Double hearing protection and speech intelligibility-room for improvement

Double hearing protection is used in many air forces around the world for protection in noisy aircraft environments, particularly in helicopters. The usual combination is foam ear plugs under headset or helmet muffs. Much of the research that spurred the introduction of foam earplugs indicated little change in speech intelligibility in persons with normal hearing. However, aircrew often complain about having to maximize intercom volume for speech understanding, causing a situation with no reserve volume and bad sound quality. In recent years, further developments have included so-called hi-fi plugs and custom made ear plugs which are claimed to improve speech communication. The aim of the present project was to investigate different types of ear plugs and their effect on speech intelligibility in helicopter noise.

Each of nine normal-hearing pilot subjects were placed in an environment of recorded helicopter noise from a BO-105 helicopter. Speech audiometry was performed under four different conditions: headset only, and three different ear plugs worn under the headset. Fitting of the ear plugs was performed by an ear, nose and throat specialist to ensure similar conditions. The sequence of test conditions was randomized and double-blind. In addition, a subjective rating scale was used.

Wearing foam ear plugs under the headset decreased speech intelligibility dramatically. The "hi-fi" plug was somewhat better than foam plugs, and the custom made ear plug provided a speech intelligibility close to the headset-only situation. Subjective rating scores coincided with these findings.

In helicopter noise, custom made ear plugs may provide a much improved speech intelligibility over conventional plugs when worn under a headset, while maintaining improved noise protection over the headset-alone situation. Custom made ear plugs might therefore be a good alternative to other forms of enhanced noise protection in helicopters.