Road to Acquisition: Preparing a MEMS Microphone Array for Measurement of Fuselage Surface Pressure Fluctuations

Preparing and pre-testing experimental setups for flight tests is a lengthy but necessary task. One part of this preparation is comparing newly available measurement technology with proven setups. In our case, we wanted to compare acoustic Micro-Electro-Mechanical Systems (MEMS) to large and proven surface-mounted condenser microphones. The task started with the comparison of spectra in low-speed wind tunnel environments. After successful completion, the challenge was increased to similar comparisons in a transonic wind tunnel. The final goal of performing in-flight measurements on the outside fuselage of a twin-engine turboprop aircraft was eventually achieved using a slim array of 45 MEMS microphones with additional large microphones installed on the same carrier to drawn on for comparison. Finally, the array arrangement of MEMS microphones allowed for a complex study of fuselage surface pressure fluctuations in the wavenumber domain. The study indicates that MEMS microphones are an inexpensive alternative to conventional microphones with increased potential for spatially high-resolved measurements even at challenging experimental conditions during flight tests.

MEMS microphone intensity array for cabin noise measurements

Aircraft cabin noise measurements in flight are used toto quantify the noise level, and to identify the entry point of acoustic energy into the cabin. Sound intensity probes are the state-of-the-art measurement technique for this task. During measurements, additional sound absorbing material is used to ease the rather harsh acoustic measurement environment inside the cabin. In order to decrease the expensive in-flight measurement time, an intensity array approach was chosen. This intensity probe consists of 512 MEMS-Microphones. Depending on the frequency, these microphones can be combined as an array of hundreds of 3D- intensity probes. The acoustic velocity is estimated using a high order 3D finite difference stencil. At low frequencies, a larger spacing is used to reduce the requirement of accurate phase match of the microphone sensors. Measurements were conducted in the ground-based Dornier 728 cabin noise simulation as well as in-flight.

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.

Applicability of MEMS microphones for environmental sound level monitoring

This paper explores the challenges associated with the integration of MEMS microphone technology into IEC 61672 classified or type-approved environmental sound level monitors. A comparison is drawn between MEMS microphones and electret condenser capsule microphones to highlight key performance differences within the technologies, and a basic integration method for both technologies is suggested. A review of the IEC 61672 and type-approval standards is conducted against the suggested integration method for a MEMS microphone; key shortcomings are reported and objectively reviewed. Development trends for MEMS microphones are explored, providing key insights into the progression of the technology against electret condenser capsule microphones. Furthermore, the evolution of environmental sound level monitoring systems is explored with a key focus on networked and sound localisation technology. The importance of MEMS microphones within the evolution of environmental sound level monitoring systems is presented alongside key arguments for the practical suitability of MEMS technology over electret condenser capsule technology.

Assessment of inter-IC sound microelectromechanical systems microphones for soundscape reporting

Acoustic parameters obtained from calibrated acoustic equipment are part of the minimum soundscape reporting requirements as stated in Annex A of ISO 12913-2. To dynamically monitor the acoustic environment of a large area, a large network of acoustic sensors could be deployed, albeit at significant cost. Micro-Electro-Mechanical Systems (MEMS) microphones offer compact, low-cost and high-performance alternatives to traditional analog microphones. In particular, the use of Inter-IC Sound (IS) communication allows MEMS microphones to be conveniently used in concert with I2S output interfaces for sound actuation. The performance of several IS MEMS Microphones was compared to that of an IEC 61094-4:1996 WS2F microphone in an anechoic chamber and a series of digital filters was designed to compensate for the differences in frequency response. The noise floor, compensated frequency response, acoustic parameter accuracy of IS MEMS were evaluated and recommendations regarding the suitability of the IS MEMS were provided.