The use of capacitive
sensors is more appropriate for applications that require high levels of accuracy and sensitivity.
These sensors are easier to calibrate, better at providing repeatable readings, and less
susceptible to wear and tear than resistive sensors, which have reduced performance over time
as the ink ages.
The measurement system used in this paper consists of highly sensitive conformable tactile
sensors (manufactured by Pressure Profile Systems - PPS) with thresholds down to pressures
of less than 2 [kPa], which are mounted on a measurement system (see Figure 3). The sensor
surface is 122 x 122mm with32x32sensors, each sensor having an area of 3.8x 3.8 mm. A
measurement test system of variable width was constructed, with a flat surface on one side and
half human dummy head on the other side, as shown in Figure3. The half dummy head adheres
to the ANSI S3.36 standard. The open earmuff width is adjusted to 145±1 mm, as given in ANSI
S12.6-2008[1] for the measurement of the headband force. The advantage of the flat surface
measurements is that they enable an absolute comparison between the earmuff comfort indices
to be performed without interference due to the complex geometry of the human (or dummy)
head. The geometry of the human head is very complex and comfort index values will show a
large variability. Therefore, a measurement taken on a flat surface is an absolute value without
the interference of human variability. The dummy head should be able to provide a useful
comfort index taking into account that in this case the human head geometry is standardized
and through comparison with the flat surface measurements the effect of this human geometry
can be determined.
Two equations were used for the calculation of the comfort index. The first equation provided
Comfort Index 1 (CI1), which is used in [7] and is given by:
{
[
∑
̅|
|
(
) ̅
]}
(1)
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where:
̅ is the average pressure
is the total pressure
is the total number of contact points
This index varies between 100 for zero variation in the contact pressure (very comfortable
muffs) to 0for a highly concentrated contact pressure or large standard deviation (very
uncomfortable muffs). The CI1values for the eight muffs obtained with this equation are very
close to each other.
The second equation provided Comfort Index 2 (CI2),which is given by:
√∑
[
]
∑
[
[
̅)
(
(
√
)
(
(2)
)
]]
These two equations are based on the standard deviation of the contact pressure distribution.
The standard deviation shows the degree of variation or dispersion from the average pressure
distribution. A low standard deviation indicates that the data points tend to be very close to the
mean (comfortable muff), while a high standard deviation indicates that the data points are
spread over a large range of values (uncomfortable muff). Other equations were also
investigated, but they provided very similar results.
Figures 3, 4 and 5 show the values obtained for the two indices(CI1 and CI2), for the dummy
head and the flat surface, for the eight earmuffs (A a H) considered. For each index the average
value(± standard deviation) was calculated from the 9 measurements(three samples measured
in triplicate) for each of the 8 earmuffs. Note that the comfort index for the flat surface is always
larger than that for the dummy head, since the latter has curved surfaces and the contact
pressure variation is greater.
Figure 3: Variable width measurement system with flat surface on one side and half
human dummy head on the other side.
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Figure 4 : Contact pressure between earmuff and human manikin (left) and between
earmuff and flat surface (right) for the 8 earmuffs.
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100
90
80
70
Dummy head CI2
60
Flat CI2
50
Dummy Head CI1
40
Flat CI1
30
Subjective
20
10
0
A
B
C
D
E
F
G
H
Figure 5: Comfort indices for the eight earmuffs, forthe dummy head and the flat
surface, calculated usingthe two equations: CI1 (Equation 1) and CI2 (Equation2).
The results obtained applying this novel technique show that the contact pressure distribution
between the earmuff cushions and dummy head or flat surface is directly related to comfort. A
more uniformed distribution gives more comfort even for a higher total headband force.
Therefore, the design of the headband point of attachment, type of headband arc and flexibility
of the cushions are very important factors in relation to earmuff design.
A comparison between the values for the measured indices and the subjective evaluation
showed a good correlation for all earmuffs (above 75%).Comfort indices do not provide
calibrated values with absolute number, but rather the relative values obtained for the different
brands (A to H). Using either the flat surface or the dummy head and also using either Equation
1 (CI1) or Equation 2 (CI2), the results gave almost parallel curves with the same comfort
ranking.
Further work is needed to quantify the uncertainty of the measurements. The uncertainty values
for the sensors used are not available from the manufacturer.
This technique for comfort evaluation can be also used to detect leakage as shown in
[7].
Conclusions
Hearing Protectors Devices (HPD) are the salvation of workers in high level noise to avoid permanent
hearing loss. HPD should provide sufficient noise attenuation. Avoiding over protection they should be
worn during all working period and therefore should be comfortable. In this paper novel method is
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presented for evaluating earmuff comfort by measurement contact pressure between Earmuff and user
face.
Acknowledgments
This research was carried out with the support of the Brazilian funding bodies (CNPq, CAPES
and FINEP) and Laboratory of Personal Hearing Protector Equipment (LAEPI) of NR
Consultancy Ltda. The guidance and orientation of Prof. Armando Albertazzi for the uncertainty
calculation used her is very much appreciated.
References
[1] ANSI S12.6-2008. Method for measuring the Real Ear Attenuation of Hearing Protectors.
[2] ISO 4869-1-5, Acoustics-Hearing Protectors.
[3] Gerges, S. N. Y.; Dias, R.A.; Geges, R.N.C. Detection and contribution of outliers for subjective
evaluation of sound. Accepted for IJSV , 2016.
[4] BIPM,IEC, IFCC, ISO, IUPAP, OIML, Guid to express of Uncertainty in Measurements,Geneva, 1995
[5] Lima, F; Gerges, S.; Zmijevski, T.; Bender D.; Gerges, R. Uncertainty Calculation for Hearing
Protectors Noise Attenuation Measurements by REAT Method. Journal of the Brazilian Society of
Mechanical sciences and Engineering . Vol.32(1), 2010, pp 28-36.
[6] Gerges, R.; Gerges, S. and Vergara F. Uncertainty of hearing protector noise attenuation based on
REAT method. ICA 2016, Buenos Aires.
[7]
] Gerges S.N.Y. Earmuff comfort. Applied acoustics. 2012; 73, 1003-12.
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Hearing protectors: State of art and emerging technologies

Transcription

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