ACOUSTIC SIGNATURES

1. ACOUSTIC SIGNATURES 080990116 Oğuzhan U. BAŞKURT 080000130 B. Sertaç SERBEST

2. Definition of Acoustic Signatures • World War II

3. Usage Areas • Propeller Noise • Ship Hull Hydrodynamics • Structurally Transmitted Noise • Ship Noise Management System • Active Noise and Vibration System • Underwater Acoustics in Oceanography and Marine Geology • Critical propagation issues in shallow-water acoustics • Geoacoustic modeling of the seafloor • High-resolution shallow water 3-D survey and inversion for geophysical parameters • Measurement of ocean-bottom

4. Propeller Noise • The cavitating marine propeller is typically the strongest single noise source on a naval ship, and propeller designer’s direct considerable effort towards delaying the onset of cavitation.

5. Ship Hull Hydrodynamics • Ship hull hydrodynamics has concentrated on prediction of propeller inflow velocities for propeller cavitations and noise prediction. To date, propeller inflow conditions have been predicted using panel methods together with thin boundary layer theory

6. Structurally Transmitted Noise • At ship speeds below cavitations inception, a warship's signature is generally dominated by structurally transmitted noise from on-board machinery

7. Ship Noise Management System • Maintenance of the machinery in a ship and details of how it is used largely determine the underwater noisesignature in many situations

8. Active Noise and Vibration Control • Reducing or controlling the underwater acoustic signatures of naval ships and submarines has historically meant the use of "passive" techniques. • Components such as rubber engine mountings, flexible pipe work couplings and joints, vibration isolators, and interior acoustic absorbers can be used to prevent acoustic energy from being coupled into the structure of the vessel. • While these methods have proven to be effective in general, there are usually low frequency limitations that cannot be overcome by passive means. • Active noise and vibration control methods, when used in combination with more conventional passive techniques, show promise for good noise control over the full frequency range.

9. Underwater Acoustics in Oceanography and Marine Geology • Electromagnetic waves are used to probe the regions above the surface of the ocean but the attenuation of these waves in water is so great that they are of little use for investigations within the ocean. • Fortunatelly sound is propagated, to great enough distances in water so that it can be used for many of the purposes served by radio, radar and light in the atmosphere • There are some usage areas of Underwater Acoustics in Oceanography and Marine Geology: • Precision Echo Sounding • Fish Detection • Acoustic Telemetering • Marine Geology • Naval Warfare

10. Critical propagation issues in shallow-water acoustics • Several sets of propagation problems in shallow-water acoustics are discussed, covering overlapping frequency ranges. • At VLF (say below 300 Hz) the bottom material is important, because energy travels in the bottom or because of attenuation associated with shear-wave coupling. • At LF (say above 30 Hz) the presence of gas in surficial sediments can change boundary conditions and bottom losses, and even allow a new type of interface wave. • At MF (300 Hz to several kHz) dispersed fish can raise the attenuation, typically by 1 dB/km. • Fish have also been observed to dominate the echo structure or reverberation, out to 100 km. • At HF (say above several 100 Hz) the sound-speed profile becomes important, partly because of channeling. • At VHF (usually above several kHz) the propagation mechanisms tend to change from boundary reflection to boundary scattering. • In addition all these effects are greatly influenced by distance and by water depth

11. Geoacoustic modeling of the seafloor • Over the past decade a number of new techniques have been developed for measuring the geoacoustic properties of seafloor sediments. • Several of these methods utilize sensors that are deployed on the seafloor and respond to seismic waves generated by different kinds of source including explosives and mechanical impact as well as the ambient pressure fluctuations generated by wave motion at the sea surface. • Since the motion that is sensed at the seafloor is a combination of many different kinds of interface and body waves (i.e., Scholte waves, Love waves, p, s[sub v], and s[sub h] refractions and reflections, etc.), inversion to obtain a geoacoustic model is a complex task that must be tailored to each individual experiment taking into account the bandwidth of the signal and the interaction between the various types of wave. • This paper summarizes some of the recent field experiments and the methods that are being used to interpret the data

12. High-resolution shallow water 3-D survey and inversion for geophysical parameters • In the shallow water environment even in subsurface depths less than 30 m, 3-D structures can affect wave propagation. • A high-resolution (500--3500 Hz) 3-D survey (53 lines; 5-km length, 10-m line spacing, 5-m shot spacing) using a Huntec deep-towed boomer source and STARFIX navigation was carried out offshore New Jersey in 1989. • Additional 3-D acquisition along with vibra coring will be conducted in the same region in the summer of 1993 • A major challenge is to combine 3-D acquisition, imaging, interpretation, and core data with the requirements of material property estimation such that a complete understanding of propagation in shallow water environment can be achieved

13. Measurement of Ocean-Bottom • Geoacoustic properties of the ocean bottom can be a significant factor influencing acoustic propagation in shallow-water environments. • Knowledge of these properties is required for reliable acoustic propagation modeling and matched-field processing

14. Sincerely Yours…