Effects of Active and Passive Hearing Protection Devices on Sound Source Localization, Speech Recognition, and Tone Detection

Abstract

Hearing protection devices (HPDs) such as earplugs offer to mitigate noise exposure and reduce the incidence of hearing loss among persons frequently exposed to intense sound. However, distortions of spatial acoustic information and reduced audibility of low-intensity sounds caused by many existing HPDs can make their use untenable in high-risk (e.g., military or law enforcement) environments where auditory situational awareness is imperative. Here we assessed (1) sound source localization accuracy using a head-turning paradigm, (2) speech-in-noise recognition using a modified version of the QuickSIN test, and (3) tone detection thresholds using a two-alternative forced-choice task. Subjects were 10 young normal-hearing males. Four different HPDs were tested (two active, two passive), including two new and previously untested devices. Relative to unoccluded (control) performance, all tested HPDs significantly degraded performance across tasks, although one active HPD slightly improved high-frequency tone detection thresholds and did not degrade speech recognition. Behavioral data were examined with respect to head-related transfer functions measured using a binaural manikin with and without tested HPDs in place. Data reinforce previous reports that HPDs significantly compromise a variety of auditory perceptual facilities, particularly sound localization due to distortions of high-frequency spectral cues that are important for the avoidance of front-back confusions.

Fig 1. Hearing protection devices (HPDs) tested in the current study.

Four HPDs are illustrated as worn inside the right ear of a test manikin (G.R.A.S. 45CB Acoustic Test Fixture).

Fig 2. Testing apparatus used in all three psychophysical tasks.

Subjects were seated in a circular array of loudspeakers and fitted with a headband supporting (1) an electromagnetic position sensor and (2) a laser pointer. Speakers were obscured by an acoustically transparent curtain, and all testing was completed in total darkness. An LED demarcated the nominal 0° loudspeaker, which served as the starting position on each trial in the localization task, was the target loudspeaker in tone detection and speech-in-noise tasks. Head position was monitored continuously via the overhead position-tracking receiver. See text for additional details.

Fig 3. Localization responses across 5 conditions (Control and 4 HPDs).

Symbols are shifted along the abscissa around each target angle for clarity. The dashed black line (unity) indicates perfect performance. Dotted black lines with negative slopes indicate responses expected for trials on which subjects experienced front-back confusions (see text).

Fig 4. Mean localization errors across subjects.

Mean root-mean-square errors (RMS; upper panel, lower panel) or very large errors (VLEs; middle panel) are given for each response angle across conditions. The lower panel gives RMS values after removal of trials on which VLEs occurred (see text). A grand mean is also given across devices for each measure, based on each subject’s mean error across target speaker angles. Error bars indicate ± 1 standard error across subjects.

Fig 5. Localization data across all source angles and device conditions, plotted in polar coordinates.

Responses from all subjects are combined on the same axes. Each radius (colors) gives the head turn response angle for a single trial. Open circles around the perimeter of each plot indicate the target azimuth. The mean response error across subjects (RMS) and number of VLEs (out of 300 possible), respectively, are shown to the upper right of each plot.

Fig 6. Mean QuickSIN score across device conditions.

Higher numbers indicate better performance. Symbols give data for individual subjects; bold points give the mean across 10 subjects ±1 standard error.

Fig 7. Tone detection thresholds across frequency (abscissa) and HPDs (parameter) relative to Control.

Lower numbers indicate better performance. Bold points give the mean across 10 subjects ±1 standard error.

Fig 8. Comfort ratings across the 3 devices tested used in all tests (Combat Arms, EB15, Hybrid).

Bold points give the mean across 10 subjects ±1 std. error.

Fig 9. Directional transfer functions (DTFs) and differences in DTFs for front and back locations, as measured in the unoccluded (“Control” condition) left ear of an ANSI test manikin (see text).

A. DTFs measured at the 10 locations that potentiated front-back confusions in the psychophysical experiment. Each curve gives the gain (right ordinate) unique to the specified location (left ordinate). The 180° curve is plotted twice to facilitate comparison to nearby locations. B. Contrasts between “front” and “back” DTFs measured in the left ear. Each curve gives the gain differences (right ordinate) across frequency that might be exploited to discriminate the specified front and back pair (left ordinate). Measurement angles are depicted in the inset graphic.

Fig 10. Hearing protective device (HPD)-related distortions of front-back directional transfer functions (DTFs).

A. Front-back DTF contrasts (see Fig 9) are given for Control measurements (as in Fig 9B) and the four tested HPDs (see legend). The black curve gives the difference (in dB) between HPD and unoccluded contrasts. B. Means across all five potential front-back confusions are given for (upper left panel) the root-mean-square (RMS) amplitude (in dB) of black curves in (A) and for (upper right panel) the rate of very large errors (VLEs) in psychophysical testing, across all tested HPDs. The lower panel plots mean VLE rate against DTF distortion, suggesting a very high correlation between the two.