Microphone Diaphragm Research Articles

Reliable metrology for infrasound sensors

The reliable assessment of infrasound has become increasingly important in the last years. Reliable measurements require calibration of the employed sensors with traceability to the SI. While there is measurement equipment capable of measuring infrasound available, there is no sufficient calibration below 10 Hz. Traceability for airborne sound is currently realized by primary calibration methods which are applicable down to 2 Hz, with limitations below 10 Hz. Against that background, a primary and a secondary calibration setup have been developed at PTB. Together they enable the traceable calibration of a variety of infrasound sensors such as microphones, sound level meters or microbarometers. Moreover in practice, procedures are necessary which ensure reliable results in a field environment even with external disturbances. A field study was conducted comparing the performance of measurement microphones and microbarometers at a wind park. Emphasis was placed on different means of protection against wind induced noise. Additionally, a laboratory study investigated the influence of small damages to a microphone diaphragm on the reliability of the microphone. In this contribution, the newly developed calibration setups for infrasound sensors and the insights from the field and laboratory studies are presented.

Simulation of Piezoelectric Transducer Microphone Diaphragm Based on Different Materials

Piezoelectric microphone which utilizes MEMS technology is a type of transducer that converts an input acoustic signal into an output electrical signal. The characteristics of the microphone diaphragm such as the diaphragm design features and the type of piezoelectric materials used will affect the performance of the microphone in terms of sensitivity. It is hard to control the stress of the diaphragm used in the MEMS transducer microphone. A modification of the diaphragm is done in this project to reduce the residual stress of the piezoelectric transducer. In addition, finite element analysis namely structural, modal and harmonic were carried out using Ansys 15.0 to simulate the mechanical and dynamic behaviour of the microphone diaphragm. Two types of diaphragm structure were designed, namely square and circular, while three types of piezoelectric material which are AlN, PZT and ZnO were used as the diaphragm material. The structural analysis findings of the diaphragm subjected to 1 Pa pressure revealed that the circular diaphragm made of AlN material exhibited the highest stress, reaching 43.05 GPa, surpassing the stresses observed in the other two materials. On the contrary, the square diaphragm composed of PZT material demonstrated the lowest stress, with only 1.55 GPa. In terms of resonance frequency, the circular AlN diaphragm achieved the highest resonant frequency, reaching 449.84 kHz, whereas the square PZT diaphragm exhibited the lowest frequency at 200.25 kHz. In general, the circular diaphragm design consistently yielded higher first resonant frequencies compared to the square design.The results show that the circular diaphragm with AlN piezoelectric materials is the ideal diaphragm in the microphone because of the highest stress generated and the first resonant frequency. The stress is related to the sensitivity of a microphone while the high resonant frequency can lead to the better optimization of signal to noise ratio control.

Microphone diaphragm thermal noise floor measurement using a laser vibrometer

The response of a microphone to thermal noise can be extremely difficult to predict and can adversely affect the performance of a given design. The direct measurement of the thermal noise floor of a microphone diaphragm could greatly improve our understanding of the parameters that affect the noise performance. A laser vibrometer can serve as a powerful tool, enabling precise and non-intrusive measurements of the diaphragm thermal noise. The experimental setup for measuring the diaphragm thermal noise using a laser vibrometer is described. Having both the noise floor, and the response to sound for a given microphone, the sound pressure-referred noise floor can be estimated. Measured and predicted results are presented for a variety of microphones. These measured results describe how design parameters, such as the size of the diaphragm, the dimensions of the backside volume, and overall geometry, impact the thermal noise floor measurements.

Highly-sensitive wafer-scale transfer-free graphene MEMS condenser microphones

Since the performance of micro-electro-mechanical system (MEMS)-based microphones is approaching fundamental physical, design, and material limits, it has become challenging to improve them. Several works have demonstrated graphene’s suitability as a microphone diaphragm. The potential for achieving smaller, more sensitive, and scalable on-chip MEMS microphones is yet to be determined. To address large graphene sizes, graphene-polymer heterostructures have been proposed, but they compromise performance due to added polymer mass and stiffness. This work demonstrates the first wafer-scale integrated MEMS condenser microphones with diameters of 2R = 220–320 μm, thickness of 7 nm multi-layer graphene, that is suspended over a back-plate with a residual gap of 5 μm. The microphones are manufactured with MEMS compatible wafer-scale technologies without any transfer steps or polymer layers that are more prone to contaminate and wrinkle the graphene. Different designs, all electrically integrated are fabricated and characterized allowing us to study the effects of the introduction of a back-plate for capacitive read-out. The devices show high mechanical compliances Cm = 0.081–1.07 μmPa−1 (10–100 × higher than the silicon reported in the state-of-the-art diaphragms) and pull-in voltages in the range of 2–9.5 V. In addition, to validate the proof of concept, we have electrically characterized the graphene microphone when subjected to sound actuation. An estimated sensitivity of S1kHz = 24.3–321 mV Pa−1 for a Vbias = 1.5 V was determined, which is 1.9–25.5 × higher than of state-of-the-art microphone devices while having a ~9 × smaller area.

The design of an optomechanical microphone using a photonic waveguide interferometer

Most commercial microphones rely on capacitive or piezoelectric readout of a vibrating diaphragm. Optical methods have also been used to read the motion of a vibrating microphone diaphragm. Most optical microphone demonstrations to date use free-space optics. We present the design of an optical interferometric microphone that uses a photonic waveguide embedded within the diaphragm. The deflection of the moving diaphragm causes a strain field within the diaphragm, and this strain in turn changes the optical path length of the waveguide. Light traversing the sensing arm of the interferometer combines with light traversing an on-chip reference arm to yield an output light power that depends on the instantaneous displacement of the diaphragm. The output of the combined beams is coupled from the chip to a photodetector to complete the interferometer. An advantage of such transduction method is the absence of a microphone backplate and its associated thermal-mechanical noise. Electrical noise associated with high input-impedance amplification, as required for small capacitive and piezoelectric sensors, is also circumvented. This embodiment does, however, introduce several implementation challenges including the routing of light onto and off the microphone diaphragm.

Calibration and testing of measurement devices at infrasound frequencies: proof from malfunctioning devices at site

With the growing prevalence of infrasound and potential for annoyance comes the need for noise assessment. Performance validation of measuring instruments is an established necessity for reliable measurement data at conventional frequencies. However, infrasound measurements are critically dependent on the integrity of the microphone. A case study is presented showing that errors in excess of 20 dB result if the microphone diaphragm is perforated, and that such a defect cannot be detected by visual examination or with a typical sound calibrator. A further laboratory study validates the findings, and a scheme is proposed for identifying when such an issue exists.

Effect of corrugation structure and shape on the mechanical stiffness of the diaphragm

Here, we studied the change in the mechanical stiffness of a diaphragm according to the corrugation pattern. The diaphragm consists of a silicon oxide and nitride double layer; a corrugation pattern was formed by dry etching, and the diaphragm was released by wet etching. The fabrication of the thin film was verified using focused ion beam and scanning electron microscopy images. The mechanical stiffness of the diaphragm was obtained by measuring the surface vibration using a laser Doppler vibrometer while applying external sound pressure. Flat squares, diaphragms with square corrugations, and circular corrugation patterns were measured and compared. The stiffness of the diaphragm with a corrugation structure was found to be smaller than that without a corrugation structure; in particular, circular corrugation showed a better effect because of the high symmetry. Furthermore, the effect of corrugation was theoretically predicted. The proposed corrugated diaphragm showed comparable flexibility with the state-of-the-art MEMS microphone diaphragm.

High-frequency acoustic impedance tube based on MEMS microphones

Acoustic impedance tubes are commonly used to measure a test specimen's acoustic characteristics, such as reflection factor, absorption coefficient, or acoustic impedance, in combination with one or two condenser measurement microphones according to associated standards. In the development process of an impedance tube, the microphone diaphragm's size has an important role in the measurement quality. On the one hand, the microphone diameter has to be large enough to ensure the possibility of measuring at low sound pressure levels (SPLs), but on the other hand, the size of the microphone diaphragm should be small in order not to influence the sound propagation through the impedance tube due to the microphone coupling. Micro-Electro-Mechanical Systems (MEMS) microphones are recently widely applied in various acoustic applications due to their small size and high sensitivity. This paper proposes the development of an acoustic impedance tube equipped with 16 MEMS microphones and an inner diameter of 8 mm with an operating frequency range between 60 Hz and 16 kHz. The bottom port MEMS microphones are connected via a 1 mm hole to the tube. The system evaluation is based on standard test specimens like empty probe adapters, rigid termination, and porous absorbers.

Theoretical investigation on the infrasonic frequency response of measurement microphones in different venting states

Abstract The demands for metrological traceability of infrasound, and to finally link with the quasi-static pressure metrology require accurate modeling and calibration of measurement microphones. However, two distinct microphone responses were obtained during infrasonic calibrations, when its vent connects to the calibrating sound field or external atmosphere, respectively. But the current theoretical model cannot explicitly characterize this phenomenon. Therefore, this paper focuses on explicitly modeling the infrasonic frequency responses of measurement microphones when its vent connects to the sound field or external atmosphere, respectively. The pressure leakage and heat conduction coupling model of the microphone is first established. Then the second coupling mechanism of the mechanical interaction on microphone diaphragm is studied and the frequency response models of the microphone in two venting states are explicitly derived. The models directly reveal the two coupling mechanisms in primary microphone calibrations. Significant differences of microphone responses between two venting states are verified by calculations.

ZnO thin film piezoelectric micromachined microphone with symmetric composite vibrating diaphragm

Residual stress is an important factor affecting the sensitivity of piezoelectric micromachined microphone. A symmetric composite vibrating diaphragm was adopted in the micro electro mechanical systems piezoelectric microphone to decrease the residual stress and improve the sensitivity of microphone in this paper. The ZnO film was selected as piezoelectric materials of microphone for its higher piezoelectric coefficient d31 and lower relative dielectric constant. The thickness optimization of piezoelectric film on square diaphragm is difficult to be fulfilled by analytic method. To optimize the thickness of ZnO films, the stress distribution in ZnO film was analyzed by finite element method and the average stress in different thickness of ZnO films was given. The ZnO films deposited using dc magnetron sputtering exhibits a densely packed structure with columnar crystallites preferentially oriented along (002) plane. The diaphragm of microphone fabricated by micromachining techniques is flat and no wrinkling at corners, and the sensitivity of microphone is higher than 1 mV Pa−1. These results indicate the diaphragm has lower residual stress.

AC actuation contribution to the induced electromotive force in the miniaturized inductive planar microphone

In this paper, we aim to analyze the gain ratio in the output induced voltage of a CMOS-compatible miniaturized electrodynamic microphone which is based on the use of two coplanar concentric inductors. These latter are comparable to a concentric transformer with a secondary situated on the microphone diaphragm top. The study has been applied when the outer inductor is actuated using an AC bias instead of DC leading to a time-variable B-field. The induced output voltage expression has been evaluated using two distinct analytic methods: the magnetic field derivation and the electric field approach. When an AC current with frequency above that of the acoustic band is used to bias the primary, results show that the induced voltage increases and becomes independent from the incident pressure wave frequency. The paper demonstrates also that with an AC bias, the microphone induced voltage values can reach the order of hundreds of µVs, as opposed to only few µVs when applying a DC bias. The gain in the induced voltage has been investigated and it is shown that it can be correlated to the AC current frequency, ?c, and the inner inductor fluctuation frequency, ?p. Moreover, this has been quantified analytically and it is shown that it equals ?c/8?p. This result will not only enhance the overall performance of the microphone, but will also make the preamplification design less complicated by increasing the SNR ratio.

Optimization of Induced Voltage From CMOS-Compatible MEMS Electrodynamic Microphone With Coaxial Planar Inductances

This paper presents an improved design of the electrodynamic CMOS-MEMS microphone. The studied microphone is based on two integrated concentric planar inductors, a fixed outer one and an inner one attached on a flexible vibrating diaphragm. First, the magnetic field, generated by the outer inductor, was evaluated. Unlike conventional electrodynamic microphones based on Faraday’s law, the induced voltage was found to be proportional to the product of the diaphragm displacement and its velocity. Even though the induced output voltage is in the range of a few microvolts, and its frequency is doubled compared to that of the incident acoustic wave. This result pushed us to adjust and improve the basic principle of this kind of microphone by trying to generate a vertical downward shift of the microphone diaphragm while maintaining the COMS compatibility. The Lorentz force appears to be a possible alternative to the wafer bonding, as it will be automatically generated when we bias the inner inductor by a dc current. First, this paper shows that the induced voltage will be linear for a vertical fluctuation magnitude lower than $10~\mu \text{m}$ with an induced voltage in the range of several microvolts. Using the developed vertical offset technique, we show that induced voltages can be increased by a factor of 10 and the dynamic response of the electrodynamic microsensor will be appropriate for a use as a microphone.

Small aperture microphone arrays

Superdirectional microphone arrays are arrays that are typically smaller than the acoustic wavelength and attain directional gains that exceed those of a classical delay-sum beamformer. Early superdirectional microphones utilized a combination of a velocity microphone along with a pressure microphone. Later, a single sensor superdirectional microphone was discovered that utilized the combination of pressure and pressure-difference (which is proportional to the acoustic particle velocity) through appropriate porting of the incident acoustic field to both sides of the microphone diaphragm. In the early 1980s, with the advent of multichannel FFT analyzers, there was a renewed interest in using superdirectional microphone arrays to estimate the acoustic intensity and to calculate the acoustic power flow from sources. The estimation of the vector acoustic intensity led to many transducer designs that utilized up to 6 pressure microphones and commensurate signal processing. In this talk we will show some of the early acoustic intensity probes that were designed to estimate the acoustic intensity. These small arrays have been utilized in many other beamforming applications. We will show one where a superdirectional beamformer is also capable of source direction finding by utilizing some simple adaptive beamformer implementations.

Towards a sub 15-dBA optical micromachined microphone.

Micromachined microphones with grating-based optical-interferometric readout have been demonstrated previously. These microphones are similar in construction to bottom-inlet capacitive microelectromechanical-system (MEMS) microphones, with the exception that optoelectronic emitters and detectors are placed inside the microphone's front or back cavity. A potential advantage of optical microphones in designing for low noise level is the use of highly-perforated microphone backplates to enable low-damping and low thermal-mechanical noise levels. This work presents an experimental study of a microphone diaphragm and backplate designed for optical readout and low thermal-mechanical noise. The backplate is 1 mm × 1 mm and is fabricated in a 2-μm-thick epitaxial silicon layer of a silicon-on-insulator wafer and contains a diffraction grating with 4-μm pitch etched at the center. The presented system has a measured thermal-mechanical noise level equal to 22.6 dBA. Through measurement of the electrostatic frequency response and measured noise spectra, a device model for the microphone system is verified. The model is in-turn used to identify design paths towards MEMS microphones with sub 15-dBA noise floors.

Advanced Optical Microphone

This paper introduces a new type of optical microphone that is characterized by structural simplicity and low cost, while retaining all the features offered by more expensive technologies such as laser interferometry. Operation of the new microphone relies on a reflective pattern that is etched on one side of the microphone's diaphragm. An image of the pattern is captured by a charge-coupled device, and pattern recognition is performed in real time to determine the diaphragm's instantaneous position. Hence, the complex task of detecting the minute variations in the diaphragm's position are performed by cheap, high speed optical electronics rather than by means of interferometry as shown in the literature.

Acoustic determination of impedance tube microphone locations

Impedance tube methods allow for convenient, rapid characterization of the normal-incidence acoustic properties of materials and constructions. Many such methods rely on the determination of precise phase relationships between microphones sensing the sound field inside the impedance tube; knowledge of the location of these microphones is crucial to the accuracy of the measurement. Due to the large physical size of typical microphone diaphragms, physical measurements of the microphone positions (such as with a ruler or caliper) are inadequate and have a large uncertainty. This paper presents a method to determine the impedance tube microphone acoustic center locations from a broadband acoustic reflection coefficient measurement of a rigid termination. Utilizing the Bayesian inference framework, the estimation procedure provides information about microphone locations and uncertainties in the position estimates.

Perceptual evaluation of the functional and aesthetic degradation of speech by wind noise during recording

This paper will present results from a systematic investigation into functional and aesthetic audio quality of speech recordings degraded by wind noise. The major source of wind noise tested comes from velocity fluctuations interacting with the transducer, generating pressure fluctuations at the microphone diaphragm. To better understand the effect of this type of noise, a perceptual experiment was designed to assess task performance and perceptions of quality when speech and simulated wind noise are presented together. A wind noise simulator was developed, which produces realistic audio from anemometer data, to allow the noise to be isolated from other ambient sounds, and also enable salient parameters to be controlled. Two key components of wind noise in recordings were evaluated, the average level and its temporal variance or “gustiness.” Eight levels of wind noise were factorially combined with three levels of gustiness. Each of these permutations was then presented with one of 24 randomly assigned, grammatically correct, nonsense sentences. Participants were asked to type the sentence they heard, rate the difficulty of the task, and indicate overall quality of the clip. Each sentence contained four keywords—correct identification of which was used for scoring performance.

A microacoustic analysis including viscosity and thermal conductivity to model the effect of the protective cap on the acoustic response of a MEMS microphone

An analysis is presented of the effect of the protective cover on the acoustic response of a miniature silicon microphone. The microphone diaphragm is contained within a small rectangular enclosure and the sound enters through a small hole in the enclosure's top surface. A numerical model is presented to predict the variation in the sound field with position within the enclosure. An objective of this study is to determine up to which frequency the pressure distribution remains sufficiently uniform so that a pressure calibration can be made in free space. The secondary motivation for this effort is to facilitate microphone design by providing a means of predicting how the placement of the microphone diaphragm in the package affects the sensitivity and frequency response. While the size of the package is typically small relative to the wavelength of the sounds of interest, because the dimensions of the package are on the order of the thickness of the viscous boundary layer, viscosity can significantly affect the distribution of sound pressure around the diaphragm. In addition to the need to consider viscous effects, it is shown here that one must also carefully account for thermal conductivity to properly represent energy dissipation at the system's primary acoustic resonance frequency. The sound field is calculated using a solution of the linearized system consisting of continuity equation, Navier-Stokes equations, the state equation and the energy equation using a finite element approach. The predicted spatial variation of both the amplitude and phase of the sound pressure is shown over the range of audible frequencies. Excellent agreement is shown between the predicted and measured effects of the package on the microphone's sensitivity.

MICROPHONE POP FILTER

A microphone pop filter for attenuating plosive artifacts utilizes a substantially acoustically transparent material configured to define multiple airfoil surfaces that are oriented non-orthogonally relative to an axis defined between an audio source and a microphone diaphragm, with the substantially acoustically transparent material disposed intermediate the audio source and the microphone diaphragm and separated from the microphone diaphragm by an airspace. Plosive artifacts from the audio source may be deflected away from the microphone diaphragm by the airfoil surfaces to reduce the impact of such artifacts on the microphone diaphragm and the resulting electronic signal output therefrom.

Numerical Study on the Transient Response of a Condenser Microphone

The vibrating of a condenser microphone diaphragm due to incident pressure causes changes in the capacitance and the energy stored in the volume of the microphone. One of the most important parameters that must be determined very precisely for any microphone is the amount of this displacement due to the executed pressure on the diaphragm i.e., mechanical sensitivity. Thus, in this paper, the generalized equations describing the motion of the diaphragm and the Reynolds equation for compressible gases have been solved in transient mode to determine with precision the mechanical response of the microphone. Unlike the past static and quasi dynamic simulations, the Reynolds equation that is used to calculate the damping and stiffness of the air near the diaphragm is considered in nonlinear form. The numerical frequency response obtained for a condenser microphone has been compared with experimental results that exist in the literature. The numerical results obtained indicate a very good accuracy of the code. The effect of the damping and stiffness coefficient on the frequency range, which is very important in designing a practical microphone, is also studied in detail. Such a dynamic analysis, unlike the past numerical static simulation, gives a deeper view into the reasons of the nonlinearity of this important measuring transducer.