Evidence of effectiveness of impulse sound hearing protection in the face of impulse noise triggered by handguns

 Fig. 1: Hearing protection plug Ear ISAF with ISL impulse sound filter

For the protection of soldiers on military missions, it is necessary to have hearing protection that not only offers effective protection against impulse sounds, but at the same time  permits detection of surrounding area noises and allows intelligible communication. The hearing protection plugs currently in use, Ear Classic II and Bilsom 303, which actually were developed for a constant wave load, do not fulfil these requirements because the acoustical perception of the environment is extensively limited due to the linear absorption of 28 and 33 dB respectively. Because of this lack of capacity, many soldiers on duty stop using the hearing protection completely in order to fend off mortal dangers; they thereby risk permanent hearing damage, possibly including deafness, in the face of unexpected noise from weapons or explosions.

This lack of capacity can be remedied for soldiers on duty through the introduction of what has been called an impulse sound hearing protection in the Bw. The German-French research institute Institute de Recherches Saint Louis (ISL) for this purpose has developed an impulse sound filter, which has been built into various hearing protection plugs (Figure 1). 

Normal to moderately loud sound events such as speaking, the approach of persons or the sound of drums make it through this filter - which normally is “open” - and thus can be perceived without impairment,. 

With increasing sound pressures, lieke from a shot or the detonation of a mine, this filter closes itself off as an instant reaction to the pressure wave originating from the sound event, and thus prevents damage to the sensory cells in the inner ear. Up to now, the impulse sound hearing protection has not been introduced in the Bw because no CE certification label can be given to this product, and therefore, according to German guidelines, no certified proof of effectiveness exists.

In the current study, commissioned by MOD FüSan as a special research project, the effectiveness of the impulse sound filters developed by the Institute de Recherches Saint Louis (ISL) was tested in shooting with handguns (pistol P8, rifle G 36, and machine gun MG 3). This was intended to answer the question as to whether the use of the impulse sound filter could reliably reduce the risk of inner ear damage from a traumatic blow. In addition, it was supposed to give evidence of improved surrounding area detection and communication as opposed to the previously used long-term wave hearing protection.

The shooting trials were carried out with a total of 60 test subjects. On each test day, before the beginning of the shooting, immediately after the shooting, and 24 hours after the shooting, sound audiometry was performed in order to determine the individual hearing curve and speech 

 Fig. 2: Audiometry vehicle of the WTD 91

audiometry was performed to test the understanding of speech. 

The audiometric tests were performed directly at the shooting range using the mobile audiometry measuring bus of the WTD 91, Meppen (Figure 2).

Final remarks

In the trials, it could be shown that the use of the hearing protection plug with an integrated ISL impulse sound filter Ear ISAF could reduce the risk of shock trauma caused by weapon noise as effectively as the use of the Ear Classic hearing protection plug used until now in the Bw. None of the subjects suffered hearing damage during the shooting trials, and the tone audiometry did not show any significant changes of the hearing curves before or after the shooting (Figure 3). However, the two hearing protection plugs behaved in a significantly different manner in the test of speech understanding. Here the new type of hearing protection plug with the impulse sound filter was far superior to the previously introduced hearing protection plugs. The speech understanding of the soldiers using the impulse sound hearing protection Ear ISAF at 60 dB was 91%, while in the comparison group with the previously introduced Ear Classic it was only 55.7% (Figure 4).

The current study thus gave proof that immediate detection of surround noises and effective communication of soldiers with each other are possible at the same time as protection against weapon and explosion noise. Based on the results of the study, the new hearing protection has already been certified for “out of country” missions like in Afghanistan. Further studies to determine the maximum allowable number of shots in practice are being carried out at the current time.

Date: 05/09/2018

Source: MCIF 1/2012