Joint Baltic-Nordic Acoustics Meeting 2004, 8-10 June 2004, Mariehamn, Åland BNAM2004-1 DENOISING AND ANALYSIS OF AUDIO RECORDINGS MADE DURING THE APRIL 6-7 2000 GEOMAGNETIC STORM BY USING A NON-PROFESSIONAL AD HOC SETUP Unto K. Laine Helsinki University of Technology Laboratory of Acoustics and Audio Signal Processing Otakaari 5 A, 02015 Helsinki, Finland Unto.Laine@hut.fi ABSTRACT An unpredictably strong geomagnetic storm took place on April 6-7 2000. A scientific study on aurora related sounds and acoustical effects had just been started. There were two possible choices: to perform audio recordings with a non-professional ad hoc setup, or, to miss a promising possibility to study the phenomena. The first choice was selected and it produced over seventy-five minutes of data, corrupted with impulsive noise from a DC/AC-inverter used to power recording equipment in the field. The data consists of ten successive recordings. This paper describes a novel method to cancel the impulses found in data without affecting the microphone signal significantly. Next, the power of the cleaned signal is computed in 1/3-octave bands. The spectral comparisons of the different recordings showed a clear increase in the average power in recording #5 that was sampled somewhat after the peak of geomagnetic activity. Specifically, the frequency band around 100 Hz showed an increase in power and also in its fluctuation during the most active geomagnetic periods. This new result supports those obtained earlier [4]. Also, the role of related infrasounds is preliminary discussed. 1. INTRODUCTION “IN the time of the Cimbrian warres (120-101 BC), we have been told, that Armour was heard to rustle, and the Trumpet to sound out of Heaven. And this happened very often both before and after those warres. But in the third Consulship of Marius (103 BC), the Amerines and Tudertes saw men in armes in the skie, rushing and running one against another from the East and West; and might behold those of the West discomfited. That the very firmament it selfe should be of a light fire, it is no marvaile at all; for often times it hath been seene, when clouds have caught any greater deale of fire.” Caius Plinius Secundus, Historia Naturalis [1]. Figure 1. Aurora Borealis above Pressburg, Hungary on February 10, 1681 [2].