Optimization of a thermal flow sensor for acoustic particle velocity measurements

Abstract

Abstract-In this paper, a thermal flow sensor consisting of two or three heated wires, the Microflown, is treated for application to acoustic measurements. It is sensitive to flow ("particle velocity"), contrary to conventional microphones that measure acoustic pressures. A numerical analysis, allowing for detailed parametric studies, is presented. The results are experimentally verified. Consequently, improved devices were fabricated, and also sensors with a new geometry consisting of three wires, instead of the usual two, of which the central wire is relatively most heated. These devices are the best performing Microflowns to date with a frequency range extending from 0 to over 5 kHz and a minimum detectable particle velocity level of about 70 nm/s at 2 to 5 kHz (i.e., 3 dB PVL or SPL, corresponding to a pressure of 3 1 10 5 Pa at a free field specific acoustic impedance).[1383]

Characterization and modeling of CMOS-compatible acoustical particle velocity sensors for applications requiring low supply voltages

“…Previous work [28], devoted to the optimization of the Microflown, suggests that considerable sensitivity improvements can be obtained by increasing the distance between the wires (L G ). Increasing the length of the wire segments (W C ) and suspending arms (L C ) improves thermal insulation with benefits in terms of sensitivity but requires longer times for the wet etching step.…”

“…The APV (u a ) produces a perturbation on the temperature distribution that can be considered proportional to u a [28]. Since the origin of this perturbation is the local oscillatory displacement of the static temperature distribution, larger effects can be expected where the gradients are higher.…”

Biomimetic lateral-line system for underwater vehicles

“…Instead of using mechanoreceptors, Snookie will use hot-thermistor anemometry to measure water velocities. Hot-wire anemometry to measure fluid velocities is well established [9]- [14], also by means of microelectromechanical devices [15], [16]. Hotthermistor anemometry has long been used to measure air and water velocities [17]- [22].…”

Study and development of an in situ acoustic absorption measurement method

“…Two 200 nm thick platinum wires are heated to about 300°C. The operation principle involves intriguing physics [62][63][64] but can be understood using hand waving arguments as follows; if there is particle velocity, the temperature distribution asymmetrically alters, and therefore cause a temperature difference between both wires. Due to the thermal resistance effect, the heat flux also induces a resistance difference between the wires, which is measured.…”

“…Several studies were dedicated to this frequency dependent behaviour. Two main reasons are given for the decreasing responsivity at high frequencies; heat diffusion effects (it takes time for heat to travel between the wires), and the thermal mass of the wires [62][63][64]93]. Up to date, an undisputable explanation for the reduced responsivity at low frequencies is lacking; although in [14] thermal boundary layer effects are suggested as the cause.…”