Underwater Acoustics

Analysis, Design and Performance of Sonar

Richard P. Hodges

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781119957492
• Edition: 1
• Topic: Ciencias físicas
• Subtopic: Ondas y mecánica de ondas

Offering complete and comprehensive coverage of modern sonar spectrum system analysis, Underwater Acoustics: Analysis, Design and Performance of Sonar provides a state-of-the-art introduction to the subject and has been carefully structured to offer a much-needed update to the classic text by Urick. Expanded to included computational approaches to the topic, this book treads the line between the highly theoretical and mathematical texts and the more populist, non-mathematical books that characterize the existing literature in the field. The author compares and contrasts different techniques for sonar design, analysis and performance prediction and includes key experimental and theoretical results, pointing the reader towards further detail with extensive references. Practitioners in the field of sonar design, analysis and performance prediction as well as graduate students and researchers will appreciate this new reference as an invaluable and timely contribution to the field.

Chapters include the sonar equation, radiated, self and ambient noise, active sonar sources, transmission loss, reverberation, transducers, active target strength, statistical detection theory, false alarms, contacts and targets, variability and uncertainty, modelling detections and tactical decision aids, cumulative probability of detection, tracking target motion analysis and localization, and design and evaluation of sonars

1

Introduction to Sonar

SONAR (SOund NAvigation and Ranging) systems have many similarities to radar and electro-optical systems. The operation of sonar is based on the propagation of waves between a target and a receiver. The two most common types of sonar systems are passive and active. In a passive sonar system, energy originates at a target and propagates to a receiver, analogous to passive infrared detection. In an active sonar system, waves propagate from a transmitter to a target and back to a receiver, analogous to pulse-echo radar. In addition to these two types, there is also daylight or ambient sonar, where the environment is the source of the sound, which bounces off or is blocked by the target, and the effects of which are observed by the receiver. This latter type of sonar is analogous to human sight.

Sonar differs fundamentally from radar and electro-optical systems because the energy observed by sonar is transferred by mechanical vibrations propagating in water, solids, gases, or plasma, as opposed to electromagnetic waves. Today, sonar refers not only to systems that detect and/or transmit sound, but to the science of sound technology as well.

In military applications, sonar systems are used for detection, classification, localization, and tracking of submarines, mines, or surface contacts, as well as for communication, navigation, and identification of obstructions or hazards (e.g., polar ice). In commercial applications, sonar is used in fish finders, medical imaging, material inspection, and seismic exploration.

Figures 1.1, 1.2, and 1.3 illustrate the basic passive, active, and daylight/ambient sonar systems.

1.1 Acoustic Waves

The term “acoustic” refers to sound waves in any medium. Acoustic waves come in two types: longitudinal or compression and transverse or shear. In fluids, only longitudinal or compression waves are supported because fluids lack shear strength. The easiest way to visualize these two types of waves is to consider a Slinky (see Figure 1.4). If the end or middle portion of a Slinky is moved side to side or up and down, a transverse or shear wave will move along it. This method displaces the material of the Slinky in a direction perpendicular to the direction of travel. As the material is moved off the axis, the spring force exerts a restoring force that pulls it back on axis. If several of the Slinky coils are compressed or stretched, then releasing them will propagate a longitudinal or compression wave along the Slinky. This method displaces the material of the Slinky along the direction of travel. Again, the restoring force will tend to push the material back into place. In this book, we will deal with transverse or shear waves only occasionally, so unless specifically stated, longitudinal or compression waves are assumed.

Figure 1.1 Passive sonar system

Figure 1.2 Active sonar system

Figure 1.3 Daylight/ambient sonar system

1.1.1 Compressions and Rarefactions

Longitudinal waves are composed of compressions, where the parts of the medium (coils of the Slinky) are closer together than normal, and rarefactions, where the parts of the medium are farther apart than normal.

Figure 1.4 Transverse (top) and longitudinal (bottom) waves on a Slinky

Figure 1.5 Pressure wave

The fundamental parameter of an acoustic wave is pressure. When water or air molecules are pushed or pulled apart, they exert a restoring force that resists the motion. The force will be felt locally as pressure or force per unit area. The amplitude of the wave will be the peak pressure reached in one cycle. The disturbance of the medium that propagates is the distance between molecules. Figure 1.5 illustrates the fundamentals of a pressure wave.

1.2 Speed of Propagation

For a nondispersive medium, one in which different wavelengths propagate at the same phase velocities (e.g., water), we would expect the same type of relationship between wavelength and frequency as with electromagnetic waves:

(1.1)

where

λ = wavelength, the distance between corresponding points (peak to peak or valley to valley)

f on a wave f = frequency, the number per unit time the wave performs a cycle

The speed of propagation for sound waves is much sl...