Interior noise in military helicopters

The Temporary Threshold Shift (TTS) or temporary hearing loss caused by excessive interior noise in military helicopters results in a critical degradation of the acoustically unprotected infantryman\u27s ability to detect the existence or the approach of the enemy by hearing under combat conditions. The overall problem of noise reduction in helicopters consists of: (1) identifying the various noise sources, and (2) employing appropriate noise control techniques. Identification of the source is accomplished by the frequency correlation of 1/10-octave band analysis of the interior sound levels measured within the helicopter cabin with near-field sound generated by the sound sources. Reduction of the overall interior helicopter noise can be accomplished by attenuation or interdiction along the noise \u27\u27path , and/or by redesign of the source to reduce the sound generated in the first place. In the case of the helicopter, attenuation or interdiction of the noise consists of: (1) padding or insulating the source with lightweight absorption materials, (2) vibration isolation of the source, and/or (3) the wearing of ear protection by the passengers. This study proposes the installation of lightweight leaded vinyl sheet as an interior measure until redesign can be accomplished to permit adequate vibration isolation. Ear protection is recommended for all crew and passengers, but it is realized that cost and storage space may be prohibitive --Abstract, page ii