Longitudinal pressure fluctuations in the atmosphere can oscillate with a large range of periods. Figure 3.1 shows a rough categorization of the different frequency bands. The frequencies intervals shown are to indicate where the different types of sound fit in the total pressure fluctuation picture and are, therefore, not fixed values.

Oscillations having periods between approximately 20 Hz and 20 kHz are audible to humans (sound). The ultrasone regime is characterized by frequencies higher than 20 kHz. Sound with frequencies lower than the minimum frequency of audibility (20 Hz) is called infrasound. The lower limit of the infrasonic domain is not strictly defined. Very low frequent events, like meteorological phenomena, are not of interest. Therefore, we adopt a value of 500 seconds as the lower limit of infrasound.

Which sources can generate infrasound?

• Volcanic eruptions
• Standing ocean waves (microbaroms)
• Explosions
• Nuclear tests
• Supersonic planes flying through the sound barrier (sonic booms)
• Exploding meteors

Female elephants are known to communicate through infrasound and pigeons use infrasound for orientation, but why should a geophysicist bother about infrasound?

• Being a seismologist at the KNMI, one has to be able to provide public information regarding vibrations. In figure 3.2 a recording on a seismometer, located in the northern part of the Netherlands (Finsterwolde), is given.
  Figure 3.2: A signal recorded on a borehole seismometer in Finsterwolde (The Netherlands)
  

  The signal shown in figure 3.2 can be experienced as an earthquake. But the signal does not have earthquake characteristics (although, at first site body and surface waves seem present). A source located in the atmosphere, rather than the earth, seems more likely. By measuring both atmospheric pressure fluctuations in the infrasonic domain and seismic vibrations, one can distinguish between earthquakes and atmospheric sources. The signal in figure 3.2 appeared to come from a plane flying through the sound barrier. Such a signal can have strong impacts on houses, making people mistake it for an earthquake.

• To verify the Comphrensive Nuclear-Test-Ban Treaty (CTBT) a worldwide network of, amongst others, 60 infrasound arrays is presently being installed. Possible nuclear tests can be identified by monitoring infrasound in the atmosphere. Research should be conducted, in this perspective, to understand infrasonic wave propagation through the atmosphere. Data processing techniques known from seismology, like frequency wavenumber analysis combined with beamforming, enable a proper treatment of the data. Applying geophysical raytracing to explain the observed data will give more insight into the behavior of infrasound. With wind being the major source of noise in infrasound measurements, research also focuses on the reduction of wind noise by analog filtering.

• Explosions are one of the sources of infrasound. By continuously monitoring the atmosphere, insight can be provided into phenomena related to large explosions. From a research and public information perspective, one can think of understanding the weather dependent inconvenience caused by explosions for test purposes.
  A series of large explosions generating infrasound, occurred in the East of The Netherlands (Enschede) on May 13th, 2000. The analyses of infrasound data enabled a clear reconstruction of the explosions' origin times.