Reconstruction of the audio environment
with an Hearing Protection Device:

Principle and concepts of solutions
for an acoustically "transparent" hearing protection

Presented by Lorenz Kroener and submitted october the 6th 2021 to the thesis committee:

Joël DUCOURNEAU Laboratoire Énergies et Mécanique Théorique et Appliquée (LEMTA), Énergie et transferts, ÉR Acoustique, Université de Lorraine, Vandoeuvre-lès-Nancy Rapporteur
François ALOUGES Centre de Mathématiques Appliquées (CMAP), École Polytechnique, Palaiseau Rapporteur
Rozenn NICOL Orange Labs, Lannion Examinatrice
Guillaume ANDÉOL Institut de Recherche Biomédicale des Armées (IRBA), Brétigny-sur-Orge Examinateur
Udo ZÖLZER Department of Signal Processing and Communication, Faculty of Electrical Engineering, Helmut Schmidt University, Hamburg Examinateur
Véronique ZIMPFER Institut franco-allemand de recherches de Saint-Louis (ISL) Co-encadrante de thèse
Alexandre GARCIA LMSSC, Le Cnam Paris Directeur de thèse
Christophe LANGRENNE LMSSC, Le Cnam Paris Co-encadrant de thèse


Many professionals, e.g. mining and construction workers, ground crews or soldiers are exposed to impulsive or constant high level noise. In order to prevent hearing loss, they depend on hearing protections devices (HPDs). On the contrary, HPDs interfere with situational awareness and sound source localization. This contradiction makes users pondering between hearing loss and situational awareness. Often last mentioned dominates over first mentioned.

This work aims to bring in line hearing protection and situational awareness. A virtual acoustic environment (VAE) with 16 circularly, horizontally arranged loudspeakers is set up. Localization performance with commercially available HPDs, including active and passive earplugs and earmuffs, is assessed in the VAE with 40 subjects. Earplugs with small geometries show better results than large-sized earmuffs. These results coincide with the study on modifications of the Head Related Transfer Function (HRTF) caused by HPDs. Earplugs preserve many individual spectral cues, while earmuffs cancel out most of these cues.

We compare methods of combining a simulated, generic HRTF with the simulated, individual Pinna Related Transfer Function. An analytic model of HRTFs, controlled by the azimuth angle, is developed. Respecting the limitations of embedded systems, regarding energy supply and computational power, 14 digital filters are defined. A headphone based listening test is conducted to rate these filters regarding subjective front-back discrimination performance, resulting in better performance with low order filters than with high order filters.

We present 4 designs of advanced HPDs which are aimed to improve the sound localization performance. Prototypes are manufactured and evaluated in a subjective listening test with 36 participants, showing that it is possible to improve sound localization of a commercially available active HPD.