|Infrasound is being measured with a microbarometer, this is a highly sensitive barometer as known from meteorology. A barometer measures air pressure fluctuations in the order of 100 hPa (hecto-pascal) while a microbarometer records down to pressure values of hundreds of pascals. Furthermore, a microbarometer can measure very fast fluctuations of seconds and less, while barometers measures values in the order of minutes. The atmosphere is sampled through the aluminum pipes on top of the sensor. An analogue noise reducer is connected to this so-called inlet box.|
|The microbarometer is a differential pressure sensor, ie. infrasonic pressures are measured with respect to a backing volume. Therefore, the microbarometer is installed beneath the earth's surface to avoid temperature fluctuations. The green coaxial cables take care of power supply and signal transport.|
|Infrasound measurements are sensitive to wind. An analogue wind noise reducers is used to reduce the noise induced by wind. The coherency length of wind is in the order of 10 cm, while the infrasonic signal of interest is coherent over much larger lengths. A infrasonic
wave of 1 Hz has a wavelength of 330 m and can considered coherent over this length. The effect of wind is reduced by averaging the pressure field over an area rather than measuring it at in one point. This can be achieved by sampling the atmosphere with porous hoses, laid out in eg. a star-like configuration. The wind noise reducer realizes a signal gain of 20 dB.
|The complete installation of a sensor in the vicinity of the KNMI. Infrasound is measured with a series of microbarometers, a so-called array. Doing so, the bearing of the infrasonic wave can be obtained from array processing techniques. Furthermore, noise is reduced by averaging the signals over the array. The sensor, as shown, is part of the six elements De Bilt Infrasound Array.|