Caesar T. Garcia

A low-noise differential microphone inspired by the ears of the parasitoid fly Ormia ochracea

A miniature differential microphone is described having a low-noise floor. The sensitivity of a differential microphone suffers as the distance between the two pressure sensing locations decreases, resulting in an increase in the input sound pressure-referred noise floor. In the microphone described here, both the diaphragm thermal noise and the electronic noise are minimized by a combination of novel diaphragm design and the use of low-noise optical sensing that has been integrated into the microphone package. The differential microphone diaphragm measures 1 x 2 mm(2) and is fabricated out of polycrystalline silicon. The diaphragm design is based on the coupled directionally sensitive ears of the fly Ormia ochracea. The sound pressure input-referred noise floor of this miniature differential microphone has been measured to be less than 36 dBA.

Performance and Modeling of a Fully Packaged Micromachined Optical Microphone

A microelectromechanical systems (MEMS) optical microphone that measures the interference of light resulting from its passage through a diffraction grating and reflection from a vibrating diaphragm is described ( JASA, v. 122, no. 4, 2007). In the present embodiment, both the diffractive optical element and the sensing diaphragm are micromachined on silicon. Additional system components include a semiconductor laser, photodiodes, and required readout electronics. Advantages of this optical detection technique have been demonstrated with both omnidirectional microphones and biologically inspired directional microphones. In efforts to commercialize this technology for hearing aids and other applications, a goal has been set to achieve a microphone contained in a small surface-mount package (occupying 2 × 2 mm × 1 mm volume), with ultralow noise (20 dBA) and a broad frequency response (20 Hz-20 kHz). Such a microphone would be consistent in size with the smallest MEMS microphones available today but would have noise performance characteristics of professional-audio microphones significantly larger in size and more expensive to produce. This paper will present several unique challenges in our effort to develop the first surface-mount packaged optical MEMS microphone. The package must accommodate both optical and acoustical design considerations. Dynamic models used for simulating frequency response and noise spectra of fully packaged microphones are presented and compared with measurements performed on prototypes.

Design and experimental evaluation of a low-noise backplate for a grating-based optical interferometric sensor

Optical grating-based interferometric sensors have been the subject of prior investigations, with recent work focused on micromachined microphone applications. The silicon structure is similar in construction to capacitive microelectromechanical-system microphones, with the exception that the microphone backplate contains an optical-diffraction grating at the center. The grating serves as a beam splitter in this system, allowing only a portion of the incident light to pass to the diaphragm and back, enabling interferometric readout of diaphragm displacements. A cited advantage of this system is the ability to design highly perforated backplates with low mechanical damping and with the ability to realize low thermal-mechanical noise. Grating backplates, however, have their own unique optical design constraints different from capacitive sensors. This paper details a rigorous finite element computational fluid dynamics model for flow resistance of a grating backplate. The model is validated for a case study backplate fabricated in the epitaxial layer of a 2-μm silicon-on-insulator wafer. The dynamics of the backplate are studied in isolation from other microphone elements by mounting the backplate in close proximity to a rigid optical-reflector and using electrostatic actuation to vibrate the backplate for extraction of compliance, resonance frequency, and quality factor.

Diffraction based optical MEMS microphones and accelerometers with active electrostatic force feedback

Diffraction‐based optical displacement detection method and its use in low noise micromachined microphones have been shown earlier. [Hall et al., J. Acoust. Soc. Am. 118, 3000‐3009 (2005), Garcia et al., J. Acoust. Soc. Am. 121, 3155 (2007)]. In these devices, the integrated electrostatic port of the sensor is uncoupled from the integrated optical sensing. This structure enables one to use this port for sensitivity tuning, self characterization, and active control to adjust the device dynamics. Given that the displacement noise of integrated optical sensor is below the thermal‐mechanical noise of the mechanical structure, one can implement force feedback methods such as active Q‐control, or adjust device stiffness without adding substantial noise to the system. We implemented micromachined optical microphones and accelerometers with integrated optoelectronics integrated in a 1.5mm3 volume. We present experimental results on force feedback Q‐control of low noise omnidirectional, and biomimetic directional ...

An integrated optical microphone test‐bed for acoustic measurements

A diffraction‐based optical detection method for microphone applications has been demonstrated previously [Hall et al., J. Acoust. Soc. Am. 118, 3000–3009 (2005)]. This method, coupled with proper integration techniques can produce precision measurement microphones with 24 dBA noise levels and suitable bandwidths. Thus far, these characterization studies have been performed using experimental setups, which would disturb the acoustic field due to size and non‐symmetric features. In these regards, previous optical microphone test beds have been inadequate experimental platforms. This has motivated the development of a more robust integrated instrumentation microphone package for future testing and characterization. In order to meet the size restrictions for such an optical microphone platform, vertical cavity surface emitting lasers are used as light sources and small photodiode arrays are used to detect intensity variations in refracted orders of the optical detection method. The overall dimensions and sha...

Commercial packaging of an optical microelectromechanical systems microphone.

A microelectromechanical systems (MEMS) optical microphone has been presented that measures the interference of light resulting from its passage through a diffraction grating and reflection from a vibrating diaphragm. [J. Acoust. Soc. Am. 122, (2007).] In this embodiment, both the diffractive optical element and the sensing diaphragm are micromachined on silicon. Additional system components include a semiconductor laser, photodiodes, and required readout electronics. In our efforts to commercialize this technology for hearing‐aids and other applications, a goal has been set to achieve a microphone contained in a small surface mount package (occupying 2 × 2 × 1 mm3 volume), with ultra‐low noise (15 dBA) and broad frequency response (20 Hz–20 kHz). Such a microphone would be consistent in size with the smallest MEMS microphones available today, but would have the noise performance characteristics of professional‐audio microphones at least 10× larger in size and 10× more expensive to produce. This paper wil...

State space modeling of surface mount silicon microphones

Most surface mount microelectromechanical system (MEMS) microphone packages are similar in construction, consisting of a printed circuit board with sound inlet, a MEMS die with a through-wafer etch aligned over the sound inlet, and cap which serves to protect the structure and render an enclosed back volume. From a lumped modeling perspective, this system is a network of acoustical and mechanical elements. Network models (i.e., equivalent circuit models) have proven to be the most common modeling technique for simulating important features of these microphones, including frequency response functions and internal noise floors. While these models have many advantages including their ability to be solved efficiently using modern circuit simulation software, they do not lend themselves well to an understanding of system dynamics as a decomposition of the fundamental mechanical modes of the packaged system. We present a state space model for complete MEMS microphone packages and present frequency response simu...

A low‐noise biomimetic differential microphone

A miniature differential microphone is described that has a noise floor that is substantially lower than that of existing devices of comparable size. The sensitivity of a differential microphone suffers as the distance between the two pressure sensing locations decreases, resulting in an increase in the input sound pressure‐referred noise floor. In the microphone described here, the two sources of microphone internal noise, the diaphragm thermal noise and the electronic noise, are minimized by a combination of novel diaphragm design and the use of low‐noise optical sensing. The differential microphone diaphragm measures 1 mm by 2 mm and is fabricated out of polycrystalline silicon. The diaphragm design is based on the coupled ears of the fly Ormia ochracea. The sound pressure input‐referred noise floor of this miniature differential microphone has been measured to be less than 36 dBA.