Hearing protection: Surpassing the limits to attenuation imposed by the bone-conduction pathways

Abstract

With louder and louder weapon systems being developed and military personnel being exposed to steady noise levels approaching and sometimes exceeding 150 dB, a growing interest in greater amounts of hearing protection is evident. When the need for communications is included in the equation, the situation is even more extreme. New initiatives are underway to design improved hearing protection, including active noise reduction (ANR) earplugs and perhaps even active cancellation of head-borne vibration. With that in mind it may be useful to explore the limits to attenuation, and whether they can be approached with existing technology. Data on the noise reduction achievable with high-attenuation foam earplugs, as a function of insertion depth, will be reported. Previous studies will be reviewed that provide indications of the bone-conduction (BC) limits to attenuation that, in terms of mean values, range from 40 to 60 dB across the frequencies from 125 Hz to 8 kHz. Additionally, new research on the effects of a flight helmet on the BC limits, as well as the potential attenuation from deeply inserted passive foam earplugs, worn with passive earmuffs, or with active-noise reduction (ANR) earmuffs, will be examined. The data demonstrate that gains in attenuation exceeding 10 dB above the head-not-covered limits can be achieved if the head is effectively shielded from acoustical stimulation.

Validity of diagnostic pure-tone audiometry without a sound-treated environment in older adults

“…We evaluated the performance of an audiometer employing passive attenuation using insert earphones covered by circumaural earcups coupled with real-time monitoring of environmental noise for air-conduction and boneconduction threshold measurement in a natural environment. Double transducer attenuation using insert foam plugs and circumaural earcups produces a significant increase in ambient noise attenuation that may actually exceed typical attenuation for transportable sound booths (Berger, Kieper & Gauger, 2003;Franks, 2001). Results of the current study confirmed statistically and clinically equivalent hearing thresholds as measured in a natural environment versus a sound-treated booth.…”

“…Active noise monitoring provides a measure of quality control. Furthermore, the system can be set to monitor noise levels according to the average attenuation provided by the test setup in a typical group of subjects as opposed to double attenuation values previously reported (Berger, Kieper & Gauger, 2003). By employing these attenuation levels, the software can be programmed to monitor ambient noise levels across octave or interactive levels, according to standards for audiometric test environments (e.g.…”

“…Forehead placement bone-conduction audiometry was conducted with both ears occluded by the deep insertion of the earphones. Placement of the insert earphones was deep with the foam tip inserted completely into the canal to improve the attenuation of ambient noise (Berger & Killion, 1989;Berger, 1983;Berger, Kieper & Gauger 2003) and to minimize the occlusion effect. Placing insert earphones down to the bony part of the ear canal reduces the occlusion effect allowing for bone-conduction evaluation with occluded ears (Dean & Martin, 2000;Stenfelt & Goode, 2005;Swanepoel & Biagio, 2011).…”

'Calibrating' the insertion depth of roll-down foam earplugs

“…At and above 2 kHz there is almost no change in attenuation as a function of insertion depth until the fit becomes so poor that depth of insertion is none at all. Except for the NI fitting, the attenuation at and above 2 kHz approaches and is limited by the flanking bone-conduction (BC) pathways to the inner ear (Berger et al, 2003) and thus varies little as insertion depth is increased. A feature of particular interest is the dip in attenuation at 2 kHz for the SI and DI fittings.…”

Effects of Hearing Protection Methods and Noise Directions on Bone-Conduction Sensitivity

“…One of the advantages of the dual system is that it enables the listener to communicate with the least intrusion from background noise, resulting in better communication. [10] In [11] These organizations sometimes face situations during which, if the rescue and military personnel hear a signal but misinterpret it, they could convey the wrong message to a partner and face capture or even death from their enemy. [8] The purpose of the present study is to find the most sensitive placement of the skull in order to perceive speech through the bone vibrator in various protection methods while being exposed to noise.…”