Measuring infrasound can be achieved via two approaches. Either a microphone can be made low frequent or a barograph can be made high frequent. At the KNMI both approaches are applied. In this chapter the high frequent barograph approach is explained, by means of the KNMI microbarometer and its field application. In figure 4.1 a schematic view of the KNMI microbarometer is given. Various elements can be distinguished.

The atmosphere is probed through the inlet box, shown in the top of figure 4.1. Possible pressure fluctuations are guided through six inlets towards the small volume inlet box. The sensing element is a Validyne® pressure transducer, which measures pressure variations differentially through a variable reluctance scheme. Differential pressure measurement means the pressure is measured with respect to a reference pressure in a backing volume. This backing volume is connected beneath the sensor as shown in figure 4.1. Doing so, all possible pressure fluctuations would be measured also those from meteorological variations. Therefore, a leak is introduced in the backing volume, being a thin capillary. The acoustical resistance of this capillary controls the low frequency cut off of the instrument by its diameter and length. The relaxation time of the instrument is tuned in such a way that the lowest period recovered equals 500 seconds. The electronics are placed in a box beneath the pressure measuring devices.

Figure 4.2 shows a picture of the KNMI microbarometer. On top the inlets and inlet box can be seen. The sensor and backing volume are located within the grey box. The white box at the bottom contains the electronics. The configuration of the KNMI microbarometer is such that a robust instrument is obtained, necessary for field applications.

A view insight the grey box, containing the sensing elements, is given in figure 4.3. The disc, of stainless steel, is the Validyne® pressure transducer. The backing volume is connected to it. In figure 4.3, the backing volume is opened to show the thin capillary. Although, the capillary is difficult to distinguish, it leads pressure fluctuations with periods larger than 500 seconds back to the atmosphere through the white rubber tube connected to the top of the backing volume. The thick walled backing volume with the capillary inside (instead of outside) garantee temperature stability.

Measuring pressure differentially introduces temperature dependence; warming up or cooling down the air within the backing volume will influence the measurements. The instrument is, therefore, mounted beneath the surface to achieve temperature stability. The instrument is also protected in this way.

In figure 4.4, a schematic view is given of the installation of the KNMI microbarometer in the field. The instrument is placed in a pvc container, creating a dust, water and insect proof environment. The latter is experienced non trivial in field applications. Extra protection of the instrument is achieved by covering it with a concrete top. To each instrument six porous are connected to reduce wind noise. Wind is coherent over only small distances, thus by sampling the atmosphere over area rather than one point, noise reduction is achieved.

Figure 4.5 shows a picture of one of the microbarometer's installations at Airfore base Deelen in the East of the Netherlands. The intergrating area is defined through six porous hoses in a spider like layout. To sample the surrounding atmosphere not too close to the instrument, closed hose (yellow) connects the porous hose (black) to the instrument.