HUMAN
`
`FACTORS,
`
`1990,32(1),9-25
`
`Attenuation Performance of Four Hearing
`Protectors under Dynamic Movement and
`Differen t User Fitting Conditions
`
`JOHN G. CASALIt and MIN-YONG PARK, Virginia Polytechnic Institute and State
`University, Blacksburg, Virginia
`
`An experiment was conducted to determine the effects of movement activities and
`alternative
`fitting procedures on protection levels afforded by four hearing pro-
`tection devices (HPDs). Psychophysical
`attenuation measurements
`at nine one-
`third-octave bands from 125 to 8000 Hz were obtained prior
`to, during, and fol-
`lowing a 2-hr wearing stint
`that
`included periods of either highly kinematic but
`controlled work activity or vigorous temporomandibular movement. The 40 sub-
`jects, who were nonusers of HPDs, initially fit the protectors
`according to either
`the instructions on the package (i.e., subject fit) or after receiving interactive train-
`ing on proper fit (i.e., trained fit). Thereafter no further protector adjustments were
`allowed during the wearing period. The subject-fit condition resulted in signifi-
`cantly lower protection levels, from 4 to 14 dB, at 1000 Hz and below for a pre-
`molded polymer earplug, a user-molded foam earplug, and a double protector
`consisting of a muff over the foam plug. The muff alone was significantly more
`resilient
`to fitting effects on attenuation
`than were the plugs. Movement activity
`caused up to a 6-dB significant
`reduction in frequency-specific
`attenuation
`over
`time for the premolded plug, muff, and muff-plug combination. The compliant
`foam earplug was largely resistant
`to either type of movement effect but did ben-
`efit more than the other devices from use of the trained-fit procedure.
`Implications
`of the results
`for hearing protector
`testing protocol, device selection, and user
`training are discussed.
`
`INTRODUCTION
`
`Hearing Loss Countermeasures
`
`Permanent hearing loss is a frequent and
`tragic
`consequence
`of exposure
`to high-
`
`to John G. Casali,
`should be sent
`for reprints
`I Requests
`IEOR Depart-
`Director, Auditory
`Systems
`Laboratory,
`ment, Virginia Polytechnic
`Institute
`and State University,
`Blacksburg, VA 24061.
`
`or
`sounds of industrial, military,
`intensity
`even recreational origin. It has long been rec-
`ognized that high-level
`industrial machinery
`noise poses a major
`threat
`to workers' hear-
`ing. More recently concern has increased for
`the hearing of those exposed to nonindustrial
`noise sources,
`such as symphony orchestra
`performers
`(Royster, Royster,
`and Killion,
`1989), rock musicians and listeners (Johnson,
`
`© 1990, The Human Factors Society,
`
`Inc. All rights
`
`reserved.
`
`

`

`100February 1990
`
`HUMAN
`
`FACTORS
`
`such as
`1987), spectators of noisy activities
`automobile
`racing, and consumers who use
`power tools.
`of
`progression
`the insidious
`To combat
`noise-induced hearing impairment,
`effective
`countermeasures must be employed, one of
`which is personal
`hearing protection. Be-
`cause of the expense,
`ineffectiveness, and/or
`feasibility of some administrative
`and engi-
`neering noise control strategies, hearing pro-
`tection devices
`(HPDs) have emerged as a
`popular defense. Furthermore,
`the U.S. Occu-
`pational Safety and Health Act has solidified
`the need for effective HPDs in general
`indus-
`try with the requirement
`that all employees
`exposed to an 8-hr time-weighted mean of 85
`dB(A) or greater
`be supplied with HPDs
`(OSHA, 1988). Based on Environmental
`Pro-
`tection Agency estimates (EPA, 1981), this re-
`quirement
`affects more than 9.2 million
`American workers,
`including military
`per-
`sonnel.
`
`The Problem of Rating Hearing
`Protector Effectiveness
`
`data and the noise re-
`Spectral attenuation
`duction rating (NRR), which is computed
`thereof, are the primary metrics by which one
`can predict whether or not HPDs will provide
`adequate protection
`and OSHA compliance
`in a given high-noise environment. However,
`these attenuation data, which are required by
`the EPA for all HPDs sold in the United States
`(EPA, 1984), often overestimate actual protec-
`tion values because, according to the stan-
`dard protocol under which they are obtained,
`"the methodology is intended to yield opti-
`mum performance values which may not usu-
`ally be obtained
`under
`field conditions"
`(ANSI, 1984, p. 1).
`tests are per-
`The standard
`attenuation
`formed under highly controlled
`laboratory
`conditions using specific procedures
`that
`in
`no way account
`for the workplace influences
`on HPD performance.
`In such tests trained
`
`and motivated subjects are seated quietly for
`a very short wearing period and tested with
`new, properly fit HPDs under optimal condi-
`tions.
`In contrast,
`in the field workers may
`wear ill-fitted and/or damaged HPDs for pro-
`longed periods while performing
`physical
`movements and exertions associated with the
`work, factors that can contribute
`to a poor
`protector
`seal. In other words,
`the standard
`procedures
`and conditions
`under which
`HPDs are tested and rated for attenuation
`are
`quite different
`from those in the environ-
`ments where HPDs are actually used. As such,
`the laboratory-obtained
`attenuation
`values
`indicate significantly higher protection than
`is typically attained in the field, as verified by
`the surveys of Lempert and Edwards
`(1983)
`and Padilla (1976). Berger (1983a) concluded
`on the basis of a review of studies
`that
`the
`NRR overestimated
`protection in the field by
`an average of 13 dB or greater, depending on
`the standard deviation adjustment
`applied to
`the calculation. When one considers
`that
`the
`range of NRRs for currently available,
`stan-
`dard (i.e., non-level-dependent)
`HPDs
`is
`about 1~35 dB, 13 dB of protection overes-
`timation is quite significant, especially if the
`ambient noise is above 100 dB(A) and a mar-
`ginal protector
`is used.
`
`Research Objective
`
`The intent of this study was to develop and
`utilize a laboratory-based
`protocol
`to esti-
`mate the influence of two important
`vari-
`ables (HPD fitting procedure
`and movement
`activity
`during wearing
`period)
`on the
`achieved attenuation
`of four different HPDs.
`The effects of these two real-world influences
`were of particular
`interest because current
`HPD testing
`standards
`(e.g., ANSI S12.6-
`1984) provide protection
`ratings only for a
`well-supervised
`fit of the HPD immediately
`after the device is donned, which is unrealis-
`tic in the application setting.
`Previous
`studies
`(e.g., Abel, Alberti,
`
`and
`
`

`

`HEARING PROTECTION
`
`February 1990--11
`
`Riko, 1982; Casali and Epps, 1986; Casali and
`Lam, 1986) have indicated that
`the attenua-
`tion achieved may be dependent on how the
`subject was trained to fit the protector. On
`the basis of that prior work, proper
`fit of in-
`sert HPDs (earplugs)
`is generally thought
`to
`be more strongly influenced by user instruc-
`tion than is that of circumaural HPDs (ear-
`muffs). For this study two fitting conditions
`that were intended to represent
`the extremes
`of fitting instruction typically encountered by
`the industrial workers were compared. These
`included naive-subject
`fit (subject
`fit) using
`only HPD package instructions
`and trained-
`subject fit (trained fit) using HPD package in-
`structions
`as well as close supervision by a
`trained experimenter.
`field influence is worker
`Another important
`movement activity. Of particular
`relevance to
`the performance
`of earplugs
`are temporo-
`mandibular
`Uaw) movements
`induced
`by
`chewing gum or tobacco, eating, or talking
`while wearing HPDs on the job. For most ear-
`plugs the data on temporomandibular move-
`ments are limited (with the exception of slow-
`recovery
`foam plugs, which demonstrate
`little or no change), but large amounts of jaw
`movement have generally resulted in reduced
`protection
`(Abel and Rokas, 1986; Berger,
`1981; Cluff, 1989). The results are even less
`definitive
`for work-related movement
`than
`for jaw movement, primarily because the ac-
`tivities during the experimental wearing pe-
`riods have been largely unspecified. Kasden
`and D'Aniello (1978)
`reported
`significant
`losses in attenuation
`over a 3-hr activity pe-
`riod for a single-flange premolded
`plug (V-
`SIR) but not for a custom-molded plug. Stud-
`ies by Krutt
`and Mazor
`(1980) and Berger
`(1981), in which subjects wearing HPDs went
`about
`their normal office or laboratory work,
`demonstrated
`small
`reductions
`in attenua-
`tion for several earplugs of the premolded
`and mineral down varieties but
`little or no
`deficit
`for slow-recovery foam plugs. Casali
`
`be-
`and Grenell (1989) measured attenuation
`fore and after subjects performed a light as-
`sembly task for approximately
`1.25 hr while
`wearing Willson 665 earmuffs. A slight drop
`in attenuation
`occurred over the wearing pe-
`riod but only at the lowest (125 Hz) test fre-
`quency. Those studies generally pointed out
`that work-related and jaw movement may de-
`grade attenuation,
`but none of them utilized
`a simulation
`of highly kinematic,
`strenuous
`work activity,
`in which hearing protector at-
`tenuation may be most
`likely to degrade to
`critical
`levels. Therefore,
`to provide
`a con-
`trolled,
`repeatable
`investigation of the activ-
`ity variable,
`the HPD wearing period in this
`study consisted of either a vigorous, whole-
`body physical work activity or temporoman-
`dibular movement activity elicited by chew-
`ing movements and forced vocal efforts.
`
`METHOD
`
`Subjects
`
`subjects participated,
`Forty paid volunteer
`with five males and five females
`randomly
`assigned to each of four HPD conditions. The
`subject group had the following characteris-
`tics:
`
`(1) age range of 19-35 years, mean age of fe-
`males = 23.1 years, of males = 24.6 years;
`(2) inexperience with HPD use (less than one use
`every six months on average) and no prior
`participation in audiological experiments;
`(3) no evidence of otopathic disorders, head le-
`sions, tinnitus, or excessive cerumen in the
`ear canal;
`(4) normal pure-tone audiogram for each ear, de-
`fined as hearing threshold levels between
`-10 and 20 dB at frequencies of 125-8000 Hz
`in octave steps (as per ANSI SI2.6-1984) and
`determined in a screening session using a Bel-
`tone Model 114 manual audiometer.
`
`Subjects read and signed an informed con-
`sent document
`indicating their willingness
`to
`participate
`and removed all headgear,
`ear-
`rings, or eyeglasses prior
`to the attenuation
`tests.
`
`

`

`12-February 1990
`
`HUMAN
`
`FACTORS
`
`Apparatus
`
`instrumentation. All REAT
`test
`Attenuation
`(real-ear attenuation
`at threshold) data were
`collected using a Bekesy (1960) psychophysi-
`cal procedure in which the subject pressed a
`control button whenever a signal was audible
`(causing it to decrease in I-dB steps at an at-
`tenuator
`rate of 5 dB/s) and released the but-
`ton whenever
`the signal was inaudible (caus-
`ing it to increase at the same rate). In effect
`the subject
`tracked the threshold for each test
`frequency, producing a tracing of threshold
`response
`on a computer monitor. Using a
`computer
`scoring algorithm,
`the threshold
`for each frequency was computed as the mid-
`point of the series of peak and valley reversals
`on the tracing for each test frequency. Bekesy
`tracings were obtained for occluded (protec-
`tor worn) and unoccluded (protector off) con-
`ditions for each subject, and the difference (in
`dB) between the occluded and unoccluded
`thresholds was taken as the attenuation
`pro-
`duced by the HPD for a given test frequency.
`Each time an attenuation
`test was taken in
`the experimental
`sequence, separate
`thresh-
`olds were obtained
`for each of nine one-
`third-octave
`noise bands, with center
`fre-
`quencies of 125, 250, 500, 1000, 2000, 3150,
`4000, 6300, and 8000 Hz, pulsed on-off at a
`rate of 2 Hz (ANSI S12.6-1984). In this man-
`ner a spectrum of attenuation
`was deter-
`mined for the HPD, as worn, for each experi-
`mental condition.
`at-
`Equipment
`to perform these real-ear
`tenuation tests consisted primarily of an IBM
`PCI AT-controlled
`Norwegian-Electronics
`Model 828 audio signal generation,
`filter, and
`aUenuator
`system, which presented
`one-
`third-octave
`band signals
`through four fre-
`quency
`response-matched
`loudspeakers
`placed at
`the corners of an imaginary tetra-
`hedron surrounding
`the subject's head. Sig-
`nals were presented in a uniform sound field
`around
`the stationary
`subject's
`head in a
`
`noise
`chamber having ambient octave-band
`levels of less than 10 dB (linear) at center fre-
`quencies from 250 to 8000 Hz and less than 24
`dB at 125 Hz, and a reverberation
`time for all
`test signal frequencies
`less than 0.20 s. Cali-
`bration was verified daily with a Larson-
`Davis 800-B one-third-octave
`analyzer
`and
`ACO 7013 microphone. The hearing protector
`test facility has been verified to be in accor-
`dance with ANSI SI2.6-1984 (Casali, 1988).
`Work task equipment. Six simulated indus-
`trial
`tasks were performed
`by the occluded
`subject, who worked in a constant 28°C envi-
`ronment. A motorized work task similator
`(Figure
`1) provided
`calibrated
`resistance
`against which the subject had to work. Using
`interchangeable manual control heads on the
`motor
`shaft, each of six different
`activities
`was performed during each work activity pe-
`riod. All activities were paced with a metro-
`nome, and physical workload was controlled
`using constant
`resistance from the simulator.
`These activities
`(with pacing in parentheses)
`consisted of valve turning (50 left/right half-
`turns per minute),
`ladder climbing (100 rungs
`per minute), crowbar work (50 push/pulls per
`minute), straight
`lever pulling (70 cycles per
`minute),
`load pushing (50 per minute),
`and
`bar
`(shoulder)
`rotation
`(50 rotations
`per
`minute). Concurrent with this work task, sub-
`jects were required
`to turn their head and
`neck approximately
`100 deg every 5 s to mon-
`itor video displays, one located to the left and
`one to the right (Figure 1). This forced rapid
`head acceleration/deceleration,
`which could
`induce HPD slippage.
`
`Experimental Design and Protocol
`
`Each subject was randomly assigned to one
`of the four HPDs and attended four experi-
`mental sessions separated by at least 24 hr, in
`which one fitting procedure
`and one activity
`condition were applied
`in each session. A
`mixed-factors,
`complete
`factorial design re-
`sulted, with each HPD (a between-subjects
`
`

`

`HEARING PROTECTION
`
`February 1990-13
`
`Figure 1. An occluded subject performing a work task activity; monitoring video displays are located in the
`rear. Shown is the valve rotation activity, one of six activities using the Baltimore Therapeutic Equipment
`Work Simulator.
`
`variable) being donned and worn under all
`four combinations of fitting and activity con-
`ditions (within-subjects
`variables). A discus-
`sion of the levels of each independent
`vari-
`able follows.
`HPDs. To ascertain whether certain HPDs
`were more susceptible than others to attenu-
`ation loss caused by fitting and wearing pe-
`riod variables,
`four diverse protector
`types
`were studied. These HPDs and their current
`manufacturer NRR values (laboratory-rated)
`are as follows:
`
`= 25 in
`(1) BUsom UFo] Universal Earmuff(NRR
`over-the-head position): a basic foam cushion
`earmuff with adjustable,
`gimballed earcups
`and headband clamping force of 10.2 N at an
`earcup
`separation
`of 14.35 cm and head
`height of 13.08 cm
`(2) E-A-R Foam Plug (NRR = 35): a cylindrical
`earplug made of slow-recovery foam that
`is
`finger-rolled
`by the user
`into
`a small-
`diameter
`cylinder, quickly inserted into the
`ear canal, and allowed to expand to provide a
`seal
`(3) E-A-R "UltraFit" Plug (NRR = 27): a pre-
`
`earplug with three hemi-
`molded polymer
`spherical
`flanges of decreasing radii
`toward
`the inserted tip end (stem provided for finger-
`tip grasp during insertion)
`(4) Combination:
`BUsom muff over E-A-R foam
`plug (no NRR): an exemplary
`combination
`protector
`for use in ambient
`noise levels
`where "double" protection is needed (no NRR
`is specified because
`the combined attenua-
`tion is less than the arithmetic
`sum of the in-
`dividual protector attenuation).
`
`Several examples of a third class of HPD-
`the ear canal cap-were
`tried unsuccessfully
`in the experiment. Most subjects complained
`of pain caused by localized pressure on the
`conchal and tragal areas of the ear and were
`too uncomfortable
`to wear
`the canal caps
`continuously for the full two-hour period. Un-
`like earplugs and muffs, canal caps are pri-
`marily useful for those who must go in and
`out of noisy areas and therefore need an in-
`termittent use device that
`is easy to don and
`doff.
`Fitting procedure. HPD fitting was accom-
`plished under two fitting conditions, with the
`
`

`

`14-February 1990
`
`HUMAN
`
`FACTORS
`
`the
`always preceding
`condition
`subject-fit
`fit
`condition so that
`the subject
`trained-fit
`was not biased by the experimenter
`training.
`Under
`the subject-fit
`condition the subject
`was handed the HPD in its standard indus-
`trial packaging and asked to insert
`the plug
`(or don the muff) according to the manufac-
`turer's printed instructions. No experimenter
`intervention or guidance was given, nor was
`the subject allowed to ask questions
`regard-
`ing the fitting procedures. This fitting proce-
`dure mimicked many industrial practices
`in
`which workers
`fit
`their own HPDs with no
`supervision. Before the attenuation
`tests be-
`gan, subjects were allowed to adjust or refit
`the HPD until
`they were satisfied with the
`placement. Once fit, the subject sat quietly in
`the chamber
`for 5 min prior
`to the first
`threshold test. This allowed time for the sub-
`ject
`to relax and become accustomed to the
`chamber
`and also for the HPD to stabilize;
`this was particularly
`important
`for the foam
`plugs, which slowly expand to conform to the
`ear canal.
`In the trained-fit condition the subject read
`the manufacturer's
`instructions,
`listened to a
`verbal explanation
`of those instructions
`by
`the experimenter
`(without embellishment
`of
`the instructional
`content),
`and could ask
`questions about
`fitting the HPD. The experi-
`menter provided verbal
`feedback while the
`subject practiced donning the HPD outside
`the test chamber but did not physically assist
`the subject
`in achieving fit. After learning the
`proper
`fitting
`techniques,
`the subject
`re-
`moved the HPD, entered the chamber,
`and
`replaced the HPD without
`the experimenter
`present. A 70 dB(A) pink noise was then pre-
`sented so the subject could adjust or refit the
`HPD to subjectively
`block the maximum
`sound, as instructed
`by the experimenter.
`Once the subject was satisfied with the fit, the
`experiment proceeded with the stabilization
`period.
`The rationale behind including a form of
`
`auditory feedback as a fitting aid was two-
`fold. First,
`in a noisy industrial
`environment
`workers may rely on the ambient noise as an
`indicator of HPD seal
`if they become cogni-
`zant of its usefulness. For instance, earplug
`users may be trained
`to cup their hands
`tightly over their pinnae in the presence of
`noise and to listen for an increment
`in atten-
`uation over
`that
`achieved with the plugs
`alone. If cupped hands result
`in subjectively
`quieter feedback, then a refit of the plugs may
`be indicated.
`Proper use of auditory
`feed-
`back as a fitting aid may be easily attained
`through training,
`though it is not necessarily
`obvious to the naive user who relys only on
`HPD package instructions. Thus the 70 dB(A)
`pink noise was included only as a component
`of the trained-fit
`condition
`applied in this
`study. Second, HPD testing standards
`(e.g.,
`ANSI SI2.6-1984), which entail quality fit of
`the devices, dictate the use of noise as a fit-
`ting aid.
`Wearing period and activity conditions. Dur-
`ing each data collection session the subject
`performed two 30-min periods of activity out-
`side the test chamber while continuously
`wearing the HPDs as originally
`fit. In the
`work activity condition,
`the Work Simulator
`movements and display monitoring task were
`performed for each 30-min period, with con-
`tinuous activity on one of the six "jobs" for
`each of six 3-min work periods, separated by
`2-min rest intervals. This task was highly ki-
`nematic, emphasized upper torso movements
`and rapid head acceleration,
`and induced
`moderate perspiration
`in all subjects.
`In the
`jaw movement
`condition
`the subject was
`seated at a desk and alternated 5-min periods
`of reading aloud with 5-min periods of chew-
`ing gum or eating a snack for each of the two
`30-min periods. During the reading portion of
`the task the subject watched an analog sound
`level meter and tried to maintain a forced vo-
`cal effort such that
`the meter response to the
`spoken word was about
`70 dB(A). This
`
`

`

`HEARING PROTECTION
`
`February 1990-15
`
`task elic-
`combination speaking-and-chewing
`ited almost continuous
`temporomandibular
`movement but no other physical activity dur-
`ing the HPD wearing period. This procedure
`provided
`a controlled means of allowing
`jaw movement alone to affect achieved atten-
`uation, much as would gum-chewing or con-
`versation
`in noise. The order of activity
`conditions was counterbalanced
`across sub-
`jects.
`sessions.
`Protocol sequence for experimental
`First
`the subject was familiarized with the
`attenuation test and activity task procedures
`and practiced in Bekesy threshold tracking.
`The subject was then audiometrically
`tested
`for temporary
`threshold shift as compared
`with the original screening audiogram;
`if no
`shift was found,
`the session proceeded. Pre-
`task unoccluded (HPD off) thresholds were
`first obtained for each one-third-octave
`test
`band. Then using the assigned fitting condi-
`tion, the subject donned the HPD and the sta-
`bilization period commenced. Once this ini-
`tial
`fit was established,
`the subject was
`instructed not to touch or readjust
`the HPD for
`the remainder of the 2-hr wearing period and
`was continuously monitored by the experi-
`menter
`to ensure
`compliance.
`Pretask oc-
`cluded (HPD on) thresholds were then ob-
`tained. The subject exited the test chamber
`and performed the appropriate
`activity task
`for the first 3D-min period. Then, following a
`5-min break,
`the subject reentered the cham-
`ber and the first posttask occluded thresholds
`were established. The second 3D-min task pe-
`riod was subsequently undertaken,
`followed
`by another
`short break and then the final
`posttask
`occluded
`threshold
`tests. At this
`juncture
`the HPDs had been worn for 2 hr
`without
`adjustment
`and attenuation
`data
`had been obtained after initial
`fit, after 1 hr,
`and again after 2 hr. The attenuation
`tests
`interspersed with the two 3D-min activity pe-
`riods accounted for the total 2-hr continuous
`wearing period. Finally,
`the subject removed
`
`the HPD and a posttask unoccluded threshold
`was obtained.
`
`RESULTS
`
`Threshold Data Reduction
`
`session and fitting of
`For each experimental
`the HPD, three attenuation
`(i.e., noise reduc-
`tion in dB) data points were computed for
`each of the nine test frequencies:
`(1) initial fit
`or pretask attenuation was computed as the
`difference between pretask occluded thresh-
`olds and pretask unoccluded
`thresholds
`at
`each test frequency;
`(2) posttask attenuation,
`following 2 hr of wearing, was the difference
`between posttask occluded and unoccluded
`thresholds;
`and (3) attenuation
`during the
`task, after 1 hr wearing, was computed as the
`difference
`between
`the second
`occluded
`threshold and the mean of the pre task and
`post task unoccluded thresholds. The result-
`ant data set was complete, with no missing
`data points attributable
`to HPDs falling out
`of the ear or subject attrition. Although none
`of the HPDs worked completely loose and fell
`off during the extended activity period, no-
`ticeable
`slippage
`did occur with approxi-
`mately 20% of the subjects using the muff and
`with a few wearing the UltraFit earplug.
`
`Attenuation Data Analyses
`
`Overall analysis of variance. Mixed-factors
`analysis of variance (ANOVA) was applied to
`the complete
`factorial design of HPD type
`(H), wearing time (T), movement activity (A),
`and fitting procedure (F) using the dependent
`measure of attenuation in dB at each test fre-
`quency. All interactions
`of the independent
`variables were included in the ANOVAs. Sub-
`jects
`(5) were treated
`as a random-effects
`variable; wearing time, movement
`activity,
`fitting procedure, and HPD type were treated
`as fixed-effects variables. Complete ANOVA
`summary tables appear
`in Park (1989).
`(p < 0.05) two-way
`Statistically significant
`
`

`

`16-February 1990
`
`HUMAN
`
`FACTORS
`
`interaction effects included Wearing Time x
`HPD at all frequencies except 8000 Hz, Fit-
`ting Procedure x HPD at all frequencies be-
`low 2000 Hz, Movement Activity x HPD at
`6300 Hz, and Fitting Procedure x Wearing
`Time at only 500 and 6300 Hz. A three-way
`interaction
`(Fitting Procedure
`x Wearing
`Time x HPD) was significant only at 500 Hz
`and was analyzed and interpreted
`further
`only in the context of its significant embed-
`ded two-way effects. For the main effects sig-
`(p < 0.05) was revealed for HPD
`nificance
`type and wearing time at all frequencies,
`for
`fitting procedure
`at all frequencies
`except
`6300 Hz, and for movement activity at only
`3150 Hz. For each ANOVA result
`the data
`were next partitioned and analyzed with sim-
`ple-effects F tests on the dimensions of inter-
`est, followed by pairwise means comparisons
`tests. Because of their
`size the F test and
`means comparisons
`test tables are not repro-
`duced here; however,
`the statistically signifi-
`cant differences are indicated by different
`let-
`ters in all mean data plots. On these graphs
`letters do not appear with the mean values at
`certain
`frequencies,
`indicating
`that
`there
`were no statistically
`significant differences
`among the means according to the ANOVA
`and,
`therefore,
`that
`the pairs of means were
`not further analyzed with pairwise compari-
`sons tests. Means for which pairwise
`tests
`were performed are labeled with letters in all
`cases. Unless noted otherwise, all tests were
`performed at p < 0.05.
`Interaction of fitting procedure with wearing
`time. The presence of this interaction
`indi-
`cated that the fitting procedure used in initial
`donning of the HPD affected the stability of
`the device over the wearing period, but
`this
`was evidenced by attenuation
`reductions
`at
`only 500 and 6300 Hz. Simple-effects F tests
`(F = MS~MSF x T x SU£) on the subject- and
`trained-fit conditions
`indicated that attenua-
`tion decreased significantly over the wearing
`period for both fitting conditions at both test
`
`fit at 500 Hz, F(2,72) =
`subject
`frequencies:
`56.82, P < 0.01; at 6300 Hz, F(2,72)
`=
`43.52, P < 0.01; trained fit at 500 Hz, F(2,72)
`= 17.20, P < 0.01; at 6300 Hz, F(2,72) =
`7.91. p < 0.01. However, according to pair-
`wise Bonferroni
`t test
`results,
`the wearing
`time effect was slightly more pronounced and
`steady for the subject-fit condition (average
`reduction of 3.2 dB attenuation
`from pretask
`to posttask measurement)
`than
`for
`the
`trained-fit condition (average reduction of 1.5
`dB from pretask to posttask). Under both fit-
`ting conditions
`the decrease
`in attenuation
`was monotonic
`across the three attenuation
`measurement
`junctures. These results are de-
`picted in Figure 2.
`Interaction
`of
`fitting procedure with HPD.
`The influence of fitting procedure on attenu-
`ation was not only time dependent but also
`device specific and was consistently
`in evi-
`dence at 1000 Hz and below in the overall
`ANOVA. The simple-effects F tests (F = MS/
`MSF x S(H»
`revealed which HPDs were most
`sensitive to subject
`fit versus trained fit dif-
`ferences. The outcome,
`shown in the lower
`half of Table I, was clear cut: all of the ear-
`plug HPD conditions,
`including the muff-plug
`combination, were highly susceptible
`to fit-
`ting procedure differences, but
`the earmuff
`was not. A post hoc comparison of mean at-
`tenuation on each individual HPD condition
`(except the muff) indicated that substantially
`lower attenuation
`occurred in the subject-fit
`condition than in the trained-fit
`condition,
`ranging from a low difference of 4.0 dB at 250
`Hz for the UltraFit
`to a high difference of 14.1
`dB at 1000 Hz for the E-A-R foam plug. As
`depicted in Figure 3, these fitting procedure
`effects were very pronounced at 1000 Hz and
`below for the combination, E-A-R plug, and
`UltraFit HPDs. Differences
`above 1000 Hz
`were apparent
`for
`the UltraFit;
`however,
`these higher
`frequencies were not analyzed
`on a post hoc basis because they lacked sig-
`nificance in the overall ANOVA. The upper
`
`

`

`HEARING PROTECTION
`
`February 1990-17
`
`Hz, as borne out by the previously discussed
`interactions.
`The difference
`in attenuation
`between fitting procedures was 2-8 dB de-
`pending on frequency, with the largest differ-
`ences occurring at 1000 Hz and below (Fig-
`ure 4; see page 20).
`Interaction of HPD with activity period. This
`interaction has particular bearing on HPD se-
`lection in that
`it revealed that changes
`in
`achieved attenuation with respect
`to wearing
`time were device specific. Based on simple-
`effects F tests (F = MSrlMST x S(H)
`shown in
`the top half of Table 1, it was evident
`that
`the
`attenuation
`provided by all HPDs changed
`significantly
`over
`the wearing
`period
`at
`nearly all frequencies, with the exception of
`the E-A-R foam plug, which showed no atten-
`uation
`changes.
`Subsequent
`Bonferroni
`t
`tests showed that except for the E-A-R foam
`plug there was a significant
`reduction from
`pretask protection to during-task and/or post-
`task protection across nearly all test frequen-
`cies (Figure 5, page 21). These reductions
`in
`for the VI-
`attenuation were quite consistent
`traFit and combination protectors and less so
`for the Bilsom muff, for which the Bonferroni
`t tests revealed differences at 125, 250, 500,
`2000, 4000, and 6300 Hz. When the Bilsom
`muff was worn over the E-A-R plug, there was
`a consistent decrement
`in spectral
`attenua-
`tion over the activity period which did not
`occur with the E-A-R plug alone and more of
`a decrement
`than occurred with the Bilsom
`muff alone.
`Interaction of movement activity with HPD.
`This interaction was isolated in the ANOVA
`to a single frequency,
`6300 Hz, and sub-
`(F = MSAI
`F tests
`sequent
`simple-effects
`MSA x S(H) were conducted to determine the
`differences between the two movement activ-
`ities
`for each HPD. Only the attenuation
`achieved by the Bilsom muff was
`signifi-
`cantly different between activities, with more
`attenuation
`(3.4 dB) in the jaw movement
`condition than in the physical work activity
`
`0
`
`10
`
`20
`
`30
`
`40
`
`50
`
`TRAINED-FIT
`
`~
`-+---
`--
`.•... -.
`
`PRE-TASK
`
`DURING-TASK
`POST-TASK
`
`A
`
`iii
`
`~ z0
`
`Zw
`
`•...•...
`<
`
`~<:
`
`;)
`
`60
`
`II-rll--r-II-rll
`250
`500
`1000 2000 3150 4000 6300 8000
`
`125
`
`o
`
`10
`
`iii
`~ 20
`
`FREQUENCY
`
`(Hz)
`
`SUBJECT-FIT
`
`PRE-TASK
`
`..-..•.- DURING-TASK
`--.- POST·TASK
`
`30
`
`40
`
`50
`
`A
`
`Z0~
`
`<::l
`...
`•...
`<
`
`ZW•
`
`60
`
`1I--r-1I-r11-r1l
`250
`500
`1000 2000 3150 4000 63008000
`
`125
`
`FREQUENCY
`(Hz)
`Figure 2. One· third-octave band attenuation
`in dB
`for each fitting procedure over the course of the ac-
`tivity period. Means with different
`letters are signifi-
`cantly different at p < 0.05 according to a Bonferroni
`t test.
`
`that
`right panel of Figure 3 clearly indicates
`there was little advantage gained in protec-
`tion with the earmuff when trained fitting
`was used instead of subject
`fitting according
`to the manufacturer's
`instructions
`alone.
`Fitting procedure main effect. The fitting
`procedure
`effect, when collapsed
`across
`HPDs, demonstrated
`that
`trained fitting af-
`forded significantly better attenuation
`than
`subject
`fitting at all frequencies except 6300
`
`

`

`18-February 1990
`
`HUMAN
`
`FACTORS
`
`TABLE 1
`
`Simple Effect F Test Summary Tables
`
`Frequency (Hz)
`
`125
`
`250
`
`500
`
`1000
`
`2000
`
`3150
`
`4000
`
`6300
`
`HPD
`
`F Ratios
`
`F tests to determine wearing time effects for each HPD (from Wearing Time x HPD interaction)
`
`Ultra Fit plug
`E-A-R plug
`Bilsom muff
`Muff/E-A-R plug
`
`85.21"
`0.29
`6.68"
`44.08"
`
`19.37*"
`1.02
`6.86"
`14.41"
`
`18.25"
`0.57
`6.79"
`25.90"
`
`25.38"
`0.10
`2.71
`20.48"
`
`25.95"
`0.13
`3.75'
`17.19"
`
`11.84"
`1.69
`0.24
`12.04"
`
`28.14"
`0.90
`6.56"
`8.46"
`
`24.28"
`2.46
`3.77'
`14.20"
`
`F tests to determine fitting procedure effects for each HPD (from Fitting Procedure x HPD interaction)
`
`Ultra Fit plug
`E-A-R plug
`Bilsom muff
`Muff/E-A-R plug
`
`9.10"
`68.68"
`0.12
`67.91"
`
`10.41"
`101.87*"
`0.54
`113.00"
`
`11.89"
`100.44"
`1.05
`84.56"
`
`10.64"
`98.61"
`1.28
`15.68"
`
`, Statistically significant at p < 0.05; "statistically significant at p < 0.Q1.
`
`condition, F(1,36) = 18.83,p < 0.01. Because
`this effect was restricted to a single high fre-
`quency for a single HPD, its explanation is
`difficult. However, it was observed that
`the
`muff appeared to slip down over the pinna
`more in the work activity task (perhaps be-
`cause of the perspiration induced) than in the
`jaw movement task, in which relatively little
`muff slippage was evident. This could have
`accounted for the lower movement activity
`attenuation, but
`it is somewhat surprising
`that
`it would be evidenced only at a single
`high test frequency.
`Activity movement and wearing time effects.
`Although the ANOVAindicated a significant
`main effect difference between jaw move-
`ment and work movement on attenuation,
`the difference was negligible (0.8 dB less at-
`tenuation
`under work activity
`than jaw
`movement) in a practical sense and occurred
`only at a single frequency (