An evaluation of sound restoration hearing protection devices and

audibility issues in mining1)

1) This is the second paper published in NCEJ on the special

topic of Noise in Mines.

,2)

2) The findings and conclusions in this report are those of

the authors and do not necessarily represent the views of

the National Institute for Occupational Safety and Health.

Amanda S. Azmana)

a) NIOSH, 626 Cochrans Mill Rd, P.O. Box 18070,

Pittsburgh, PA 15236, USA; email AAzman@cdc.gov.
 

and Roberta L. Hudakb)

b) NIOSH, 626 Cochrans Mill Rd, P.O. Box 18070,
 

Pittsburgh, PA 15236, USA; email: RHudak@cdc.gov.
 

Despite advancements in engineering noise controls and the use of administrative controls, miners are still dependent on hearing protection devices for

prevention of noise-induced hearing loss. However, miners often complain of

reduced audibility or confusion about identifying spoken words when wearing

conventional hearing protectors. This leads to an increased risk of miners being

struck by moving equipment or errors in communication with co-workers.

Miners will often remove their hearing protectors to overcome these obstacles.

To address this problem, electronic technology exists that allows some amount

of sound to pass through the hearing protector, therefore restoring some audibility of the passively attenuated sounds. This paper will present the results of

testing completed on a selection of four sound restoration hearing protection

devices, with the objective of determining if they provide improved speech intelligibility to workers near certain types of mining equipment. VC 2011 Institute of

Noise Control Engineering.

1 INTRODUCTION

The mining workforce experiences high levels of

noise exposure and in turn suffers from high rates of

noise-induced hearing loss (NIHL). In fact, the mining

sector has the highest prevalence of hazardous workplace noise exposures (76%) among all industrial sectors1

. Despite engineering and administrative controls

implemented to reduce noise, miners continue to exhibit a high prevalence (24%) of hearing difficulty2

.

Therefore, it remains necessary for miners to use hearing protection devices (HPDs) to reduce their chances

of acquiring NIHL.

HPD manufacturers have recognized the problems

associated with working in environments with high

noise levels and the need for workers to hear for proper

job performance and safety. In response, technology

has been developed aimed at selectively restoring some

of the sounds that are normally blocked when conventional HPDs are worn. The devices into which this

technology has been integrated are referred to as sound

restoration hearing protection devices (SRHPDs).

Because miners often work in noise while still needing to be aware of safety hazards around them, they

could benefit from information about the applicability

of SRHPDs to their situation. This paper evaluates a

selection of SRHPDs to determine if they provide

improved speech intelligibility to workers near certain

types of mining equipment.

1.1 Background

Hearing protection devices have several drawbacks,

including unreliable attenuation unless worn correctly,

discomfort for some users, and attenuation that blocks

desired sounds along with noise. Specifically, miners

have expressed concerns that wearing HPDs inhibits

their ability to communicate with co-workers and

reduces the audibility of alarms and other auditory

warnings. Decreased audibility is not only a communication nuisance but can also contribute to unsafe working conditions. For miners who exhibit hearing loss,

the degradation in audibility while wearing hearing

protection can be even greater.

Audibility is also degraded by the presence of background noise. This occurs because background noise

interferes with the human auditory system’s ability to

distinguish and process speech and other meaningful

sounds. This phenomenon is known as masking. Masking is the interference with a person’s ability to detect
and process one sound because of the presence of a

second sound. Further, the masking effect of noise

becomes more pronounced as the sound pressure level

of the background noise increases and the frequency of

the background noise decreases. When the background

noise reaches a certain level, speech becomes totally

inaudible3

.

3) SRHPD specifications vary by manufacturer, therefore

the actual limit to amplification varies between 80 and 82

dB Lp on either a linear or A-weighted scale.

The high levels of noise that are often present in the mining environment can mask both speech

and warning signals, leading to confusion and potential

danger.

1.2	 Current HPD and SRHPD Technology

and Research

SRHPDs are just one of several types of electronic

hearing protectors available today. Each type of electronic hearing protector has a fundamentally different

mode of functioning and different intended purposes,

therefore an important distinction between the types

must be made.

SRHPDs, also known as level-dependent hearing

protectors, have one or two microphones outside of the

ear cup and a speaker inside. This design allows for the

ambient sound to be selectively amplified, thus enabling the audibility of certain sounds while maintaining

protection to the worker from hazardous noise levels.

The devices are designed to amplify sounds that are

considered to be safe for the human ear, typically those

below 80 or 82 decibels3) sound pressure level (Lp).

Below this sound pressure level threshold, the devices

provide adjustable amplification to potentially improve

audibility. As ambient sound approaches the device’s

threshold of amplification, the electronic circuitry performs limiting, compression, or other processing to

keep the sound transmitted to inside the protector

below hazardous levels. The National Institute for

Occupational Safety and Health (NIOSH) definition of

a hazardous noise level is any noise exposure reaching

at least 85 dBA, as an 8-hour time-weighted average.

Most of these devices can also be worn in a passive

mode with the electronics turned off, functioning as a

conventional (passive) hearing protector.

A second common type of electronic hearing protection device involves technology called active noise

reduction (ANR), which uses the concept of phase cancellation to reduce the presence of noise under the ear

cup. ANR technology reduces or cancels unwanted

noise by the addition of a 180-degree out-of-phase

electronically generated sound wave. Some ANR devices are designed to provide passive sound isolation and

attenuation, while others provide little noise reduction

when turned off.

A third common type of HPD technology uses electronic processing to provide protection from impulse

noise. These HPDs are designed to block short duration (less than 1 second), high amplitude sound bursts

such as gunfire, explosions, or industrial stamping

processes. The current study evaluated SRHPDs, but

not ANR or impulse HPDs.

Previous research has examined the benefit of

SRHPDs for enhanced audibility as compared to conventional hearing protectors or the open ear. Two studies

using the Hearing in Noise Test (HINT) found that performance of SRHPDs was approximately the same4 or

worse5 by comparison to conventional hearing protectors. Other research has shown that subjects with normal

hearing perform better on word recognition tasks when

using SRHPDs than with the open ear6 when tested in a

loud environment. It is important to note, however, that

these studies varied greatly in their research methods,

devices tested, and subject population.

1.3	 Objective

Based on the above cited research on HPDs and

SRHPDs, it has been proposed that SRHPDs may

improve audibility for miners as well as for workers in

other noisy industries. The objective of the current

research is to determine the effect of four types of

SRHPDs on the speech intelligibility scores of normal

hearing people exposed to three background noises

common in the mining environment. Specifically, it is

hypothesized that subjects will perform better with an

SRHPD in its active electronic mode than when it is

functioning as a passive protector. It is also hypothesized that subjects will perform better when exposed to

a lower volume background noise than a higher volume background noise. Finally, it is hypothesized that

the device with the most sophisticated processing system would have the highest speech intelligibility score.

The current study is the second of two NIOSH studies of the performance of SRHPDs in mining noise. In

the first study, an instrumented acoustic test fixture

(ATF) was used to measure the output of nine

SRHPDs and predict audibility in various background

noises with various device settings using the Speech

Intelligibility Index (SII)7

. The experimenters sought

devices that had a stated purpose (in their marketing

materials or packaging) of improved audibility or communication in high noise environments. Candidate

devices were identified through an extensive search of

the Internet and contact with known manufacturers of

HPDs and other safety products which took place during the spring and summer of 2009. All devices were
purchased in new condition from standard consumer

vendors. The results obtained from the SII were used

to select a reduced set of devices (from the original

group of 9) and conditions for human subject testing in

the second stage of research, described here. Because

the first study required unweighted sound pressure levels for application of the SII, the second study was also

conducted using unweighted values for consistency in

methods and results. All references to decibels or

sound pressure level (Lp) throughout the paper are

unweighted unless otherwise specified.

2 METHODS

2.1 Subjects

This laboratory experiment evaluated the audibility

performance of four types of SRHPDs set to specific

outputs as tested on human subjects in mining background noise. Potential subjects were recruited from a

subject pool maintained by NIOSH. The pool consisted

of non-NIOSH employees who had been recruited

through postings at local businesses and by word of

mouth. An otoscopic exam of each potential subject’s

ears was conducted to rule out any blockage, infection,

or other condition that could affect audibility. Normal

hearing sensitivity was required for participation. To

ensure that each potential subject had hearing sensitivity better than or equal to 20 dB HL, audiometric testing that followed the modified Hughson-Westlake

procedure8 was conducted using a Welch Allyn GSI 61

2-channel clinical audiometer9 and TDH-50 earphones.

Middle ear function was assessed by a licensed audiologist using a Welch Allyn GSI 33 middle ear analyzer.

All potential subjects with normal hearing sensitivity

also exhibited middle ear function within the range of

normal. All screening procedures were conducted by a

licensed audiologist or CAOHC-certified occupational

health nurse. The NIOSH-required Human Subject

Review Board approvals were obtained prior to the start

of subject testing. Thirty subjects aged 18 to 55 (mean

32 years old, median 30 years old, with 22 females and

eight males) passed the required screening measures

and were enrolled in the study. The subjects were reimbursed at a standard rate for their participation.

2.2 Devices Under Test

Based on the results from the prior evaluation of

nine devices by Azman and Yantek7

, four devices

were selected for subject testing. These devices were

labeled as devices 1, 2, 3, and 4. Two of the devices

had output adjustment knobs and two had buttons. Device 1 was the most expensive with many proclaimed

advanced features. Device 2 was from a widely known

hearing protector manufacturer. Device 3 is permissible for use in underground coal mines. Device 4 was

manufactured by a company that produces a variety of

mining health and safety products.

Three samples of each type of device were used, for

a total of 12 sample devices. All 30 subjects were

tested using all four devices in all research conditions.

A subject used the same device sample for all tests of

that particular device, and each device sample was

used by 10 subjects.

2.3 Experimental Paradigm and Stimuli

The Modified Rhyme Test (MRT) was the testing

paradigm implemented. The procedure in ANSI S3.2­

200910 was used as guidance; however deviation from

the standard was necessary. The lengthy practice and

training time required within the standard was not

implemented because it, in addition to multiple conditions of testing would have led to an overly long test

protocol that could have created concerns for subject

fatigue. Even without the training and practice time

stated in the standard, the test sessions neared six hours

for some subjects. Also the minimum number of talkers stated within S3.2-2009 was not implemented

because it requires an equal number of talkers and listeners. For this situation, it was more important to have

a consistent speech stimulus and was not realistic to

have 30 talkers to equal the 30 listeners (subjects) participating in the study.

The MRT is useful for this purpose because it is

designed to determine speech intelligibility of communication systems. This closed-set response test was

developed by House et al.11 and has been shown to

yield self-consistent and reliable results12. The MRT

has demonstrated sensitivity to degraded speech and

has been used successfully with human and digitally

altered speech stimuli both in reverberation and in

noise. Other standard hearing protector tests, such as

ANSI S12.6-200813, determine the attenuation of hearing protectors, but were not implemented because

quantification of attenuation was not the purpose of

this study.

The MRT consists of 50 six-word sets of monosyllabic English words that vary on either the first or the

last sound. The listener is shown a printed list of 50

six-word sets from which to choose a pre-recorded,

spoken target word, which is always among the six

listed words. The target word is presented, then there is

a brief pause while the listener chooses between the

options, then the next target word is spoken. This is

repeated for all 50 targets of a particular list. Figure 1

illustrates an example of three six-word sets from an

MRT list that was used in this study. A full test list
contains 50 of these sets. The 50 six-word sets were

randomly ordered into 40 lists, each set with a

randomly-determined target (spoken) word.

Fig. 1—Example of the first three six-word

sets of list 7 of the modified rhyme

test. One word from each set is the

spoken target word which was circled

or highlighted by the subject.

Listener responses can be scored as (1) the number

of words heard correctly; (2) the number of words

heard incorrectly; or (3) the frequency of particular

confusions of consonant sounds. For the purposes of

the current study, the MRT was scored by percentage

of correct responses (e.g., 35 correct out of 50 stimuli

would be 70% correct). This provides a simple means

for comparing performance across devices.

The MRT stimuli were recordings of spoken target

words generated during previous NIOSH work. For the

current project the MRT target words were adjusted to

82 dB Lp as measured by a Bruel and Kjaer 4188-A

microphone hanging in the center of the booth routed

through a National Instruments PXI 4462 4-channel

data acquisition system, and analyzed by a Nelson

Acoustics Trident acoustic analysis software package.

The Trident software provides a real-time display of

the sound pressure levels in the booth as well as onethird octave band data.

It is recognized that this stimulus sound pressure

level is louder than the volume at which speech would

typically occur. However, speaking with a raised voice

is a realistic occurrence in noisy environments, and

this level is in accordance with the standard speech

spectrum level of 82 dB Lp identified as “shout” in

ANSI S3.5-199714. This was also the speech level

used for the prior NIOSH SII study; therefore the same

level was used in the current human subjects study for

consistency.

Sounds from the mining environment served as the

background noise for the MRT testing. These noises

were recorded by NIOSH researchers during earlier

field studies. The recordings consisted of noise from

continuous mining machines (CMM) and roof bolting

machines (RBM), which are common sources of noise

overexposure in underground coal mining. The original recordings consisted of sound from operating

modes of the CMM and RBM, but also occasionally

included workers’ voices and other extraneous sounds.

The sound files were edited for time and content to

contain 30 seconds of the sound emitted during the

desired mode of operation of the machine. The background noise levels were adjusted to 80 dB 30-second

Leq for the CMM cutting=conveying and the RBM drilling, and to 78 dB 30-second Leq for the CMM tramming. The range of Lp within the 30-second Leq for the

CMM cutting=conveying and the RBM drilling was

79.5–80.5 dB while the range of Lp for the CMM tramming was 76.5–79 dB. These levels were measured in

the center of the booth by the same 4188 microphone

and the Trident software. The 30-second clips were

looped to yield background noise files that were 4.5

minutes in length. Spectral analysis of the noise files

showed that the noises varied in spectral content as

well as Lp. Figure 2 shows that the RBM noise contains relatively more high frequency content than the

two CMM noises.

The background noises were then digitally merged

with MRT stimulus words so that synchronization of

the words in relation to the start and stop of the background noise was consistent for all test lists. This was

done to yield 40 test lists that had equal timing for the

start and stop of the target words as well as equal timing for the start of each individual test word in relation

to the background noise. The background noise started

10 seconds prior to presentation of any target words of

the MRT test list and stopped 10 seconds after the end

of the list. The combined background noise=MRT

stimulus test lists were saved as 40 individual files for

use during the subject testing.

2.4 Testing Apparatus

All research took place in the Auditory Research

Laboratory located at the NIOSH Pittsburgh facility.

All testing occurred in a double-walled sound-treated

hearing test booth. The stimuli sound sources were

stored as lossless digital files and were played from a

desktop personal computer. Sound sources were routed

into the test booth through an MPX 300 amplifier and

a GSI clinical audiometer. The audiometer was used to

adjust stimulus intensity to 1 dB accuracy. The soundtreated booth was outfitted with two diagonallysituated Cambridge Soundworks Model 6 speakers

from which the test stimuli were presented. Sound

pressure levels of the combined stimulus words and

background noise inside the test booth were confirmed

using the same microphone and data acquisition system mentioned previously.
Fig. 2—Unweighted frequency spectra of the 3 background noise sources. Notice that the CMM

noises contain more low frequency content than the RBM noise.

3 PROCEDURE

Subjects were tested using the MRT while seated in

the center of the test booth in a non-absorptive chair

with a small desk on which their response sheet and

writing tool were located. All SRHPDs were fitted on

the subjects by a researcher. The researcher also

manipulated the output control of the SRHPD to the

desired position. Each subject was tested in nine conditions for each of the four devices across either one or

two sessions. The conditions for testing each of the

four devices were the three noise types (CMM cutting

and conveying 80 dB, CMM tramming 78 dB, or

RBM drilling 80 dB) by the three device settings (passive, 1= 3 4 output, =4 output) which resulted in 36 conditions. For each condition, a separate test list was used.

Four additional test lists were used as practice lists

before beginning testing.

In accordance with the MRT, the subjects selected,

by circling or highlighting, a spoken target word from

a closed set of words that were visually displayed on

sheets of paper. The test lists were presented from the

speakers in the test booth. The spoken target words

began after 10 seconds of background noise with each

utterance lasting approximately 1.5 seconds with a
 

3-second pause between each utterance. The carrier

phrase mark the ____ now was used in all conditions.

During the 3-second pause between words, the background noise continued. Subjects were instructed to

mark the target word from a response list containing

the 50 six-word sets. The sets were numbered 1–50

and matched the auditory presentation order. There

was a 1–3 minute break between each word list while

the hearing protector and response sheet were changed

by the researcher. Testing was monitored through a

window in the test booth to ensure that the subjects

remained on the correct six-word set as the list progressed. Subjects were instructed to guess at the word

when they could not discern it. This procedure was

done for all 36 test lists and the practice lists. The list

order, SRHPD model used, SRHPD setting used, and

background noise were all randomized. Although subjects often made comments regarding test conditions or

their performance, no feedback was given by the

researchers regarding correctness of responses or difficulty of listening conditions.

The study resulted in 1080 response lists derived

from testing 30 subjects on 36 different test lists (the

practice lists were not used in the final analysis). The

paper copies of all results were scored by two research

assistants. There was no threat of bias in scoring based

on knowledge of the test procedure, condition, or subject because the chosen word was either correct or

incorrect. The results were coded and entered into electronic format by another researcher with 20% of the

results checked for accuracy by the primary researcher.

Devices were coded as 1, 2, 3, and 4; noises were

coded as 1 for CMM tramming, 2 for RBM drilling,

and 3 for CMM cutting=conveying; settings were

coded as 0 for off=passive, 1 for 1=4 of output, and 3 for

3=4 of output.

4 RESULTS

A 3x3x4 analysis of variance (ANOVA) was used

to test the hypotheses with score as the dependent variable and noise, setting, and device as the independent

variables. ANOVA can also be used to detect interaction effects, although none were hypothesized. Table 1

shows the mean performance scores represented as

speech intelligibility scores for each category of the
independent variables. The largest difference in mean

speech intelligibility scores was found between the categories of the noise independent variable (22% change

from highest to lowest score). The difference is much

less from the highest to lowest mean scores within the

setting and device independent variables (3% for setting and less than 2% for device).

Table 1—Mean performance scores represented

as speech intelligibility for each level of

the independent variables when collapsed across all other variables.

INDEPENDENT

VARIABLE

Mean Performance

Score

Standard

Deviation

NOISE

Tramming

Drilling

Conveying

88.59

73.14

66.18

5.17

6.79

8.65

SETTING

Passive

1=4 output

3=4 output

76.84

76.90

73.95

11.19

11.60

12.06

DEVICE

1

2

3

4

76.15

76.60

76.20

74.92

11.66

11.28

11.33

11.76

While the ANOVA found that all three main effects

were statistically significant, no interaction effects

reached significance. The first hypothesis predicted

that performance scores would be the lowest (poorest)

with setting 0 (passive) compared to settings 1 (1=4

output) and 3 (3=4 output). The analysis showed that

the effect of setting reached statistical significance, F

(2, 1044) 1⁄4 10.734, p < .001, however the effect was

small from a practical standpoint. The Bonferroni post

hoc test15 revealed that subjects scored 3% lower when

using setting 3 (3=4 output) than either settings 0 (passive) or 1 (1=4 output). This result is contrary to the

hypothesis that subject performance would improve

when the SRHPDs were in the active mode.

The second hypothesis predicted that performance

scores would be higher (better) in the background

noise with the lower Lp than in the other background

noises. Specifically, performance would be better

in the CMM tramming background noise than the

RBM drilling or CMM cutting=conveying background

noises. The analysis revealed a statistically significant

effect for noise F (2, 1045) 1018.251, p < .001. The

Bonferroni post hoc

1⁄4

 test revealed that subjects scored

15% better when exposed to background noise 1

(CMM tramming) compared to noise 2 (RBM drilling)

and 22% better in background noise 1 than background

noise 3 (CMM cutting=conveying). The difference in

average scores of 7% between background noises 2

and 3 also reached statistical significance.

The third hypothesis predicted that performance

scores would be higher (better) when subjects used device 1 than when using devices 2, 3, or 4. The analysis

revealed a statistically significant effect for device F

(2, 1045) 1018.251, p < .001. Contrary to hypothesis

3, the Bonferroni

1⁄4

 post hoc test revealed that subjects

using device 2 scored 1.7% higher than subjects using

device 4. There were no other statistically significant

differences between devices.

5 DISCUSSION

The most notable differences in speech intelligibility

occurred between the three noise sources. When collapsed across all variables, better performance was

obtained in the lower level background noise as illustrated in Fig. 3.

Fig. 3—Graph indicating that speech intelligibility results varied the most by

background noise source.

The best speech intelligibility was

obtained in the CMM tramming at 78 dB background

noise regardless of device type or setting. Again, this

was the background noise with the lowest presentation

Lp. This suggests that regardless of the device used or

the setting of the device, better performance occurs in

relatively low background noise situations, which is

contrary to user expectations that the devices are most

beneficial in high noise settings. Also evident in Fig. 3

is that within each background noise, mean performance varied by approximately 5% or less. That is, little

difference can be noted by device or device setting

within each type of noise.
Smaller differences are noted in speech intelligibility scores between device settings; however, these differences are contrary to an expectation that higher

settings would increase performance. The best performance was found when the device was turned off or

at 1=4 of its output. In working conditions, many users

will be tempted to turn the devices to maximum output

under the impression that they will receive maximum

benefits. Clearly, this is not the case, as can be seen in

Figs. 4, 5 and 6.

Fig. 4—Speech intelligibility scores for the

various devices and settings in background noise source CMM tramming

78 dB. Small improvements can be

==

=

noted with some devices from off to

1 4 on, but no additional improvement

is noted from 1 4 on to 3 4 on.

==

Fig. 5—Speech intelligibility scores for the

various devices and settings in background noise source RBM drilling 80

dB .No improvements can be noted

from off to 1 4 or 3 4 on.

=

=

=

Fig. 6—Speech intelligibility scores for the

various devices and settings in background noise source CMM cutting=conveying 80 dB. Small

improvements can be noted with some

devices from off to 1 4 on, but no additional improvement is noted from 1 4

on to 3 4 on.

For all of the noises and devices evaluated, performance was the worst when the device was

at the maximum output setting.

While the results indicate statistical significance,

one must also consider the practical significance of

these findings. Many of the performance differences

were on the order of 1–2 words, or 2–4 percentage

points. This small difference in speech intelligibility is

unlikely to have a dramatic effect on the ability of a

miner to comprehend spoken language. Further, none

of the devices or conditions neared 100% speech intelligibility, but they all remained at 50% or better.

Although there is a clause within S3.2-2009 to calculate MRT scores with a correction for subjects

guessing the correct word, this calculation was not

included in the analysis. The application of the correction factor reduces scores by a ratio of the number of

incorrect choices. The correction for guessing would

be applied to all test conditions and would affect all of

them to approximately the same degree. Therefore, it is

unlikely that that inclusion of the correction factor

would change the relative results based on comparisons across conditions.

The intent of this study was to evaluate SRHPD performance in various mining noises. The main noise

characteristic of interest was the Lp at which the background noises were presented, however the frequency

spectra varied as well (see Fig. 2), which is common

with real-world noises. While the best performance was

observed for the lowest noise Lp (as expected), a smaller

effect was also found between noises at the same Lp,

but with different spectral content. The current study

was limited to just the three different noise samples,

which is insufficient to fully explore the interaction

between noise spectral content, Lp, device processing

characteristics, and audibility. Future studies should

consider including noise samples from more types of
machinery that encompass a broader range of spectral

content and Lp. To gain more control over withinsubject performance, future studies should also consider

acquiring a baseline of speech intelligibility in a quiet

setting to identify each subject’s decrement due to noise

and improvement (if any) due to SRHPD processing.

A final limitation lies within the presentation of the

background noise and MRT words within the test

chamber. The mining background noises and the MRT

words were digitally mixed and presented as a monaural signal through the two loudspeakers in the test

chamber. This presentation eliminated any opportunity

for subjects to use spatial cues to differentiate between

the speech and noise sounds. In some mining settings,

workers are very close to a machine noise source while

speech occurs at a distance. In other cases, machine

noise may be at a distance while the worker is listening

to speech from a nearby coworker. In each of these

cases, directionality cues and cues related to the acoustic environment (e.g., reverberation) could affect the

relative audibility of speech in noise. Greater control

over these factors could be attained by simulating the

environment through a multi-loudspeaker arrangement.

Near field sounds (e.g., speech from a nearby colleague) could be represented by having the sound emanate from a single loudspeaker in the array. Although

the simpler two-loudspeaker monaural configuration

used in the present study could have lowered overall

absolute audibility scores, it is unlikely to have resulted

in any systematic bias in the relative findings. Because

the study was intended to assess relative performance

of SRHPDs in various conditions, a factor that has a

uniform effect is not a significant concern. Still, a more

realistic multi-loudspeaker setup would be useful in

future studies, especially if devices are developed with

spatial processing capabilities.

6 CONCLUSIONS

Sound restoration hearing protection devices, or

level-dependent hearing protectors, do not appear to

provide a simple across-the-board solution to improving audibility for workers in mining. The devices show

small potential benefits when worn in specific ways in

certain noise situations, but those situations must be

carefully assessed for workers to enjoy the benefits of

wearing SRHPDs, as follows:

•	 SRHPDs provide the most benefit for speech

intelligibility in background noise levels that

may not cause a worker to reach the NIOSH

recommended exposure limit (85 dB(A) as an

8-hour time-weighted average with a 3 dB

exchange rate). This testing was done with

background noises of 78 or 80 dB Lp. The previous instrumented SII study6 employed a

background noise of 95 dB Lp, which is a more

typical sound pressure level for operators of

continuous mining machines. With the CMM

noise at 95 dB, it was estimated that speech

cues would be almost totally inaudible despite

the processing technology because of the

powerful masking effect of the noise.

•	 SRHPDs provide the most benefit at lower output settings. Wearers must be aware of the output levels to which they adjust the devices.

Wearers should listen to ambient sounds

through the devices while they adjust them and

recognize that they can make additional output

adjustments as the ambient noise changes.

A recent study investigating the benefits of active

versus passive hearing protectors has revealed similar

results16. The results of this study found that speech

recognition of active and passive HPDs was highly dependent on the background noise implemented. Additionally, the passive hearing protectors improved

speech recognition at higher stimulus sound pressure

levels. Variation among devices and settings was

found to which the authors recommended trial of the

devices in various sound environments.

Although limited benefits may be obtained from

SRHPDs by workers who are constantly exposed to

high levels of noise, one must also consider the potential benefits of SRHPDs for workers who are intermittently but repeatedly exposed to hazardous noise

throughout their working shift, or those who have noise

exposure at the lower end of the hazardous continuum

(85–89 dBA 8-hr TWA). Research has shown that in

various heavy industries such as road construction,

sheet metal stamping, and lumber milling, the use of

hearing protection is highest when noise exposure is

relatively high and exposure is continuous17. Workers

who are only intermittently noise-exposed or those at

the lower exposure levels are less likely to maintain

HPD use, especially in situations where they feel that

the noise has dropped below a hazardous level. The use

of SRHPDs would allow for those workers to maintain

some amount of audibility while protecting their hearing in intermittent or lower-level hazardous noise.

The results of this study show that when exposed to

lower level noises, wearers of SRHPDs continue to

identify speech well when the devices are set to a low

output. Because the devices allow ambient sound to

pass through, with proper training on device use, workers may be less likely to remove their hearing protectors to communicate when they are in relatively quieter

conditions that they might consider non-hazardous.

Continued use of hearing protectors in these conditions

has two benefits to the worker. First, they will not
forget to reapply their hearing protectors, nor reapply

them incorrectly, if they have not been removed. Second, and perhaps most importantly, workers will not

have to make judgments regarding the “safety” of a

particular noise environment. Workers can continue to

wear their hearing protectors and maintain levels of

communication and protection in situations where they

may have previously removed their HPDs.

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