Acoustic Test Fixtures (ATFs) are widely used to evaluate the performance of Hearing Protection Devices (HPDs) against high-level impulsive sounds. The attenuation conferred by HPDs is partially limited by inertial and compressive transmissions, which could manifest as earmuff or earplug movements. The external longitudinal displacements of an earplug inserted in the ear canal of an ATF have previously been measured using a laser Doppler vibrometer in order to correlate them with the protected ear canal acoustic pressure. However, at high impulsive levels, a tilting motion of the ATF can combine with the earplug's displacements. To quantify this effect and assess its impact on the earplug displacements measurements, the displacements of a modified ATF across levels ranging from 156 to 180 dB peak have been evaluated using two different sensors. First, the protocol employing a laser-vibrometer to assess earplug displacement has been replicated. Second, an accelerometer combined with various signal processing techniques has been tested. Both sensors consistently detected the tilting motion of the ATF, with peak displacement amplitudes ranging in average from 1 mu m at 156 dB peak to 100 mu m at 180 dB peak. These results demonstrate that the maximum amplitude of ATF displacement increases exponentially with impulse peak level. However, due to a time delay in the occurrence of this phenomenon, limited to no compensation for ATF tilting is required. These findings support an enhanced experimental protocol for investigating transmission characteristics of HPDs when evaluated with an ATF exposed to high-level impulsive sounds.

This study presents a method for real-time capable localization of impulsive acoustic events in urban terrain, omitting the need for precomputation phases, such as in time matching or impulse response matching. Using ray casting, solutions of the wave equation are computed using a boundary discretization of intersected objects, forming a weighted graph using line of sight checks, and deploying an A* search graph traversal algorithm for the calculation of the propagation time. Subsequently, the wave amplitude along the identified eigenrays is computed. The study discusses the properties and suitability of cost functions based on time differences of arrival, amplitude differences of arrival, and a combined cost function for iterative ray-based localization. The method is validated using pressure data of the muzzle blasts of various firearms collected via unattended ground sensors during an experimental campaign at the military urban combat facility in Walenstadt, Switzerland.

The performances of Hearing Protection Devices (HPD) facing high-level impulse noises is limited by the transmission through the protector. This transmission can be evaluated by measuring the earplug's displacement along the ear canal axis. Previous measurements highlighted a displacement proportional to the peak acoustic pressure in the protected ear canal. To better understand the consequences on the HPD performances, this evaluation requires to be extended. Thus, various configurations are studied with Acoustic Test Fixture (ATF) external ears and earplugs of low and medium hardness. The earplug's peak displacements have been measured with a laser-vibrometer and the acoustic pressure with the ATF microphone. The impulse waves employed to excite the earplugs ranged from 150 dB to 180 dB peak. A medium hardness earplug inserted in a low hardness ear led to the worst performances with a plug displacement of 200 µm and a peak noise reduction of 17.8 dB(SPL) for an impulse noise of 177 dB(SPL). For the best case, a maximum displacement of 66 µm and peak noise reduction of 28.8 dB(SPL) have been measured. These results suggest new perspectives to improve the HPDs but also their evaluation through the choice of judicious ATF materials that conform to biological tissues.

Although urban combat is nothing new, recent conflicts have shed light on Western armies' difficulties when operating in such environments. Thus, new surveillance and hazard detection challenges in complex environments are addressed. A new decision aid tool for acoustic sensor positioning has been developed to improve reconnaissance performance. The software's sensor location optimization kernel is based on a genetic algorithm developed by our partners from FKIE. It resolves successive localization problems using pre-computed Times Of Arrival (TOA) between an impulsive acoustic source and various surveillance microphones. The present study discusses the pertinence of the Time-Domain Finite-Element Method (TD-FEM) to simulate wave propagation within an urban area to obtain TOA and resolve localization problems. The approach is compared to experimental measurements in an urban area, and the pros and cons of the TD-FEM for this configuration are discussed.

Hearing protection devices facing high-level impulse noises provide an attenuation, generally, between 20 and 40 dB. One reason for this limitation is the direct interactions between the protection device and the impulse waves. In the case of earplugs, direct transmissions through the earplug occur. These direct transmissions combine with the already well-studied indirect transmissions arising from wave propagation in the external ear's tissues (skin, cartilage, and bone). To evaluate the transmission induced directly by the earplug, an experimental protocol using a laser Doppler vibrometer was developed. Thus, the earplug's outer lateral face (OLF) displacements and acoustic pressure at the eardrum were measured simultaneously. Two earplugs (polyurethane foam and acrylonitrile butadiene styrene) inserted in an acoustic test fixture were stimulated with impulses ranging from 137 to 180 dB-peak. A slight earplug OLF movement in the ear canal varying from 1 μm to 0.1 mm could be observed, which is likely related to ear canal longitudinal compression. The earplug's OLF displacement and acoustic pressure variation at the eardrum strongly depended on the earplug type. These direct transmissions and underlying consequences considerably alter the protection efficiency.

High-level impulse noise can permanently damage the ear and protection is therefore essential. Using hearing protectors to occlude the external ear canal, reduces air conduction but provides only partial attenuation. This is because the pinna converts airborne sound into mechanical vibrations, which propagate through body tissues to the occluded ear canal. To better understand the propagation pathways and mechanisms, which remain unclear, we simulated a preliminary two-dimensional transient model of the external ear in COMSOL Multiphysics. It shed light on how the waves travel through the tissues to reach the occluded ear canal, illustrating phenomena that are usually not visible experimentally. We also conducted an experimental evaluation of the time delay of wave propagation between the external ear tissues and the auditory canal air to confirm the numerical observations. Thus, we measured delays to reach the ear canal during stimulation with a transducer at the back of the concha and at the tragus of 0.37 ms and 0.32 ms respectively. This matched the numerical simulations and corroborated transmission through the skin. The model also highlighted coupling between the skin and the plug leading to pressure transmission in the ear canal. These preliminary results pave the way for the improvement of hearing protection devices by demonstrating the impact of the protection device's material and coating on the radiative phenomena resulting from the tissue conduction.

In noisy environments, hearing protection is used to occlude the ear canal's external meatus to limit air conduction. However, acoustic waves travel through several pathways such as bones, cartilages, and soft tissues, which impact protections' effectiveness. Physical properties of cartilage, as well as soft tissues can generate airborne sound in the occluded ear. To assess the part of the cartilage which contribute to the limitation of hearing protection, Bekesy audiometries with electrodynamic transducers are performed. Four zones (ear tragus, scaphoid fossa, ear lobe, behind the concha) are excited by a pulsed signal sweeping from 125 Hz to 8 kHz. The frequency response tends to denote that the ear tragus has the best transducer-cartilage coupling efficiency. The occlusion outcome is also evaluated at the mastoid and ear tragus, showing cartilage ability to transmit sound, especially at low frequency. For higher frequencies, the contribution of cartilage decreases, and the involvement of direct air conduction from transducer acoustic radiation increases, prominently when the ear canal is not occluded. Nevertheless, considering the transducer-skin coupling effects evaluated with airborne radiation measure and the contribution to the hearing perception of the earlobe, composed mainly of skin and adipose tissue, this paper raises the question of the conduction of the sound in soft tissues, and in a first step, in the understanding of the pathway of the sound when the ear canal is occluded. (C) 2021 Elsevier Ltd. All rights reserved.

Solid-state transducers are nowadays integrated in communication headsets that open the way to a new category of headsets that are of interest in both military and civil applications. Sounds are stimulated to the inner ear directly through the bones and cartilage of the skull. The main advantage of this technology is to offer the user the possibility to have the ear clear to remain alert to his environment or to use earplugs with a high level of protection while continuing to communicate via a radio system. Different types of transducers are used in a measurement protocol to determine the influence of the bearing force and position of the transducer on the propagation from the skin to the reception by a listener. The measurement setup includes laser vibrometry measurements on the skin and solid-state hearing threshold measurements.

Recent advances in the development of hearing protection devices open new fields of applications on the battlefield. While the TCAPS (Tactical Communication And Protective Systems) protect against acute acoustic traumas, they maintain information relative to the acoustic environment of the soldiers. Today, these systems are efficient for both hearing protection and communication. We propose to use the microphones equipping the TCAPS headsets in order to detect and localize shooters on the battlefield. The microphone underneath the hearing protection is used in order to detect the shock and muzzle waves generated by supersonic shots. A meshed network between the TCAPS deployed on the field allows transmitting asynchronous information relative to the detected waves to data fusion nodes that allow estimating the shooter’s position. Solutions are proposed in order to compensate the effects of the presence of the head between the microphones underneath the hearing protection. Results concerning the estimation of the time difference of arrival of a transient wave in free field and in the presence of an artificial head are presented. The data fusion process is tested thanks to simulations in various deployment configurations.

This paper presents the latest work on hearing protectors developed in the French-German research institute of Saint-Louis. This new generation of hearing protection devices has been specifically designed to maintain an efficient protection against transient (weapon) and continuous noise while providing a better perception of the acoustic environment. Spatialized communication and warning messages provide an intuitive feedback of the information transmitted to the user. In addition, sound environment reproduction at a controlled level, and active noise reduction algorithms for enhanced continuous noise attenuation performance are integrated. Sound recorded directly in the ear canal (underneath the physical protector) is used for voice transmission, real time dosimetry, and source detection and localization. In order to improve the acceptance of the hearing protection device by the user, various numerical filters have been designed. An evaluation with a group of listeners was performed in order to determine the filters giving the “most natural” perception. In parallel, a study of the source localization when using the inner microphones of the protectors has been initiated. For these tests, the earplugs were fit into the ear canals of an artificial head. The experimental protocols and first results are discussed.

Localizing the axis of the Mach cone created by the supersonic displacement of a bullet in a reverberant environment is a challenging task, not only because of the high velocity of the moving source, but also because of the multiple wave reflections off of the walls. Although time reversal (TR) techniques allow static acoustic source localization in a reverberant space, they have not been explored yet on non stationary waves caused by supersonic displacements in urban canyons. The acoustic wave produced by a supersonic projectile has a conical wavefront and a N-shaped acoustic pressure signature. In this paper, this acoustic wave is reproduced using a line array of point-like sources (simulations) and loudspeakers (experiments). During the propagation of this conical wave in an urban canyon, the resulting pressure signals are measured using a time reversal array flush mounted into the ground. These acoustic signals allow to automatically retrieve with a high accuracy the location of the Mach cone axis using time reversal techniques. This inverse problem is solved using the maximization of a fourth-order statistical criterion of the backpropagated pressures. This criterion allows to estimate the intersections between the Mach cone axis and several vertical planes in the urban canyon. These estimations are then fitted to a 3D trajectory with a robust three dimensional interpolation technique based on the Random Sample Consensus (RANSAC) algorithm. This method allows to automatically retrieve the axis of the supersonic source with an angular accuracy of less than 0.5° and a misdistance of 0.5cm for both numerical simulations and experimental measurements.

The last version of 3MTM Combat ArmsTM earplugs was designed to meet the hearing protection needs of the French armed forces. This earplug uses triple-flange-design fits in three sizes (small, medium, and large) allowing the best adaptation to all morphologies. The individual choice of the size of the earplug is the major problem to have an adequate protection: how to choose the ideal size? Three methods were assessed in the current study. The first method was based on the observation of tympanometry tips fit. The second was a commercial device allowing to check the attenuation of the earplug in situ. The last one was based on the detection of leak in low frequency with an instrumented earplug. The objective of this study was to compare these three different methods. An assessment test with 106 soldiers was performed. Each soldier was tested with the three methods, in a different order. The results of the different methods are presented and compared.

Localiser l'axe du cone de Mach cause par le deplacement supersonique d'un objet balistique dans un environnement urbain reverberant represente un defi scientifique et technique considerable, notamment en raison des reflexions multiples sur les murs. Le retournement temporel rend possible la localisation de sources en milieu reverberant mais n'a pas encore ete teste lorsque les sources sont en mouvement supersonique. Le present article expose lapremi ere approche d'un telprobi eme dans le cadre d'un modele geometrique simple, reproduisant un espace reverberant constitue de deux murs et d'un sol d'impedance infinie. Le principe de Huyghens permet de synthetiser un cone de Mach par superposition de fronts d'ondes monopolaires. Dans le cadre de cette etude, le principe est utilise pour des simulations numeriques et pour une validation experimentale en laboratoire, o` u une ligne de haut-parleursemetparleurs´parleursemet le cone de Mach artificiel. Une methode faisant appel aux fonctions de Green des sources images modelise la reverberation pour la propagation directe et la retro-propagation par renversement du temps, grâce a un reseau de microphones deploye sur le sol. Un calcul numerique retro-propage ensuite les donnees mesurees jusque dans des tranches verticales intersectant l'axe du cone de Mach. La maximisation d'un critere statistique d'ordre 4, qui supprime les forts niveaux dus aux microphones, determine le point d'intersection en question. Une methode de tri permet de garder les meilleures estimations servantaservant`servanta l'interpolation geometrique de l'axe. La methode proposee presente une precision angulaire de 1 • et une distance entre les axes de 1 cm, ` a la fois sur les simulations et les mesures experimentales.

On the battlefield, but also during training, a soldier is continuously exposed to various types of noise (impulse and continuous). This exposure is not only noise generated by his own weapon but also by weapons or vehicles of close by troops. The exposure levels are between 160 dB peak for small arms and 190 dB peak at the soldier's ear for some anti tank weapons, with A-durations from 0.3 ms (small caliber) to 4 ms for large caliber weapons (e.g., Howitzers). In order to protect the soldier to noise exposures which may induce hearing loss, damage risk criteria (DRC) are implemented, and proposed for the prediction of the potential risk due to a certain noise exposure. Depending on the type of criteria (Pressure-Time-History or A-weighted Energy based), the recording and evaluation of different physical signal parameters has to be done in accordance to the used DRC. The paper will present the problems which may arise when recording impulse (weapon) noise with very high peak pressure levels and discuss measurement techniques compatible with the used DRCs. The paper will also discuss problems which may arise during the use and development of portable noise dose meters for the use in the military environment.

OBJECTIVE:We tested middle-ear functioning in humans following intense exposure to noise. Noise generated by small caliber firearms was thought to have no effect on the middle-ear.DESIGN:A cross-over design. We measured middle-ear impedance, acoustic reflex, distortion product otoacoustic emissions (DPOAEs), and transient evoked otoacoustic emissions (TEOAEs) before and after practice rounds performed twice per day.STUDY SAMPLE:Fifty-nine soldiers equipped with earplugs undergoing regular training for a special mission. The mean noise exposure (LAeq8h) was estimated to be 106 ±1 dB SPL.RESULTS:Impedancemetry revealed a significant increase in the compliance and gradient of the tympano-ossicular chain after impulse noise exposure in the right and left ears. Acoustic reflex reactivity did not show a significant change. DPOAEs and TEOAEs were slightly decreased at 2 kHz, and showed a marked asymmetry in disfavor of the left ear. In soldiers with initial high reactivity of acoustic reflex, increased compliance was associated with a significant decrease in left TEOAEs at 1.5 and 2 kHz.CONCLUSION:Our results suggest that the use of small-caliber firearms, even while wearing earplugs, affects middle-ear function and may play a role in the early stage of auditory fatigue encompassing tinnitus.

Although the European noise regulation seems to be well adapted to the industrial context and, in the military context, problems concerning exposure to continuous noise may be resolved by using better performing Hearing Protection Devices (HPDs), problems subsist for impulse noise. In certain countries (e.g. France) the European noise regulation has been implemented in a way that, for weapon noise, the requirement regulation cannot be met with existing HPDs. But, if double hearing protection is used, the soldier will be isolated from his acoustic environment. The consequences are that more accidents during training on the firing range will occur and/or the use of HPDs will be refused during combat. This is the reason why it is important to adapt the effective regulations for specificity of military noises. In the same time, it is necessary to characterize the HPD’s attenuation for impulsive noise exposure. This presentation will present the problems that arise due to the present implementation of the European recommendation (2003/10/EC) in France. It will also show how HPDs are tested at the ISL with impulse noise at very high peak pressure levels. These procedures allow to characterize the nonlinear behavior of the HPDs at peak pressure levels which will be experienced during training and combat. A reflection concerning a new metric describing the nonlinear behavior of HPDs in impulse noise will be discussed.