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