First description of the sound pressure and particle velocity components of the ambient noise and boat noise recorded at the WWF- Miramare Natural Marine Reserve (Trieste, Italy)

The underwater acoustic background noise, and the noise produced by a small outboard-engine boat moving at 6 knots, were recorded in the core zone WWF-Natural Marine Reserve of Miramare waters (Trieste, Italy), an area in which fish density is high, by using a tetrahedron-shaped 4-channels hydrophonic probe placed on the sea bottom. For the first time, a new 4-channels recording system was employed (“Soundfish”), capable of storing on an SD card, in standard WAV format, the signals coming from the 4 hydrophones. The collected data have been analyzed by means of the Brahmavolver software, a modification of the software initially developed for Brahma, a 4-channels recorder system developed for aerial applications. As result, vectorial decomposition of the sound field was obtained, containing a set of 4 signals, corresponding the 0th and 1st order spherical harmonics of the sound pressure field. In practice, the 0th-order components is the sound pressure, and the three first-order components are the three Cartesian components of the particle velocity. Hence, the instantaneous Sound Intensity vector can be computed, from which the three-dimensional direction-of-arrival of the sound can be obtained, and the position of the predominant noise source can be tracked continuously. A graphical tool such as Surfer ™, was used for charting the vectorial information over a marine map of the area, evidencing the estimated position of the sound source at every instant. This allowed for the construction of the trajectory of a moving sound source, such as a boat. This data can be employed for an analysis of the cause-effect relationship, as at every instant the position of the source, relative to the receiver, is known. The paper provides a detailed description of the Soundfish recorder, presents the result of a preliminary test of its angular accuracy obtained by experiments conducted inside a large test pool on a automated turntable, explains the mathematical and physical details of the signal processing performed by Brahmavolver, and presents the analysis of the recordings collected at the Miramare Reserve, showing the estimated iso-level contour maps of the boat passages, and, for the first time, statistics about the spatial properties of the sea background noise. It is also planned to employ the multichannel recordings for making in-air or in-pool playback of the recorded sound, employing a 3D surround loudspeaker system (working following the principle of th Ambisonics method), in order to reproduce the underwater acoustical spatial soundscape recorded in the field, something actually impossible to obtain from mono recordings containing just the sound pressure information.