A 213 meters high tower was built at Cabauw, situated in the central part of the Netherlands, in 1972. The tower was specifically constructed to obtain knowledge on the state of the atmospheric boundary layer (ABL). At 10 levels, each 20 meters, instrumentation can be installed to densely sample the ABL with for example wind and temperature measurements. Various measurements are also made on the observation field surrounding the tower including temperature, heat flux and radiation observations.

CIA is installed to complement the observations with information on pressure fluctuations within the ABL. CIA is constructed such that it is able to detect both gravity waves and infrasonic disturbances. Gravity waves are of interest because of their destabilizing effect on the ABL. Furthermore, the signature of infrasonic waves is dependent on the state of the ABL, determining the waveform coherency. The obtained knowledge will contribute to the applicability of infrasound as monitoring technique.

The KNMI microbarometer (mb) is a differential pressure sensor based on a Validyne transducer. A backing volume is mounted to the transducer with a capillary included. This capillary functions as a leak to the atmosphere and determines the low frequency cut-off of the mb. Traditionally in infrasound a lower limit of 500 seconds is adopted. This period was raised to 1000 s by increasing the length of the capillary for the purpose of measuring gravity waves, essentially increasing the acoustic resistance. Infrasonic noise is proportional to the reciprocal frequency, therefore, extra insulation measures were taken. The mb is also installed beneath the surface to guarantee temperature stability. Validyne's electronics will be redesigned to reduce its noise. The sensitivity of the mb ranges from hundreds pascals (Pa) up to orders of ten Pa.

The infrasonic frequency band has as upper limit the human hearing threshold of 20 Hz. The reduction of noise due to wind is of main concern in this high frequency part of the spectrum of interest. By using arrays, the noise is reduced through summing of the signals, ideally leading to a one over "the number of instruments" increase in signal-to-noise ratio (snr). The noise is further reduced at each array element (i.e. mb) individually by applying an analog noise reducer. The principle is based on the coherency length of the signals. Wind noise has a small coherency length of a couple of centimeters while the infrasonic is coherent of large distances concerning its large wavelength (e.g. 330 m at 1 Hz). Wind noise will cancel out by sampling the surroundings of the mb over an area rather than at one point. The signal of interest will remain unaffected and will constructively add up in the summing manifold of the noise reducer. Such a noise reducer can consists of a pipe array with discrete inlets or porous hoses.

The array layout is chosen such that an optimal array response is achieved. Optimal means that the array will sample the atmosphere homogeneously without any directionality. Beamforming will be applied to find the direction-of-arrival and propagation velocity of energy of interest. A general description of array design and response can be found under "related reading". CIA consists of a primary array and secondary infrasound instrumentation. The primary array is configured with six mb's as pentagon with central element. In addition, four mb's are placed at various distances from the array to measure signal properties like coherency and to determine its dependence on distance.

During the first part of the installation, instrumentation will be placed in the observation field. In a later stage, mb's will be mounted in the tower, leading to a 3D infrasound array. The strength of the wind increases with height and will seriously affect the quality of the measurements. But it will be one of the first opportunities to gain knowledge on the characteristics of the infrasonic waveform as function of height.

For questions regarding CIA contact Läslo Evers and/or Fred Bosveld.