MEMS Microphone Array on a Chip for Turbulent Boundary Layer Measurements

Abstract

A MEMS microphone array has been designed and applied to the measurement of wall pressure spectra under the turbulent boundary layer in flow duct testing at Mach numbers from 0 to 0.6. The array was micromachined onto a single chip in the PolyMUMPS surface micromachining process, allowing high spatial resolution and low surface roughness. The chip measures 1 cm by 1 cm, and is flush mounted into the wind tunnel wall. Individual elements are 0.6 mm in diameter, with element to element spacing of 1.11 mm in the crossflow direction and 1.26 mm in the flow direction. The 64 element array has 59 working elements, 58 of which are matched to within ± 2.5 dB at 1 kHz. Phase matching between the 59 elements is ± 6.5 degrees at 1 kHz. The array has been calibrated from 100 Hz to 4 kHz in a plane wave tube. The transducer bandwidth is greater than 400 kHz as determined by laser vibrometry measurements. Sensor nonlinearity of less than 0.36% is observed at a sound pressure level of 150 dB SPL. Board level electronics allow the array to be reconfigured on the fly using computer controlled CMOS switches. Multipoint wall pressure spectra were measured in 38 array configurations at the wall of a 6 inch by 6 inch flow duct at Mach numbers from 0.0 to 0.6. The array shows excellent agreement with Kulite and Bruel & Kjaer microphone measurements in the 300 Hz to 10 kHz band, and appears to be able to measure turbulent pressure spectra at frequencies as high as 40 kHz.

Influence of pressure gradients on wall pressure beneath a turbulent boundary layer

“…Forest (2012) suggested that the dimensionless pinhole diameter d + should be smaller than 18 to ensure no sensor attenuation occurred for frequencies up to ω + = 1. The development of sensors based on microelectromechanical systems (MEMS) (Löfdahl, Kalvesten & Stemme 1996;Löfdahl & Gad-el Hak 1999;White et al 2012) is a possible way to obtain very small sensors with d + < 10. In SONOBL experiments, d + ∼ 45 and Corcos correction was used to obtain the autospectra up to ω + = 2.…”

CMOS-MEMS technology with front-end surface etching of sacrificial SiO<inf>2</inf> dedicated for acoustic devices

“…was dedicated to the diaphragm perforation in acoustic short-circuit in a low acoustic pressure equalization. Others authors propose openings and perforation in diaphragm [8][9][10][11], but in their work either technology was not used [8,9], was not presented [10,11]. This paper is structured as follows.…”

Implementation of a Virtual Microphone Array to Obtain High Resolution Acoustic Images

“…[13]. Arrays of MEMS microphones are specially designed for acoustic source localization [14]; however, they are also used in other applications such as speech processing [15], turbulence measurements [16], identifying geometric dimensions and internal defects of concrete structures [17], or acoustic imaging [18,19]. Due to the high diversity of the applications of these arrays, the authors widened these uses to another field, such as the industrial one [20].…”