Elsevier

Hearing Research

Volume 122, Issues 1–2, August 1998, Pages 119-124
Hearing Research

Intrinsic differences in hearing performances between ears revealed by the asymmetrical shooting posture in the army

https://doi.org/10.1016/S0378-5955(98)00104-XGet rights and content

Abstract

Left ear noise-induced hearing losses are dominant in the army. The common explanation is the asymmetrical effect of the shooting posture. However, firm evidence to support this possibility is still lacking. In the French army shooters, eye preference rather than hand preference for shooting determines the side of shooting and hence the ear more exposed to noise. To test whether left-right asymmetry of hearing thresholds really relates to the shooting posture, we analyzed audiograms from 644 officers of the infantry and artillery branches. The interaural differences reached 7 dB for right-eyed subjects, and less, about 5 dB, for left-eyed subjects at 6–7 kHz, both with disadvantage for the left ear. In contrast, hearing thresholds of both groups in the low frequency range were significantly better for left ears. Our results suggest each ear has different intrinsic characteristics. The right cochlea might be a less sensitive but more robust sensor than the left cochlea, which might be a finer sensor but more sensitive to noise.

Introduction

In the military environment it is known that noise-induced hearing losses at 4–6 kHz are, on average, higher in the left ear than in the right ear (Ward, 1957; Buffe et al., 1986; Sorri et al., 1983; Prosser et al., 1988). In the French army, the gun commonly used is a long gun called FAMAS. More than half of acoustic traumas are related to the use of this gun (Mingoutaud, 1996). With long guns, noise exposure is asymmetrical and the explanation for the left ear inferiority seems evident because of the predominance of right-side shooters in the population: when a person fires on the right side, the impulse noise is directed more to the left than the right ear, because the right ear is positioned in an `acoustic umbra', somewhat protected from noise by the head inclination during sighting and shooting (Fig. 1) (Odess, 1972; Anttonen et al., 1980; Ylikoski, 1989). As a consequence a left-side shooter should have the opposite deleterious effect, namely a poorer right ear. Because of the small proportion of left-side shooters, tests of this hypothesis are lacking, although one large study of the possible effect of handedness on hearing threshold asymmetry has been done in a random population (Pirilä et al., 1991). The study concluded that handedness cannot be responsible for the average inferiority of hearing threshold in the left ear.

In the French army, the side of shooting is determined according to sighting eye preference, not to hand preference. Soldiers are trained to use the eye, hand and shoulder of the ipsilateral side and this side is determined according to the sighting eye, whatever the handedness.

Here, we tested in a large-scale epidemiological audiometric study in the army whether the observed asymmetry of hearing thresholds between ears does indeed relate to the shooting posture or is due to other factors.

Section snippets

Subjects

A sample of 644 young military officers of the infantry and artillery branches were tested yearly for an audiometric survey during the years 1988–1990 at Officer School. They were aged 25 years (S.D. 2.3) and had a military past of 6 years (S.D. 3.2). Before enlisting in the army subjects were tested for hearing: normal audiogram (<20 dB at all frequency ranges) and no otological disease.

Experimental design

For each subject, data are the mean audiograms of the three years (1988–1990).

All audiometric tests were

Results

In acute acoustic traumas due to the FAMAS long guns, scotomas on audiograms are essentially found at 4 or 6 kHz (Mingoutaud, 1996). On this basis we defined a `PTS' group and a `no PTS' group and took into account this covariant variable in the global ANOVA analysis so as to better underline the effect of exposure. We found about 100 of these young officers with a mean (4-5-6-7 kHz) hearing loss over 15 dB HL (tail of the histogram distribution) (Fig. 2).

We checked that right-eyed and

Discussion

This study provides evidence that the asymmetry of hearing thresholds between ears in military shooters reflects intrinsic differences between ears, irrespective of the shooting posture. Whatever the sighting eye and hence the shooting side, left ears are less efficient than right ears at high frequencies and slightly superior to right ears at 0.7–1 kHz. These results are consistent with the findings of Pirilä (1991). Using a different testing method and different audiometers there was a left

Acknowledgements

This work was supported by a grant from DGA (Direction Générale de l'Armement). We particularly thank Dr. Gorzerino at ETAS for providing the sound-proof trailer and the technical preparation and also Colonel Cabanis and Ker Vella for their helpful assistance in the testing period.

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