Theoretical and practical implications of the effects of hearing protection devices on localization ability

Training to Improve Spatial Hearing and Situation Awareness when Wearing Hearing Protection

INTRODUCTION

Hearing protection devices (HPDs) are standard personal protective equipment in military settings, but many service members may choose to not use HPDs because they impair spatial hearing and situation awareness. In an effort to reduce barriers to compliance by improving situation awareness while wearing HPDs, this study investigated whether brief training could counteract spatial hearing deficits when wearing HPDs. Participant's ability to correctly apply the HPDs across days was also examined.

MATERIALS AND METHODS

Young adults were randomly assigned to one of two groups: training or control (n = 25/group). Participants in each group performed a spatial hearing task while wearing HPDs and in an open ear condition without HPDs. Individual targets were battlefield sounds or white noise presented from a speaker array that surrounded the participant in the horizontal plane. After presentation of each target sound, the participant then controlled a white noise "auditory pointer," which they moved to the perceived location of the target. The two primary measures were the percent of trials with very large errors (> 45°), which were usually due to confusing front and back locations, and absolute localization, which is the difference between the pointer location and the true sound location. Both groups were tested on Days 1 (baseline) and 5 (post-test). On Days 2 to 4, the training group wore HPDs while receiving auditory and visual feedback after each trial.

RESULTS

Across all participants on Day 1, wearing HPDs increased the frequency of very large errors by about 3× and impaired localization by about 40%, relative to the open ear condition. When comparing performance at baseline (Day 1) and post-training Day 5, the training group with HPDs had significant reductions in very large errors and improved absolute localization (P values < .001). The training group also had significant improvements from Days 1 to 5 in the open ear condition. When the control group wore HPDs, there were also significant improvements from Days 1 to 5 (fewer very large errors and better localization), with smaller effect sizes vs. the training group. Controls did not have significant improvement in the open ear condition, but had similar trends. Most participants consistently applied the HPDs, but a subset of ∼20% frequently failed to achieve the criterion attenuation of 15 dB (over 0.25-4.0 kHz) in both ears.

CONCLUSIONS

These findings show that simple, relatively brief practice and training can substantially reduce HPD impairments on spatial hearing and situation awareness. The gains from training and practice can inform the development of relatively simple, brief methods to reduce HPD spatial hearing impairments, potentially leading to increased HPD compliance. Longitudinal data show that a subset of participants would not have received the full benefit of hearing protection because of improper application of the HPDs.

Perforated Concave Earplug (pCEP): A Proof-of-Concept Earplug to Improve Sound Localization without Compromising Noise Attenuation

Combat soldiers are currently faced with using a hearing-protection device (HPD) at the cost of adequately detecting critical signals impacting mission success. The current study tested the performance of the Perforated-Concave-Earplug (pCEP), a proof-of-concept passive HPD consisting of a concave bowl-like rigid structure attached to a commercial roll-down earplug, designed to improve sound localization with minimal compromising of noise attenuation. Primarily intended for combat/military training settings, our aim was an evaluation of localization of relevant sound sources (single/multiple gunfire, continuous noise, spoken word) compared to 3M™-Combat-Arms™4.1 earplugs in open-mode and 3M™-E-A-R™-Classic™ earplugs. Ninety normal-hearing participants, aged 20-35 years, were asked to localize stimuli delivered from monitors evenly distributed around them in no-HPD and with-HPD conditions. The results showed (1) localization abilities worsened using HPDs; (2) the spoken word was localized less accurately than other stimuli; (3) mean root mean square errors (RMSEs) were largest for stimuli emanating from rear monitors; and (4) localization abilities corresponded to HPD attenuation levels (largest attenuation and mean RMSE: 3M™-E-A-R™-Classic™; smallest attenuation and mean RMSE: 3M™-Combat-Arms™4.1; pCEP was mid-range on both). These findings suggest that the pCEP may benefit in military settings by providing improved sound localization relative to 3M™ E-A-R™-Classic™ and higher attenuation relative to 3M™-Combat Arms™-4.1, recommending its use in noisy environments.

Audiovisual training rapidly reduces potentially hazardous perceptual errors caused by earplugs

Our ears capture sound from all directions but do not encode directional information explicitly. Instead, subtle acoustic features associated with unique sound source locations must be learned through experience. Surprisingly, aspects of this mapping process remain highly plastic throughout adulthood: Adult human listeners can accommodate acutely modified acoustic inputs ("new ears") over a period of a few weeks to recover near-normal sound localization, and this process can be accelerated with explicit training. Here we evaluated the extent of such plasticity given only transient exposure to distorted inputs. Distortions were produced via earplugs, which severely degrade sound localization performance, constraining their usability in real-world settings that require accurate directional hearing. Localization was measured over a period of ten weeks. Provision of feedback via simple paired auditory and visual stimuli led to a rapid decrease in the occurrence of large errors (responses >|±30°| from target) despite only once-weekly exposure to the altered inputs. Moreover, training effects generalized to untrained sound source locations. Lesser but qualitatively similar improvements were observed in a group of subjects that did not receive explicit feedback. In total, data demonstrate that even transient exposure to altered spatial acoustic information is sufficient for meaningful perceptual improvement (i.e., chronic exposure is not required), offering insight on the nature and time course of perceptual learning in the context of spatial hearing. Data also suggest that the large and potentially hazardous errors in localization caused by earplugs can be mitigated with appropriate training, offering a practical means to increase their usability.

Situational Awareness: The Effect of Stimulus Type and Hearing Protection on Sound Localization

The purpose of the current study was to test sound localization of a spoken word, rarely studied in the context of localization, compared to pink noise and a gunshot, while taking into account the source position and the effect of different hearing protection devices (HPDs) used by the listener. Ninety participants were divided into three groups using different HPDs. Participants were tested twice, under with- and no-HPD conditions, and were requested to localize the different stimuli that were delivered from one of eight speakers evenly distributed around them (starting from 22.5°). Localization of the word stimulus was more difficult than that of the other stimuli. HPD usage resulted in a larger mean root-mean-square error (RMSE) and increased mirror image reversal errors for all stimuli. In addition, HPD usage increased the mean RMSE and mirror image reversal errors for stimuli delivered from the front and back, more than for stimuli delivered from the left and right. HPDs affect localization, both due to attenuation and to limitation of pinnae cues when using earmuffs. Difficulty localizing the spoken word should be considered when assessing auditory functionality and should be further investigated to include HPDs with different attenuation spectra and levels, and to further types of speech stimuli.

Practice Effects for Auditory Localization: A Test of a Differentiation Theory of Perceptual Learning and Development

Practice and transfer of practice in an auditory localization task were observed for three different information conditions (normal, transformed and reduced) to examine four aspects of Gibson's (1969) differentiation theory of perceptual learning and development. Support was found for the propositions that: (i) dimensions of difference, rather than prototypes, are learned; (ii) specificity is a more appropriate response measure than number of correct responses; (iii) perceptual learning will occur regardless of whether feedback is given or not; (iv) transfer of practice will be greater if dimensions of difference (as opposed to prototypes) are learned. It was also found that feedback during practice did not affect transfer. In an apparent contradiction though, specificity increased in the absence of dimensions of difference. This increase was interpreted as being associated with the detection of non-task-related information and was seen as lending support to Gibson's hypothesis that, perceptual learning is motivated by intrinsic cognitive drive and is terminated by a reduction in subjective uncertainty.

The impact of hearing protection on sound localization and orienting behavior

The effect of hearing protection devices (HPDs) on sound localization was examined in the context of an auditory-cued visual search task. Participants were required to locate and identify a visual target in a field of 5, 20, or 50 visual distractors randomly distributed on the interior surface of a sphere. Four HPD conditions were examined: earplugs, earmuffs, both earplugs and earmuffs simultaneously (double hearing protection), and no hearing protection. In addition, there was a control condition in which no auditory cue was provided. A repeated measures analysis of variance revealed significant main effects of HPD for both search time and head motion data (p < .05), indicating that the degree to which localization is disrupted by HPDs varies with the type of device worn. When both earplugs and earmuffs are worn simultaneously, search times and head motion are more similar to those found when no auditory cue is provided than when either earplugs or earmuffs alone are worn, suggesting that sound localization cues are so severely disrupted by double hearing protection the listener can recover little or no information regarding the direction of sound source origin. Potential applications of this research include high-noise military, aerospace, and industrial settings in which HPDs are necessary but wearing double protection may compromise safety and/or performance.

Effects of hearing protectors on auditory localization in azimuth and elevation

An experiment was conducted to determine the effects of two types of hearing protectors on auditory localization performance. Six listeners localized a 750-ms broadband noise from loudspeakers ranging in azimuth from -180 degrees to +180 degrees and in elevation from -75 degrees to +90 degrees. Independent variables included the type of hearing protector and the elevation of the source. Dependent measures included azimuth error, elevation error, and the percentage of trials resulting in a front-back confusion. Performance on each of the dependent measures was found to be mediated by one or more of the independent variables. Actual or potential applications include the generation of improved design guidelines for hearing protectors and workplace alarms.

Effects of earplugs and protective headgear on auditory localization ability in the horizontal plane

The purpose of this study was to determine how well humans localize sound sources in the horizontal plane while wearing protective headgear with and without hearing protection. In a source identification task, a stimulus was presented from 1 of 20 loudspeakers arrayed in a semicircular arc, and participants stated which loudspeaker emitted the sound. Each participant was tested in 8 conditions involving various combinations of wearing a Kevlar army helmet and two types of earplugs. Testing was conducted at each of 2 orientations (frontal and lateral). In the frontal orientation, overall error was slightly greater in all protected conditions than in the bare-head control condition. In the lateral orientation, overall error score in the protected conditions was substantially and significantly greater than in the bare-head control conditions. Most errors in the lateral orientation were accounted for by front-back confusions, indicating that the protective devices disrupted high-frequency spectral cues that are the basis for discriminating front from back sound sources. The results have practical implications for the use of protective headgear and earplugs in industrial or military environments where localization of critical sounds is important.

Spatial perception of speech in various signal to noise ratios

OBJECTIVE

The study was designed to assess the effects of background noise level on the detection and localization of speech.

DESIGN

The phrase "Where is this?" was presented either in quiet or in a diffuse noise field, through loudspeakers arranged in a 360 degrees azimuth array. The noise conditions included 11 signal to noise ratios (SNRs) ranging from -18 dB SNR to +12 dB SNR in 3 dB increments. Seventeen normal-hearing subjects, aged 18 to 29, participated in the study.

RESULTS

Results revealed that in all listening conditions the signal was most easily detected when presented through a loudspeaker positioned at 90 degrees or 270 degrees azimuth. Although the actual level for 50% detection varied as a function of loudspeaker location and SNR, 85% and 100% of all presentations of the signal were detected at -9 dB and -6 dB SNR, respectively. Localization accuracy improved as the SNR increased, ranging from 18% accuracy at -18 dB SNR to 89% at +12 dB SNR. Localization accuracy in quiet was 95%. The data are discussed in reference to patterns of responses at each loudspeaker location.

CONCLUSIONS

Detection of the target signal deteriorated as background noise level increased and was dependent on the source location of the incoming signal, as expected. Localization accuracy of the target signal was highly dependent on the SNR and spatial location of the signal source. Detection and localization accuracy data were found to be repeatable across test sessions and response patterns were found to be symmetrical on the right and left sides of the horizontal plane.

Sound localization. The interaction of aging, hearing loss and hearing protection

The effect of conventional ear plugs and ear muffs, and muffs with limited dichotic amplification on the ability to localize one-third octave noise bands was investigated under semi-reverberant listening conditions. Forty-eight normal-hearing subjects, half over 40 years of age, and 23 subjects with bilateral sensorineural hearing loss participated. Sound localization was assessed using an array of six loudspeakers surrounding the subject at azimuth angles 60 degrees apart. One block of 120 forced-choice speaker identification trials was presented for each of 16 listening conditions defined by ear condition (unoccluded, E-A-R plug, E-A-R muff, and Bilsom 2392 muff), stimulus frequency (500 Hz and 4000 Hz), and background (quiet and continuous 65 dB SPL-white noise). Plugs and muffs, particularly active muffs, resulted in decrements in right/left judgments based on interaural intensity but not time-of-arrival differences. High-frequency front/back discrimination was affected more by muffs than by plugs. Error patterns for the conventional and active muffs were dissimilar. Aging resulted in a decrement in unoccluded front/back discrimination. Trends for the impaired subjects were the same as those for normal subjects at 500 Hz. Many could not hear 4000 Hz with conventional protectors. Their performance was no different from normal with the active muffs.