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An acoustical wave consists of two elements, the acoustic pressure and the acoustic flow. Till now to determine the acoustic flow one has had, to measure the pressure and calculate the flow. So it would be convenient to have a sensor which is able to measure acoustical flows. At the University of Twente a novel device has been developed which fulfils this need. In this paper a short introduction to the governing principles of this dynamic flow sensor and some of its interesting applications will be presented. This micromachined device is called the microj7own.

This paper reports on a novel method to measure three-dimensional sound intensity and the fabrication of a miniature three-dimensional sound intensity probe. Verifying measurements where performed with three separate micro-machined particle velocity probes and one pressure microphone. A three-dimensional sound intensity probe has been realised based on a threedimensional micro-machined particle velocity microphone, a 3D Microflown, and a miniature pressure microphone.

The Microflown is an acoustic sensor that measures particle velocity instead of pressure, as conventional microphones do. This paper presents an analytical model describing the physical processes that govern the behaviour of the sensor and determine its sensitivity. Forced convection by an acoustic wave causes a small, asymmetrical, perturbation to the temperature profile around the two heated wires of the sensor, so that a temperature difference between these wires occurs. This temperature difference, to which the sensitivity is proportional, is calculated with a perturbation theory. Subsequently the frequency-dependent behaviour of the sensitivity can be analysed; it is found that there are two important corner frequencies, the first related to the time constant of heat diffusion, and the second related to the heat capacity of the heaters. A thorough description has already been given for the realization of the Microflown in a channel, i.e. with fixed walls acting as heat sinks near both heaters. Here, an analytic and two-dimensional model is presented that describes the situation of the present sensor without walls above and under it. Contrary to the previous model, this analytic model allows easy understanding of the sensor and is especially useful for engineering purposes due to its relative simplicity. Especially for small wire separations, the developed analysis appears to be in good agreement with experimental results and the model therefore offers the possibility of optimizing the sensor.

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