Peter C. Herdic

A human surrogate was recently developed to emulate the acoustic transmission path of a human ear. This physical model was used to study soldier exposure to impulsive small weapons fire. The ear response of the surrogate and human data are in very good agreement. The effect of the helmet and body armor equipment on a soldier's hearing was determined in this report through laboratory and firing range measurements. A sound pressure level (SPL) reduction of 2 to 3 dB on both left and right inner ears of the surrogate was found when a helmet was worn. This reduction was probably due to the helmet blocking the direct acoustic path from the weapon muzzle to the ear. Body armor appears to have very little effect on the sound reaching the inner ears. This implies that reflections from the torso are a second order effect when compared to the direct acoustic path. The physics of the direct acoustic path from the weapon muzzle to the ears appears to be the most important acoustic mechanism affecting the inner ear SPL levels. The surrogate developed here may serve as a standard for human hearing evaluation of infantry combat equipment and weapons systems deemed relevant to the warfighter.

The Laboratory for Structural Acoustics (LSA) at NRL consists of an indoor cylindrical tank (17 m dia. by 15 m deep) filled with ̃ 1 million gallons of deionized water. Key features include: 1) vibration isolation, 2) active temperature control, and 3) anechoic materials. This unique laboratory is instrumented with sophisticated mechanical, electronic and optical systems, that include large workspace in‐water roboticscanners to generate nearfield acoustical holography (NAH) databases. We discuss such a database consisting of the underwater near‐field pressure measured on a two‐dimensional surface conformal to an internally driven complex structure floating at the air‐water interface. Various inverse approaches are discussed to image the normal velocity and intensity of the structure at its interface to the fluid, as well as the total power radiated, revealing mechanisms of radiation related to the internal structure. These inverse approaches consist of the equivalent source method compared with the well established Fourier acoustics methods of NAH. This work was supported by the US Office of Naval Research.

The application and experimental results are presented for the Passive and Advanced Vibro-Acoustic Treatments Demonstration, Part B (PAVAD-B) conducted at the Laboratory for Structural Acoustics, Naval Research Laboratory (NRL) during the winter and spring of 2001-2002. The goal of this demonstration is to utilize the most advanced treatments developed in the first part of the program (PAVAD-A), improve their design, and apply them to the Sponsor Supplied Structure. In the initial PAVAD-A study, advanced treatments were applied to an idealized simple supported plate to evaluate the noise reduction performance and optimize the treatment designs. Nearfield Acoustic Holography (NAH) measurements of the untreated Sponsor Supplied Structure were also made and previously reported. These holography data illustrate the structural response and provide a means to determine the optimized PAVAD-B treatment locations. The performance of the improved PAVAD-B advanced treatments is evaluated using these same NAH techniques on the Sponsor Supplied Structure and reported in this document.