![]() However, near the surface, these lower frequencies are severely attenuated or canceled by a boundary interaction phenomenon known as the Lloyd’s Mirror Effect. Only very low frequency sounds with wavelengths larger than stern dimensions can diffract around the ship’s hull to the bow. Ship noise with wavelengths less than the ship stern dimensions are reflected back off the stern and reflect to the sides but do not propagate forward to the bow. ![]() The majority of ships that kill whales have propeller configurations above keel depth, and this shields the sounds produced by the propellers. It’s caused when the sound rays from a ship’s propellers are blocked from projecting forward by the ship’s hull. Perhaps the most confounding acoustical challenge to both whales and manatees is acoustical shadowing at the bow. Some individuals have been hit 50 different times by boats they cannot hear approaching. The incidence of collisions with small boats is so prevalent that individual manatees are routinely identified by their characteristic scars patterns from multiple encounters. Most survive collisions with smaller boats, while collisions with larger slow moving barges are fatal. Measurements of controlled ship passages through vertical hydrophone arrays demonstrate a confluence of propagation factors and near surface effects that obscure the sounds of approaching vessels which then pose serious detection challenges for marine mammals.įigure 2. The problem is a major concern for the remnant population of North Atlantic right whales and other marine mammals, such as the West Indian manatee. Whales, manatees and other animals cannot react to sounds that become indiscernible from ambient background noise nor can they react to sounds that never reach their ears. Here the acoustical laws of reflection and propagation significantly limit the ability of marine mammals to hear and locate the sounds of approaching vessels. A common denominator is that they all occur near the surface. Though more commonly identified and reported in busy coastal areas, collisions are not restricted to shipping lanes or shallow water environments. With increased worldwide commerce and international shipping, vessel collisions with marine mammals have become a global concern. Whales and other marine mammals are vulnerable to boat, barge and ship collisions. Finally, some comments are made about calibration system requirements for implementing new measurement techniques.Parametric Projectors Protecting Marine Mammals from Vessel CollisionsĮdmund R. Evaluation of sonar transducers and some of the ongoing research in calibration measurement methodology is also discussed. This paper covers the measurement facilities for carrying out the metrology function and the measurement methods for setting the primary standards and the loan standards. Calibration, test, and evaluation services and standard transducer loan services cover the frequency range of 0.2 Hz to 2 MHz. It functions as a ‘‘National Bureau of Standards for Underwater Acoustics’’ by providing two types of reference services for a fee. The Underwater Sound Reference Detachment of the Naval Research Laboratory evolved from this beginning and now serves as the focus for underwater acoustic metrology in the United States. At the beginning of World War II, the United States recognized the need for establishing systematic methods for calibrating and evaluating sonar transducers and established the Underwater Sound Reference Laboratories. ![]()
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