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  Table of Contents    
ORIGINAL ARTICLE  
Year : 2017  |  Volume : 19  |  Issue : 86  |  Page : 1-9
Using Auditory Steady-State Responses for Measuring Hearing Protector Attenuation

1 Department of Mechanical Engineering, École de technologie supérieure, Université du Québec, Montréal, Québec, Canada
2 Rotman Research Institute of Baycrest Centre, University of Toronto, Toronto, Ontario; Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, Ontario, Canada

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Date of Web Publication31-Jan-2017
 
  Abstract 

Introduction: Present methods of measuring the attenuation of hearing protection devices (HPDs) have limitations. Objective measurements such as field microphone in real-ear do not assess bone-conducted sound. Psychophysical measurements such as real-ear attenuation at threshold (REAT) are biased due to the low frequency masking effects from test subjects’ physiological noise and the variability of measurements based on subjective responses. An auditory steady-state responses (ASSRs) procedure is explored as a technique which might overcome these limitations. Subjects and Methods: Pure tone stimuli (500 and 1000 Hz), amplitude modulated at 40 Hz, are presented to 10 normal-hearing adults through headphones at three levels in 10 dB steps. Two conditions were assessed: unoccluded ear canal and occluded ear canal. ASSR amplitude data as a function of the stimulation level are linearized using least-square regressions. The “physiological attenuation” is then calculated as the average difference between the two measurements. The technical feasibility of measuring earplug attenuation is demonstrated for the group average attenuation across subjects. Results: No significant statistical difference is found between the average REAT attenuation and the average ASSR-based attenuation. Conclusion: Feasibility is not yet demonstrated for individual subjects since differences between the estimates occurred for some subjects.

Keywords: Attenuation, auditory steady-state responses, hearing protection devices

How to cite this article:
Valentin O, John SM, Laville F. Using Auditory Steady-State Responses for Measuring Hearing Protector Attenuation . Noise Health 2017;19:1-9

How to cite this URL:
Valentin O, John SM, Laville F. Using Auditory Steady-State Responses for Measuring Hearing Protector Attenuation . Noise Health [serial online] 2017 [cited 2017 May 26];19:1-9. Available from: http://www.noiseandhealth.org/text.asp?2017/19/86/1/199238

  Introduction Top


The National Institute for Occupational Safety and Health estimates that 22 million U.S. workers are exposed to hazardous noises high enough to cause hearing loss.[1] According to a study, based on data from the National Health Interview Survey between 1997 and 2003, 24% of the cases of hearing disorder in the United States are due to occupational exposures.[2] Since it is often difficult, for technological or economic reasons, to reduce noise at its source, the most commonly used solution to protect workers from noise exposure consists in using hearing protection devices (HPDs).

Unfortunately, the use of HPDs has not been rigorously adopted. One obstacle is the difficulty in providing an appropriate attenuation level required by an individual’s work environment since it is difficult to measure an individual effective attenuation of HPDs.[3] HPD attenuation can be measured by numerous methods (see Berger[4] for a comprehensive review), such as the subjective real-ear attenuation at threshold (REAT) method,[5] the objective microphone in real-ear (MIRE) method,[6] or its variant, the field microphone in real-ear (F-MIRE) method.[7],[8]

The subjective REAT method is commonly considered the “gold standard” and REAT results are included in many worldwide standards for rating hearing protector performance. REAT calculates HPD attenuation as the difference between open-ear and occluded-ear hearing thresholds for human subjects. The REAT method takes into account all relevant sound paths to the inner ear but has two main limitations. Firstly, REAT results are considered biased due to low-frequency masking effects from test subjects’ physiological noise. This masking has been shown to lead to an overestimation of attenuation (up to 5 dB) in the lower frequency bands such as 125 and 250 Hz.[9] Secondly, the results are also influenced by the variability of the subjective response, that is, the ability of a subject to “track” his or her hearing threshold levels during different portions of the REAT test.

The objective MIRE method consists of inserting a miniature microphone, either wired or in a probe-tube form, in the ear canal to measure the actual sound pressure level (SPL) at a given location, usually close to the tympanic membrane. The difference between two of these measurements on a given individual, in open-ear and occluded-ear conditions, gives the classical insertion loss. The F-MIRE method uses two miniature microphones to perform simultaneous sound pressure measurements at two locations across the HPDs. One microphone resides in the residual ear canal, and the other is located outside, on the external side of the HPDs. The difference between the two SPLs yields a measurement of noise reduction (NR). The NR is obtained from simultaneous measurements at these two microphone locations while presenting a loud pink noise from an outside reference sound source (frontal incidence, median plane). Compared to REAT measurements, MIRE and F-MIRE measurements are faster, more reliable and do not suffer from the physiological noise bias. The main limitation of these two objective methods is that they do not take into account the bone conduction sound path, which is another means of transmitting sound to the cochlea.

This paper presents an attempt to overcome the limitations of these current subjective and objective methods through the recording of electrophysiological responses. Few studies have examined the use of electrophysiological responses in the measurement of HPDs attenuation. Wilde and Humes[10] found that attenuation of HPDs may be measured effectively using transient auditory brainstem responses (ABRs) but only when using pure tones at 4000 Hz because ABRs are generally more useful for frequencies above 1000 Hz. Zera et al.[11] also verified the possibility of using ABRs to measure HPD attenuation but only with using pulse signals (clicks) and wide-band noise. Auditory steady-state responses (ASSRs) have not been previously used to assess HPDs attenuation, and may offer advantages over transient ABR techniques. ASSRs can be measured for stimuli whose frequencies are below 1000 Hz, also ASSR stimuli are not as brief as those used in traditional ABR measurements and may provide a more stable assessment of HPDs attenuation that is more related to the ongoing level of a stimulus than to stimulus onset.

ASSR techniques have been largely developed to overcome the various difficulties in the assessment of hearing thresholds of very young children, older patients or those with cognitive deficits or behavioral disorders.[12],[13] ASSRs are electrophysiological responses, recorded from the human scalp, and often evoked by one or more carrier frequencies (Fc) that are amplitude-modulated at a specific frequency (Fm). In practice, when a subject is exposed to such a stimulus, spectral power of the electroencephalography (EEG) frequency spectrum of the subject that is related to the stimulus will be manifest at Fm, and may also appear at its harmonics.[14] An interesting characteristic of ASSR is that its amplitudes increase with the level of the stimulus. This increase is not always linear with the stimulation level: the curve for the variation in ASSR amplitude has shown two distinct regions with two different slopes.[15],[16]

Historically, ASSRs were first recorded at modulation frequencies near 40 Hz. Since 40 Hz ASSRs are sensitive to arousal affects, a large number of studies using ASSRs for threshold estimation have concentrated on modulation frequencies in the 70–110 Hz range. In the case of alert/awake adults, using the 40 Hz modulation frequency provides higher ASSR amplitudes and better signal-to-noise ratios which may enable the ASSRs to be detected more rapidly.[17],[18] Because of the time consuming nature of our experimental design (e.g., the full study, including earplugs molding + REAT + F-MIRE + ASSR, took about 2 h for each subject), the present technical feasibility study examined ASSRs obtained with a 40 Hz Fm. Additionally, we only assessed two carrier frequencies which may potentially be influenced by the low-frequency masking effect: 500 and 1000 Hz. ASSR results were then compared to the results obtained with the subjective REAT method and the objective F-MIRE method, which was preferred to the MIRE method as it is easier to implement.


  Subjects and Methods Top


Participants

Ten volunteers (eight males, two females) with ages from 20 to 29 and thresholds below 20 dB SPL (from 125 to 8000 Hz) were assessed. The study was reviewed and approved by the Ethics Committee of the École de technologie supérieure of Montréal. Informed consent was obtained from all subjects prior to their entrance into the study.

Hearing protector

In our experimental design, the stimulation levels were adjusted so that the SPL in a subject’s ear canal was approximately the same in both open and occluded conditions. In other words, we increased the stimulus level proportionately to the expected attenuation provided by the occlusion to obtain a zero-net change of stimulus level between the two conditions. Stimulation levels were limited not to exceed 75 dB SPL to ensure, in case of bad fit of the earplug, that a subject would not be unintentionally exposed to an excessive noise level for a long period of time. Therefore, attenuation at 500 and 1000 Hz should be moderate (e.g., 10–20 dB) to maintain ASSRs with a good signal to noise ratio. Lastly, because of the somewhat long duration of the experiment (e.g., about 2 h), the earplugs should be comfortable.

The hearing protectors chosen for this study were Sonomax Self-Fit™ earplugs [Figure 1]. They are well suited for this experiment because they can provide the required moderate attenuation through the insertion of a filter. These are fitted to the user’s ear by injecting medical-grade silicon between a rigid core and an expandable membrane. The earplugs are designed to include an inner bore of constant length and diameter that permits the temporary insertion of a microphone probe and the permanent insertion of a passive acoustical filter. We used the brown-colored filter, in the set of filters provided by Sonomax, which provides an attenuation of 11 dB at 500 Hz and 15 dB at 1000 Hz. The characteristics of the filter are listed in [Table 1].
Figure 1: Overview of the Sonomax Self-Fit™ earplug: practical diagram (reproduced and modified with the authorization of Voix and Laville[24]) and exploded view (reproduced with the authorization of Sonomax). When using such an earplug, sound can travel to the middle ear along two paths: a solid path which corresponds to the sound going through the HPD material itself and an air path which corresponds to the sound going through the filter

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Table 1: Manufacturer data for the attenuation of Sonomax’s brown filter. Data was obtained by using REAT measurements in accordance with ANSI S3.19-1974 standard

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Experimental procedure

The experimenter must first accomplish the molding of custom earplugs for each subject and proceed with the F-MIRE measurement (task 1). The earplugs were then removed by the subject so that the filter could be installed into the HPDs. After placement of the electrodes used for the recording of ASSR, the earplugs were reintroduced by the subject, and the experimenter obtained the occluded-ear REAT measurements (task 2) and ASSR measurements (task 3). The experimenter then instructed the subjects to remove the earplugs so that the same two measurements (REAT and ASSR) could be obtained in the open-ear study condition. During these measurements, volunteers sat in a comfortable ergonomic chair inside a double-walled audiometric booth.

Task 1: Custom earplugs molding and objective measurements (F-MIRE) − 10 min

Objective attenuation has been computed after the molding of the earplugs by using the software “SONOPASS™” which measures NR (simultaneous SPLs inside and outside the HPDs) and applies compensation factors derived from laboratory testing to provide an effective individual attenuation rating. The stimulus used to conduct F-MIRE measurements was a pink noise computer generated and presented at 85 dB SPL via a loudspeaker in frontal incidence.

Task 2: Subjective measurements (REAT) − 20 min

Subjective attenuation has been computed at each frequency as the difference between the normal and occluded thresholds of hearing measured using the LabVIEW™ based “REAT MASTER™” (Nelson Acoustics) software. “REAT MASTER™” is an automatic Bekesy audiometer program designed to provide stimulus and track responses for hearing threshold determination for both normal and occluded conditions. For each frequency of each measurement, the subject must push a button as long as she/he hears a sound and stop pushing when the sound is inaudible. Three approximate results are needed to determine each hearing threshold. The stimuli used to conduct REAT measurements were warble tones at 500 and 1000 Hz computer generated and presented via a Sennheiser™ headphone. Because of the time consuming nature of our experimental design, only one subjective measurement was performed for each subject.

Task 3: Physiological measurements (ASSR) − 72 min

ASSR-based physiological attenuation has been calculated as the average difference between the normal and occluded conditions. To calculate the physiological attenuation, an example is illustrated in [Figure 2]. Each experimental value recorded with earplugs (conditions “O1” to “O3”) is projected parallel to the x-axis on the regression line[19] computed from the three experimental points recorded without earplug (D), and the corresponding abscissa is calculated. The difference between the two abscissae is calculated for each point with earplugs. The same calculation is computed with the experimental value recorded without earplug (conditions “U1” to “U3”) on the regression line computed from the three experimental points recorded with earplugs (D′). The mean of these six values is taken as the physiological attenuation.
Figure 2: Method of computation of the physiological attenuation. Unoccluded condition results are represented by the lines with “▲” markers and occluded condition results by the lines with “▪” markers. The straight lines D and D′ are obtained from linear least-squares regressions for each condition

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ASSR recordings were first collected for the occluded condition (with earplugs) and then for the un-occluded condition. Since a subject’s arousal state can modulate the amplitude of the 40 Hz responses, subjects were asked to stay awake and watch a subtitled movie during the recordings of these responses. During the experiment, visual contact was maintained with the volunteer through a glass window of the audiometric booth, and between each recording the experimenter asked the subjects whether they were okay and/or needed anything. In the middle of the experiment, coffee or tea was provided to any subjects who indicated they felt drowsy.

The LabVIEW™ based “MASTER SYSTEM” (Rotman Research Institute, www.mastersystem.ca) software[20] was used for both ASSR stimulus generation and response recording/evaluation. The frequencies of both the carrier and modulation signals were adjusted so that an integer number of cycles occurred within each recording section. This allowed the individual sections to be linked together without acoustic artifact and to be interchangeable during artifact rejection.[21] For example, a modulation frequency of 40 Hz was adjusted to 40.039 Hz. For simplicity, the frequencies will henceforth be reported to the nearest integer value.

The stimuli consisted of pure tones, amplitude modulated with a depth of 100%. The carrier frequencies of 500 and 1000 Hz were modulated at 40 and 41 Hz, respectively, and individually presented to each subject. Each stimulus was amplified using an Interacoustic™ audiometer (model AC40) and presented binaurally via Sennheiser™ headphones (model HD 280 Pro). Calibration of test stimuli was done using linear-weighting with a G.R.A.S. Sound & Vibration™ Acoustical test fixture (model 45CB). Because amplitudes do not always increase linearly with the stimulation level,[15],[16] ASSRs were recorded at three stimulation levels to improve precision for the computation of ASSR-based attenuation. Accordingly, stimuli were presented at 45, 55, and 65 dB SPL in the three un-occluded experimental conditions and at 55, 65, and 75 dB SPL (i.e., 10 dB higher) in the occluded conditions. To reduce the effects of stimulus artifact that may be present in the EEG when recording ASSR with headphones, stimuli polarity was inverted for half of the 22 ASSR recordings made for each stimulus condition.[22],[23]

The setup used for the recording of ASSR is illustrated in [Figure 3]. ASSRs were collected from an electrode placed at vertex (Cz) using an electrode on the back of the neck (below the hair-line) as reference. An electrode on the clavicle served as ground. All interelectrode impedances were below 3 kΩ at 10 Hz. ASSRs were filtered using a band-pass filter of 0.3–300 Hz (12 dB/octave) and amplified 50,000× (10,000× in GRASS Technologies LP511 AC Amplifier; 5× in National Instruments USB-6259 BNC). All data were collected using an Analog-to-digital (A/D) conversion rate of 1000 samples per second. In each recording, individual data epochs of 1024 points each were collected and linked together into sweeps (16 epochs per sweep for lasting 16.384 s each). A typical ASSR recording required a total of 264 sweeps across 72 min: two frequencies (500 and 1000 Hz) × six different stimulation levels (three un-occluded and three occluded) × two recordings of 11 sweeps. To minimize artifacts and other interference, epochs containing signals exceeding ±90 μV were rejected.
Figure 3: Overview of the experimental setup of ASSR recording. All components are monitored by a single PC. The stimulation signals from the DA output of the NI-USB 6229 board are attenuated by an operational amplifier with a gain of −0.5, so that they may be delivered to the “tape input” of the audiometer, which enables the operator to adjust the levels of stimuli delivered by the headphones. In parallel, ASSRs are scalp-recorded on the electrodes (placed between vertex (+) and hairline (−), with clavicle as a ground) and are then amplified by an EEG amplifier, before reaching the AD input of the data acquisition board. Data is processed online through the MASTER SYSTEM software

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ASSR sweeps of data were averaged in the time domain and then analyzed on-line in the frequency domain using fast fourier transform (FFT). Amplitudes were expressed in nanovolts (nV). An F-ratio statistic was calculated by the MASTER system which estimated the probability that the amplitude of the ASSR was significantly different from the average amplitude of the background noise in adjacent frequencies (within ±60 bins of the modulation frequency). A response was considered to be present if P < 0.05.


  Results Top


[Figure 4] (for 500 Hz) and [Figure 5] (for 1000 Hz) present the individual plots of ASSR amplitudes as a function of stimulation level. [Figure 6] (for 500 Hz) and [Figure 7] (for 1000 Hz) show the comparison between the ASSR attenuations and the REAT attenuations. [Table 2] summarizes the attenuation values obtained with F-MIRE, REAT, and ASSR methods.
Table 2: Individual and average attenuation measurements obtained with F-MIRE, REAT and ASSR methods

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Figure 4: ASSR amplitude as a function of stimulation level for 500 Hz. Each graph refers to a subject. Unoccluded condition results are represented by the lines with “▲” markers and occluded condition results by the lines with “▪” markers. The dotted straight lines are obtained from linear least-squares regressions for each condition. Attenuation values and their standard deviations, indicated on each graphs, have been calculated from the mean difference between the two lines

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Figure 5: ASSR amplitude as a function of stimulation level for 1000 Hz. Each graph refers to a subject. Unoccluded condition results are represented by the lines with “▲” markers and occluded condition results by the lines with “▪” markers. The dotted straight lines are obtained from linear least-squares regressions for each condition. Attenuation values and their standard deviations, indicated on each graphs, have been calculated from the mean difference between the two lines

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Figure 6: Left: Individual ASSR results versus individual REAT results, at 500 Hz. Each symbol represents a subject. The dotted line represents the line y = x. Right: Comparison between the average attenuation obtained with REAT (in gray) and ASSR (in white) methods at 500 Hz. The error bars represent the 95% confidence interval (2 SD of the average attenuation)

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Figure 7: Left: Individual ASSR results versus individual REAT results, at 1000 Hz. Each symbol represents a subject. The dotted line represents the line y = x. Right: Comparison between the average attenuation obtained with REAT (in gray) and ASSR (in white) methods at 1000 Hz. The error bars represent the 95% confidence interval (2 SD of the average attenuation)

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  Discussion Top


Objective microphonic attenuation (F-MIRE)

As seen in [Table 2], F-MIRE results across the individual subjects of this study are the same at 500 Hz, and almost the same at 1000 Hz. The small inter-subject differences may be due to a poor fit and/or an earplug “resonance effect” at 1000 Hz.

As seen in [Figure 1], when using Sonomax Self-Fit™ Hearing Protectors, sound can travel to the middle ear along two paths: a solid path and an air path. The solid path corresponds to the sound going through the HPD material itself and the air path corresponds to the sound going through the filter. During F-MIRE measurements, the microphone is plugged into the hole for the filter and the HPD is “fully-blocked.” The measured attenuation reflects the reduction of sound through the HPD. If the filter is not a fully-blocking filter (as was the case with the brown filter that was used in the current experiment), then the F-MIRE software automatically decreases the attenuation value to match that of the filter.[24]

For this reason, the F-MIRE method does not measure the actual attenuation of the ear plug with a filter, but rather gives a prediction of the attenuation for such an earplug, which is less precise than an actual measurement. Consequently, the F-MIRE measurements performed in this study have been only used to verify the conformity of the custom HPDs and to provide an initial prediction of the REAT.

Subjective psychophysical attenuation (REAT) versus objective physiological attenuation (ASSR)

The REAT average attenuation, computed on 10 subjects, shown in [Table 2], is in agreement with the REAT average attenuation given by the manufacturer, shown in [Table 1] (the average attenuation computed on 10 subjects is part of the one sigma confidence interval). This finding confirms the validity of the methodology used to perform the REAT measurements.

As seen on individual graphs [Figure 5], ASSRs’ amplitude increased as stimulus level increased (except for subject S3 at 500 Hz).

As expected at the 500 and 1000 Hz frequencies, where the low frequency masking effect in the REAT value is negligible, results in [Table 2], and [Figure 7] indicate that the average physiological attenuation is not significantly different from the average REAT values.

This observation is confirmed at 500 Hz by Wilcoxon tests, computed by using R 3.1.0 with packages MASS 7.3-35,[25] which failed to reject the null hypothesis at a 0.05 significance level (P value >0.05) and conclude that the physiological attenuation is not significantly different from the psychophysical attenuation. Two one-sided test (TOST) procedures[26] also conclude that the average physiological and psychophysical attenuations are equivalent at ±5 dB (P value <0.05).

However, at 1000 Hz, Wilcoxon tests reject the null hypothesis at a 0.05 significance level and conclude that the physiological attenuation is significantly different from the psychophysical attenuation, because of the difference between both attenuations in some subjects (number #5, #7, and #10). Finally, TOST procedure concludes that the average physiological and psychophysical attenuations are equivalent at ±6 dB (P value <0.05).

These results demonstrate the feasibility of measuring mean earplug attenuation using ASSRs. However, the feasibility of measuring individual earplug attenuation is not demonstrated since, for 50% of the subjects at 500 Hz and 40% at 1000 Hz, the ASSR-based estimates of attenuation were significantly different from the REAT-based estimates of attenuation.

Conclusion and recommendations

Although other electrophysiological methods have been used for measuring the attenuation of HPDs,[10],[11] no study has explored the use of ASSRs. The present study assessed the feasibility of objectively measuring HPD attenuation using ASSRs collected in the same subject both with and without protectors. The technical feasibility of measuring earplug attenuation was demonstrated for the group average attenuation across subjects but was not supported for the individual subjects: significant differences between REAT-based attenuation and ASSR-based attenuation were found for some subjects.

Several recommendations can be formulated based on this feasibility study:

  1. Since the F-MIRE tests for a filtered earplug do not provide an accurate measure of the individual attenuation, they have not been used for comparison with ASSR. The recommendation is, therefore, to utilize the more accurate MIRE tests, instead of F-MIRE tests, to provide an objective measurement with less variability than subjective measurements done by REAT.
  2. The ASSR procedure used in this study for measuring the attenuation is particularly time consuming: the 72 min needed for testing two frequencies is indeed longer than the required duration to perform a complete 7-frequency REAT. Consequently, this method would work for a laboratory investigation, but not for a routinely conducted test in the workplace. This somewhat long duration of the experiment caused the present feasibility study to be conducted at only two carrier frequencies (500 and 1000 Hz) which were chosen because, at these frequencies, the results might be potentially influenced by the low-frequency masking effect. Further research should try to reduce this long duration of the ASSR measurement while extending the frequency range. The use of broadband noises should be investigated as they would provide results over a wider frequency range that is closer to the reality of industrial environment. Additionally, such a stimulus would reduce the total duration of the ASSR measurement since it would not be necessary to discretize the 250–8000 Hz frequency range with several stimuli.
  3. ASSR-based “physiological attenuation” has been calculated as the average difference between the normal and occluded conditions using linear least-square regression of ASSR amplitude data. Each linear regression has been computed on a set of three points (one per stimulation levels) because amplitudes do not always increase linearly with the stimulation level. Individual results presented in this study suggest that computing these linear regressions should use a minimum of three data-points, since, for a same subject, there may be significant differences between the slopes of the occluded and non-occluded stimulation levels versus amplitude data. A more extensive study with more than three points could enable whether using more points could lead to more robust estimates of attenuation for individual subjects.
  4. Finally, with respect to the feasibility of using this method to measure individual attenuations, a more extensive study might investigate the effect of intra-individual variability of each measurement by including test–retest assessment to determine the relative contributions of signal-to-noise issues and amplitude-slope non-linearity in relation to failing to obtain clinically useful results for individual subjects.


Acknowledgements

The authors wish to express their appreciation to Natural Sciences and Engineering Research Council of Canada (NSERC) for their support of this project and to Sonomax Hearing Healthcare Inc. (Montréal, Canada) for providing the custom earplugs.

Financial support and sponsorship

Natural Sciences and Engineering Research Council of Canada (NSERC).

Conflicts of interest

There are no conflicts of interest.

 
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Correspondence Address:
Olivier Valentin
École de technologie supérieure, Département de génie mécanique, 1100 rue Notre-Dame Ouest, Montréal (QC), H3C 1K3
Canada
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1463-1741.199238

Rights and Permissions


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]
 
 
    Tables

  [Table 1], [Table 2]



 

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