Development of a Custom, 3D-Printed, Multi-Microphone, Noise-Cancelling, Hearing Protection Device with a Magnetically Attached Printed Ear Canal for Sound Localization Preservation
Hearing loss is a prominent health issue that affects a significant fraction of the global population. One of the main causes behind hearing loss is exposure to harmful noise levels in the workplace. There have been multiple efforts to prevent hearing loss from excessive noise in occupational settings, such as hearing conservation programs and use of hearing protection devices (HPDs), however, these are often ineffective. Although HPDs are frequently available in the workspace, they are not used as intended since they usually generate discomfort and hindered sound localization. The loss of sound localization can be hazardous in occupational settings, since it restricts the user from detecting the originating source of useful noises such as auditory warning signals. Anatomically, the outermost part of the ear, denominated the pinna, is responsible for sound localization since it filters sounds to determine its originating direction. With the expertise of GN Resound, prior work was developed to prove the hypothesis that embedding an artificial ear pinna could preserve the auditory localization properties on HPDs. Since the pinna is an intricate structure, conventional manufacturing is not convenient in its reproduction. Thus, additive manufacturing (AM), which can create complex geometries, was used to replicate the human pinna. Initial iterations of a noise-cancelling, sound-preserving headset were manufactured and tested, demonstrating that the embedded pinna aided sound localization. A new version of the device was designed focusing on the improvement of user experience, convenience, and comfort. The latest device design, denominated V14, included features not found in its predecessors, such as multiple microphones and a magnetic coupling. The magnetic coupling was designed to replace the fixed attachment that existed between the muffs of the device and the printed ear canal (PEC) that inserts the device’s speaker into the anatomical ear canal. This magnetic attachment consists of two main parts, a membrane PEC and earpiece PEC, that house magnets to achieve a clasp, and contact surface plates, which transmit signals from the digital sound processor (DSP) in the muff to the speaker in the user’s ear canal. A test was conducted to verify if the magnets would interfere with the acoustic performance of the speaker, and the results showed that there was only an average difference of 1.56 dB between the setup with and without the presence of the magnetic coupling, which is approximately 50 % below the SPL changes barely perceivable by humans. The cavities for two additional microphones were added to the computer aided design (CAD) model of the headset to improve the sound localization quality of the device by increasing the sound capturing capacity of the same. Of the three microphones incorporated in the design, only two were connected to the DSP according to its capacity. The noise-cancelling, sound-preserving headset has hearing aid electronic components integrated in it for its advanced functionality. A standard 312-battery pill has been used as a power source since such batteries are often used for hearing aids. To increase the battery life of the headset, the prototype for a rechargeable battery circuit was constructed and tested. A voltage within the working range of the DSP was achieved, and the battery life was measured to last a minimum of 40 hours. Embedment of the circuit into the headset CAD models was not possible at this stage due to its dimensions. The square surface of the printed circuit board (PCB) used for the prototype did not fit into the available area in the muff, therefore, the PCB was trimmed to adequately fit within the muff. The rearrangement of the circuit in the CAD models of the headset was recommended for future work and to fully implement the rechargeable battery. Results for the acoustic testing of the previous version of the device, V13, were analyzed and compared against an anthropometric manikin designed for acoustic testing without any device donned (Unaided KEMAR manikin). Favorable results on the preservation of sound localization in V13, such as an average difference in directivity index of 2.18 dB in relation to an Unaided KEMAR manikin, were obtained. The results of the acoustic testing for signal amplitude were also examined. The average difference from Unaided KEMAR was calculated for four principal azimuths, yielding results lower than the 5 dB in sound pressure level (SPL) changes that are noticeable for human beings. Sound attenuation results demonstrated that the device requires an increase in its noise reduction capabilities, since it reached an attenuation of 18 dB in contrast to the 25 dB required to match the performance of HPDs in the market. Finally, a design requirements list was introduced to standardize the specifications that the device should fulfill by determining quantifiable objectives. The design requirements were established by researching other HPDs and the changes in SPL audible to human beings. Overall, the differences in amplitude and directivity index of V13 from Unaided KEMAR were below the values set in the design requirements list presented in this thesis. The battery capacity for V14 is also within the specifications described in said list. However, the attenuation levels were 7 dB below the indicated goal in the design requirements list, demonstrating a need for further development in the noise-cancelling properties of the device.
Valadez Mesta, Brenda Leticia, "Development of a Custom, 3D-Printed, Multi-Microphone, Noise-Cancelling, Hearing Protection Device with a Magnetically Attached Printed Ear Canal for Sound Localization Preservation" (2022). ETD Collection for University of Texas, El Paso. AAI29324256.