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| 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. |
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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. |
 |
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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. |
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