Low Frequency Noise Fields and Signal Characteristics

Abstract

Very low frequency (VLF) (2 to 20 Hz) and low frequency (LF) (20 to 200 Hz) physical noise models and previous measurements are discussed and compared to low frequency, high resolution noise measurements. These data show long-term persistent directional characteristics associated with distant shipping lanes and density patterns. Short-term averages show a temporally dynamic field composed of resolved distant shipping and uncorrelated background noise. Calibrated low frequency transmission loss and signal coherence length estimates are presented for down-slope to deep ocean and bottom-limited sound channels. The transmission results compare favorably with previous measurements and computations performed with range-dependent water column and sediment characteristics. These results emphasize the importance of the coherent contribution from coastal shipping to the mid-ocean noise field. Vertical directionality measurements by Anderson et. al. (1972) show a broad angular distribution of noise intensity near the horizontal at low frequencies and a peaked distribution about the horizontal at high frequencies. This broad angular distribution near the horizontal is consistent with sound propagating down-slope via a bottom reflectivity which favors lower frequencies. The frequency variation near the horizontal was found smooth and indicates that in addition to surface ships, environmental noise influences the vertical directionality.