Realization of a High Sensitivity Microphone for a Hearing Aid Using a Graphene–PMMA Laminated Diaphragm

Abstract

Microphones for hearing aid systems are required to have high sensitivity, an appropriate bandwidth, and a wide dynamic range. In this paper, a high sensitivity microphone, 4 mm in diameter and using a multilayer graphene-PMMA laminated diaphragm that can be applied in hearing aids, is designed, optimized, and implemented. Typically, polyphenylene sulfide (PPS) has been used for the diaphragm of electret condenser microphones (ECM), and this method provides simple, low cost mass production. Generally, the sensitivity of the commercial 4 mm diameter ECM is about -30 to 35 dB (0 dB = 1 V/Pa). A microphone using a nanometer-thick graphene diaphragm has been found to have higher sensitivity than the conventional ECM. However, nanometer-thick multilayer graphene is vulnerable to large mechanical shocks or high sound pressures, and the practical production of nanometer-thick diaphragms also poses a challenge. However, if a multilayer graphene diaphragm of the same thickness as the conventional ECM is used, displacement during diaphragm vibration will be severely attenuated due to the high elastic modulus of graphene, and the microphone sensitivity will be greatly reduced. In this paper, we fabricate a multilayer graphene/poly(methyl methacrylate) (PMMA) laminated diaphragm with sensitivity higher than that of any other microphones currently available for hearing aids, with the appropriate bandwidth in the auditory range. The high sensitivity arises from the laminated structure of the thin graphene membrane with high elastic modulus and from the PMMA membrane with lower elastic modulus and higher dielectric constant. The optimal thickness ratio of the graphene-PMMA layered diaphragm was studied by both analytical and experimental methods, and then a fabricated diaphragm was assembled in a 4 mm diameter microphone package. The performance of the implemented microphone was evaluated, including the sensitivity and total harmonic distortion. It is demonstrated that the microphone using a multilayer graphene-PMMA diaphragm has an excellent sensitivity of -20 dB and a dynamic range of 90 dB, which is on average 9 dB higher than the microphone using the conventional ECM diaphragm.

Influence of the Nasometric Instrument Structure on Nasalance Score

“…In addition, the correlation coefficient between the collected speech signals through the separation plate-type and mask-type nasometric instruments is expressed by Equation (9). In general, the correlation coefficient can be calculated using the sample mean x and y, and standard deviation of the two signals x and y [4,12], where x and y are the voice signals measured through a microphone in the nasal and oral directions, respectively. Correlation coefficients were analyzed using the two measured vocal signals through the oral and nasal microphones when speaking the same word.…”

“…The most important components of nasometric instruments are the microphones that collect external sound signals and transmit them to the signal processing device. The microphones are required to have excellent sensitivity and a wide frequency band as input devices [12][13][14]. Figure 2 is an experimental block diagram for analyzing the microphone characteristics using the nasometric instrument structure, and a photograph of the experimental environment is shown in Figure 3.…”

A Review of MEMS Capacitive Microphones

“…The first direction is the employment of new materials to make the acoustic diaphragm. We have seen recent works that use graphene [115,119,125,126,128], silicon carbide (SiC) [120], and composite materials [116,127]. Graphene is employed as researchers are 'riding on the wave' of this material.…”

Realization of Closed Cavity Resonator Formed by Graphene-PMMA Membrane for Sensing Audio Frequency

“…In particular, monolayer and few layer graphene membranes could be damaged easily. Therefore, graphene-based membranes in previously reported acoustic sensors have been thickened by increasing the graphene layer number from 67 [10] to 1800 [12] or by attaching 200 nm to 3 µm thick PMMA layer [12], [13], in order to increase the robustness of the membrane. The advantage of using PMMA as the supporting layer instead of increasing the graphene layer number by 1 to 3 orders of magnitude, is that lower resonant frequency can be achieved from the graphene-PMMA bilayer due to the lower density and elasticity of PMMA.…”