THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

410 15. Underwater Acousticsof much greater magnitudes than those of the target’s dimensions, i.e., the probesignal waves should ideally be plane. In terms of ray acoustics the incident soundwave must be substantially parallel in the region of the target that they strike, andreflected sound rays should also be parallel over the area of the sonar receiver.In the military use of sonar, the applicable spectrum range covers the ultralowfrequency to the megahertz region. Acoustic mines detect the pressures below 1 Hz,which are generated by moving ships. These mines detect the acoustic radiationand explode when the acoustic level reaches a certain level in their bandpass.Such mines can be destroyed harmlessly through the use of a minesweeper that isbasically a powerful signal source towed behind a minesweeping vessel.In passive detection, the acoustic radiation of both water-surface and underwatervessels, are sensed by a hydrophone array mounted on the spy vessel or submergedat the bottom a long distance away. The receiving array must be directional in orderto be able to locate the target through sensing of somewhat higher frequencies.Modern echo-ranging (active) sonar consists of an elaborate array of equipmentto send out signals in the form of long, high-power pings in designated directionsvertically and horizontally, and newer signal processing techniques present theechoed data to the observer. Transducer arrays are often enclosed in separatehousings that are towed underwater behind the surface vessel, so that shallowthermal gradients 1 can be penetrated, and the sonar can probe in the stern (forward)direction, a procedure that cannot be achieved with a hull-mounted sonar.Peaceful uses of sonar expanded greatly immediately after World War II. Originallydeveloped for depth sounding, sonar is now being used to find fish, study fishmigration, map ocean floors, locate underwater objects, transmit communicationsand telemetric data, serve as acoustic speedometers, act as position-marking beacons,and monitor well-head flow control devices for undersea oil wells. Passivesonar provides marine biology researchers a window for tracking sounds made bycetaceans (whales, dolphins, and porpoises).15.2 Some Basic Concepts Pertaining to Underwater SoundIn an acoustic plane wave passing through a fluid medium of density ρ, the particlepressurep relates to the fluid particle velocity u as followsp = ρcu.The fluidic parameter ρc is called the specific acoustic resistance. Its value, onthe average, for seawater is 1.5 × 10 5 g/cm 2 sor1.5× 10 6 kg/m 2 s. In contrast,ρc = 42 g/cm 2 s for air. From our previous chapters we recognize that this parametercan also assume a complex value in its role as the specific acoustic impedance.1 In the 1920s and 1930s, it was observed that good echoes were obtained with the early versionsof shipboard echo-ranging equipment in the morning and poor or no results were obtained in theafternoon. This was due to the shifts in the seawater thermal gradients that caused sound to refracttoward the sea bottom and thereby place a target in the “shadow zone.”