The curtain effect in a multiple convergence zone environment: Implications for ambient noise and bottom and surface reverberation levels

Abstract

Over multiple convergence zone propagation ranges the relative spreading loss per zone, although initially high, decreases with range. The other major component of propagation loss, attenuation, remains unchanged and eventually becomes greater than the rate of spreading loss. The range at which this crossover occurs—the curtain effect [Browning et al., J. Acoust. Soc. Am. Suppl. 1 80, S54 (1986)]—is highly frequency dependent. At low frequencies the curtain effect occurs at long ranges; this allows sources or scatterers in the second convergence zone or beyond to impact the received level since they suffer only a relatively small additional loss compared to the spreading loss to the first convergence zone. For a given distribution of noise source levels or scattering strengths it is estimated what would be the resulting background level and the relative importance of events, for example, a comparison between medium strength multiple events at medium ranges and a strong event at long range. This analysis is then extended to higher frequencies where the curtain effect occurs at a range of one convergence zone or less. [Work supported by NUSC.]