One of the most common methods of detecting the presence of surface and submarine vessels is the use of sonar equipment. Sonar can be classified into two basic types: i.e. active or passive sonar. Equipment using active sonar detects the presence of a vessel by emitting a short pulse of sound ("ping") and monitoring the return of the pulse as it is reflected from the hull of the vessel. Passive sonar equipment, on the other hand, detects sound waves generated by the vessel as it moves through the water, typically vibrations produced by the vessel's propulsion system and other machinery on board.
Both active and passive sonar systems are, if the correct sophisticated equipment is used, able to produce information relating to both the position and size of the vessel detected. As such, sonar can be used to identify the vessel and, as a result, the sonar characteristics of the vessel are often called its acoustic signature. It can, particularly in military applications, be important to mask or alter the acoustic signature of a vessel, as such masking will reduce the chances of detection and identification thereof.
Many approaches have been developed to minimize the transmission or reflection or waterborne acoustic energy with the introduction of sound ranging and detection. Since the advent of the submarine, the ingenuity of designers has been particularly drawn upon to frustrate the detection capabilities of an adversary by making or otherwise obliterating the reflection of a probing acoustic interrogation signal and also sound radiated by a vessel. The change of hull configuration has been utilized to reduce the reflected signature, however, this change has not been very successful since weight and pressure considerations had to be unduly compromised and the problem of evasion by the vessel was necessarily left to a commander's discretion.
Acoustic coatings bonded on to exposed surfaces of a submersible have also been used. Other items, such as various types of coatings and compliant layers have been attached onto the hulls of undersea craft. Most have been not entirely successful because of difficulty in applying them to and to retaining them on the exposed surfaces, often they do not conform readily to irregular contours and often are unstable in a submersible's environment with temperature and pressure variations. Many also experience limited time usefulness due to fatigue and failure. Further, for use underwater they are very expensive, most deteriorate rapidly in a seawater environment, they are vulnerable to damage during docking, and they exhibit inconsistent frequency responses over wide spectra.
Yet another method of masking the acoustic signature of a vessel is to install acoustic tiles in selected areas on the hull of the vessel. These tiles come in two basic varieties: (i) decoupling tiles which deaden the sound produced by the vessel and interrupt its transmission of sounds into the water and thus help to defeat identification by passive sonars; and (ii) anechoic tiles which defeat active sonar by reducing the amount of sonic energy reflected from the hull by the "ping" produced by this type of sonar.
Unfortunately, although effective in masking the acoustic signature of a vessel, the acoustic tiles that have been used to date are relatively thick (2-2.5 inches) and heavy (25-27 lbs. for a 21.times.2' tile). As a result, the tiles are both expensive to produce and expensive to install; the high installation costs being due primarily to the additional labor costs and other complications caused by the weight and thickness of the tiles. Furthermore, the tiles are difficult to secure in place during installation and require the use of complicated and expensive equipment.
There is thus a need for a lightweight and easy-to-install acoustic tile which effectively and efficiently masks the acoustic signature of a surface or submarine vessel. One attempt to overcome the limitations imposed by the prior art is an acoustical energy absorbing baffle for minimizing sound reflection and providing isolation from noise producing sources wherein the acoustical energy absorbing baffle has a pair of restricted orifice screens rigidly secured in parallel spaced relation by a lattice stiffener, said stiffener assembly is immersed in a viscous fluid contained within a tank sealed with an elastic diaphragm. Incident acoustical energy is transmitted through the diaphragm and translated into energy absorbing motion of the fluid through the restrictive screens. The fluid and screen are designed to match the impedance of water. An acoustically compliant layer is coupled to the fluid to augment fluid particle velocity through the screens and thus to further absorb energy. The viscous fluid utilized is a silicon oil. However, no baffle enhancement effect is obtained at the most desired and necessary low frequencies.
A basic method of preventing the transmission of sound in a medium requires the introduction of a significant density discontinuity in the medium. For example, sound attenuation in a low density medium requires the introduction of a high density material such as a slab of steel to create a high density discontinuity in the low density medium. Similarly, in a high density medium, sound attenuation can be achieved by introducing a low density material such as air as a discontinuity in the high density medium. Thus, a water/air interface would serve as an effective sound attenuator in water.
A captive arrangement of air bubbles can serve as a sound attenuator in a water medium. To be effective, the hydrostatic water pressure of the medium must not exceed the pressure required to collapse the bubbles. A layer of rubber containing air cavities has been used successfully as a sound attenuator in a water medium at hydrostatic pressures less than approximately 150 pounds per square inch (psi). Such sound attenuators are commonly referred to as air-rubber baffles.
An enclosure stiffer than rubber is required to attenuate sound in a water environment at pressures higher than 150 psi. At such pressures, the air bubbles would collapse, and thus no sound would be attenuated. Enclosures stiff enough to withstand high levels of hydrostatic pressure can at the same time offer little or no resistance to sound pressure. Thus, if a stiff enclosure is constructed to exhibit resonant vibrations (with accompanying changes of the enclosed volume) at prescribed frequencies, the enclosure in effect becomes "soft" in the presence of sound pressure fluctuations at these prescribed frequencies. In other words, the enclosure is statically "stiff" but dynamically "soft."
Sonic reflectors configured for deep water marine environments are subject to elevated hydrostatic pressures and as a result have been subject to operational difficulties, particularly where it is desired that low frequencies be reflected. A high pressure baffle configuration finding acceptance in deep water environments is the so-called squashed tube baffle.
A second type of high pressure baffle configuration is the compliant tube baffle. The vibration of the stiff enclosure absorbs the sound energy at the resonant frequency, but does not transmit this energy through the low density space inside the enclosure. An enclosure so designed thus acts as an efficient barrier against sound propagation at the prescribed resonant frequencies and therefore is considered a "tuned resonant baffle," commonly referred to as a "compliant tube baffle." The compliant tube construction consists of a boxlike structure possessed of a length substantially in excess of the width or thickness thereof. Each of these compliant tube structures have longitudinal elements such as a pair of plates disposed in a generally parallel plane relationship. The compliant tubes are covered with plies of an elastomeric encapsulant with an elastomer imparting to the elastomeric encapsulant plies the desired acoustic properties. The plates of the compliant tube are supported at the sides and are deflected in deep water but will maintain a space within the boxlike structure to reflect sonic frequencies emanating from the vessel on which the tube is mounted. However, means are needed for dissipating energy to reduce reflections off the sonic reflector and to provide for a wider frequency spectrum.
In recent years, a perforated soundproof structure for insulating sounds by the Helmholz resonance principle by oppositely arranging an internal plate having a number of through-holes formed on the whole surface and an external plate through an air layer have attracted attention. In such a perforated soundproof structure constituted based on the general equation of the Helmholz resonance principle as in the past, the absorption coefficient to noises of frequencies other than the resonance frequency f can be extremely lowered depending on the way to combine the parameters. Therefore, it sometimes cannot exhibit sufficient sound absorbing performance to noises containing a plurality of frequencies as peak components.
Accordingly, the conventional structures, as described above, have the problem that noises of a wide frequency bandwidth cannot be sufficiently insulated because the sound absorbing performance to noises other than the resonance frequency is often extremely inferior. They also have the problem that experimental manufacture must be repeated until parameters for excellent sound insulating performance can be obtained in the determination of parameters based on the above-mentioned general equation. On the other hand, a drive mechanism such as engine is not only a generating source of noise but also a generating source of mechanical vibration. At this time, even if designed according to the general equation of the Helmholz resonance principle, the noise-proof cover is excited by the vibration of the drive mechanism, and the noise-proof cover itself, as a result, vibrates to generate noise. Accordingly, its soundproof performance is insufficient as a noise-proof cover for automobile that is mechanically excited, too.
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