Improved communications and hearing protection in helmet systems: The communications earplug

Military Medicine, Jun 2003 by Powell, John A, Kimball, Kent A, Mozo, Ben T, Murphy, Barbara A

Despite significant advances in hearing protection and compliance with protective standards, military personnel are still subject to noise-induced hearing loss in many combat and combat support operations. Although the Army has experienced a decrease of some 15% in primary hearing loss disability cases since 1986, a fiscal year 2000 report documents a 27.5% increase in audiograms, which demonstrated significant threshold shifts in assessed personnel (N = 841/1,077). Compensation for noise-induced hearing loss disability for the Army alone exceeded $180 million in 1998. Thus, communications and hearing protection remain critical issues for personnel involved in Army operations. Aircraft, ground vehicles, and weapons produce noise levels in excess of the limits defined in current hearing conservation standards. Performance of most helmets, improved over the years, remains marginal with regard to speech intelligibility. Furthermore, these helmets do not provide adequate hearing protection. The communications earplug, which consists of a high-quality earphone coupled with an earplug protector, provides the needed extra protection. It weighs less than 15 g and is comfortable when worn over extended periods. It is considered highly acceptable by seasoned Army aviators and crewmembers.

Introduction

Soldiers are frequently exposed to high levels of noise and routinely face the challenge of effective voice communications. Although the Army has experienced a decrease of some 15% in primary hearing loss disability cases since 1986, a fiscal year 2000 report documents a 27.5% increase in audiograms, which demonstrated significant threshold shifts in assessed personnel (N = 841/1,077).1 In addition, compensation for noise-induced hearing loss disability for the Army alone exceeded $180 million in 1998.2 Noise levels inside many military systems have been found in many instances to exceed noise exposure limits established by Department of Defense Instruction 6055.12, "Department of Defense Hearing Conservation Program" as implemented by Army regulation 40-501.3,4 For example, noise levels in military helicopters with high load capacities (such as the CH-47, CH-46, and H-53) are extremely high and often exceed the aircrews' helmet's capacity to provide adequate hearing protection.5,6 Similarly, both mounted and dismounted soldiers occupy and/or operate systems that generate high noise levels (e.g., armor, artillery, and small arms). Serious noise and speech communication problems are evident in these systems. The Bradley fighting vehicle is known to be one of the noisiest environments in the Army vehicle inventory. An infantry squad riding in the back of a moving Bradley fighting vehicle is exposed to noise levels as high as 117 dB, which is more than 30 dB higher than the 85 dB permissible steady-state noise limit set by Military Standard-1474D, Department of Defense Design Criteria Standard, Noise Limits.7 Although a properly inserted yellow foam earplug provides protection for the user to operate in that environment for approximately 2 hours, it completely obscures verbal communication. On the other hand, the DH-132 combat vehicle crew helmet that is used in tactical ground vehicles provides marginal hearing protection and allows speech communication. However, the practice of issuing helmets to the vehicle rather than to the individual also poses significant problems in attaining proper fit, which compromises both protection and communications.

Impulse noise from artillery firing is also a major cause of hearing damage. For example, a survey of the firing crews testing the XM198 155-mm towed howitzer indicated that they were experiencing adverse health effects associated with high zone firing of the cannon. In fact, field measurements showed the blast overpressures to be in the range of 170- to 183-dB sound pressure level, well in excess of the 140-dB peak pressure unprotected limits referenced in Military Standard-1474D.8 In some of these circumstances, double hearing protection is required to avoid hearing damage. However, when double protection is needed, voice communications are severely degraded, which then creates the potential for compromise in mission performance and safety.

Noise damage is insidious; hearing loss can overcome some individuals before they are aware of their impairment. The Veteran's Administration compensation for hearing loss, as a result of Army noise exposure, continues to increase each year. In fact, in 1998, the Army reported 34,851 hearing loss cases, representing compensation costs of more than $180 million.2 The cost of hearing loss can be described in dollars of compensation after retirement. But the hidden cost of property loss, lower performance, and loss of productivity are more difficult to determine. Soldier's commitments to serve their country should not result in decreased quality of life caused by preventable hearing loss.

There are three fundamental classifications of hearing protective devices. Inserts or earplugs are inserted directly into the ear canal, creating a barrier between the user's internal ear and the outside ambient noise environment. Circumaural hearing protectors encircle the pinna and are held tightly to the head of the wearer. The canal cap is a supra-aural protector that covers the external ear canal opening. There is usually a soft cushion between the head of the user and the earcup of the canal cap; this cushion serves as an interface to improve comfort. The efficiency of the barrier depends on the interface of the earplug to the ear canal, or the cushion to the head, as well as the attenuation characteristic of the earplug, canal cap, or earcup. There is a fine balance at the interface; tightness or rigidity can cause discomfort for the user, whereas looseness can allow too much sound energy through to the ear.