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The author's interest in infrasound emerged from a decade of studies of wave phenomena in the ionosphere. These phenomena appeared frequently on radio-recordings of signals from artificial satellites, which started to populate space at the end of the 1950´s. These oscillations in the electron density, influencing the propagation of radio signals, were evidently a result of the propagation of low-frequency pressure waves through the atmosphere. When infrasound and its unusual propagation properties was rediscovered during the 1960's in connection with atmospheric nuclear tests, it became obvious that the recording of infrasonic waves at the ground level could contribute to a better understanding of wave phenomena in the atmosphere.
This book is a summary of more than 35 years spent by the author and his co-workers within the field of infrasound. The work was strongly biased towards development of detection techniques with highest possible sensitivity and accuracy of pinpointing direction of origin.. The infrasound studied here, with frequencies between 0.5 and 10 Hz, at the levels occurring in the environment, is completely undetectable by human hearing. We are thus surrounded by a world of non-audible sounds, carrying vast amounts of information, which we cannot apprehend. It has therefore been a very exciting journey through a process of identifying new signal types and their properties, and finally finding their origins. This journey is, apparently, far from over. We continually find new infrasonic signatures, which cannot be immediately explained.
The main purpose of this book is to present for the interested reader the history of work in the area of infrasound at the Swedish Institute of Space Physics. It contains previosly published, as well as unpublished, material. Another purpose of this book is to serve as a manual for users of our home page containing both the real-time data from the Swedish Infrasound Network (SIN),as well as an archive of infrasonic data (complete time-series) since 1994.
This book would not have been possible without the help of my co-workers. The early, fixed frequency, 2Hz recording system was developed by Kjell Englund. The present broadband system was developed (and is constantly improved) by Pär-Ola Nilsson, Jan Karlsson and Fredrik Rutqvist. The people who encouraged me both in my work within the field of infrasound and also to write this book are former and present directors of the Swedish Institute of Space Physics, Professor Bengt Hultqvist, Professor Rickard Lundin and Dr. Lars Eliasson. Professor Bengt Hultqvist also read and commented an early draft of this book. The author is indebted to Mrs. Lisa Holmström who reviewed the manuscript of this book. I am also grateful to my family for their patience during all the years I spent working within the area of infrasound, and later with this book.
IRF Scientific Report 291
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