Importance of Sound Speed - Assignment Example

1.4 Importance of Sound Speed An understanding of how the speed of sound varies as it moves through shallow water is essential when performing underwater operations. The way that sound refracts in different tidal conditions in these shallow water areas will be especially helpful to people who work in mine warfare and sonar operations (Thain & Priestley, 2002). One of the most important tools in anti-submarine warfare is the use of sonar, which detects and locates items underwater by reflecting these sound waves off of underwater surfaces. The use of sonar is possible because sound waves travel as a series of compressions at equal speed to their frequency and wavelength, providing predictable patterns for measurement. Wave pressure is expressed by both time and distance. Sound moves in a straight line in a medium of equal density (Funk & Wagnall, 1979), but sonar depends on the reflection of sounds that are conducted underwater. Whether sound is reflecting (throwing sound back from a surface) or refracting (bending the normally straight path of sound toward a new direction) is of great importance to sonar applications. Levels of stratification and levels of salinity greatly change how sound travels through shallow water. Accurate calculations and research will give sonar operations more accurate and reliable results. 1.5 Sound Speed Theory The speed and velocity at which sound travels through water was first researched by Sir Isaac Newton in 1687. He began these investigations when he discovered that measurements of sound as it travels through fluids relied only on the physical properties of the fluid, such as its elasticity and density. The first accurate measurements of the speed of sound in water were made in 1826 by the French mathematician Jacque Sturm. Further studies of how sound originated and was carried underwater became crucial from a military standpoint in World War I with the introduction of the submarine. Great progress was made in our understanding of sonar during World War II and the issue has received increasing attention in more modern times (Funk & Wagnalls, 1979). 1.6 Sound Speed in the Sea The speed of sound in water depends on different factors including temperature, salinity and wave depth (Derencin, 2002). There is a positive relation between water temperature and depth – as the depth increases, the water temperature decreases. The term ‘isothermal’ is used to describe a uniform water temperature (Standards and Curriculum Division, 1944). 1.6.1 Density Sound travels slower as it encounters denser gas or fluid; therefore, the speed of sound in water is four times that of its speed in air thanks to the other factors that are affecting it. Density is a hydrodynamic principle that indicates pressure will increase downward and remain constant horizontally. Sea water density is recognized by its temperature and its salinity. Salinity has a greater effect on water density than its does on sound velocity. Sound speed is also determined by the medium’s flexibility. In the open ocean where there is a more consistent salinity, the lighter water is the warmer water and temperature would be constant or decrease with greater depth. Different salinity could also control density distribution. This is the reason sound waves vary from place to place in the ocean and this differentiation is an important characteristic influencing sound transmission. 1.6.2 Salinity Ocean salinity ranges from 32 to 38 ppt fixed in the open ocean. Changes in the salinity will be accompanied by changes in density. Differences in salinity can be found in oceanic fronts separating water stacks. These differences could be anticipated in river deltas, heavy ice and heavy rainfall. As has been mentioned, changes in salinity will change sound speed by influencing the density of the water. According to Thain and Priestley (2002), there have been several factors influencing the research into the sound velocity in the Dart estuary including the changes in salinity as the “fronts” move and the temperature differences between upper layers and lower layers. These three things alone will affect the levels of sound velocity greatly. 1.6.3 Pressure Pressure in many cases is more important than salinity because it changes constantly causing changes in density and increases in the sound wave formation. Because pressure can have the effect of bending, or refracting, the sound beam, these constant changes can have far-reaching effects. 1.6.4 Temperature Temperature, however, emerges as the major factor influencing sound decreasing in both depth and speed in consequence with 3 m/sec per oC. If temperature is constant below 1000 m, then pressure is the major factor influencing the speed of sound. As the temperature decreases, the density increases, slowing down the sound wave. However, temperature often varies with density at variable rate. Changes in temperature differ from a change in another point in scale. The temperature effect on sound speed is larger compared to the other factors, but there should be a change of about 165 meters to make a similar change in the speed of sound. Thus temperature is the sole factor measured upon operational conditions. All processes include the passage of time as it produces progressive changes in temperature distribution. Temperature distribution in the sea is the outcome of the interplay of all processes. These complicated distributions are also the outcome of intermittent action of all other processes. Although temperature is main factor, a sound-speed profile is a compound of pressure, salinity and temperature profiles. To use temperature data only in ocean fronts may cause up to 4.2 meters error every second in calculating sound speed. (Federation of American Scientists, 1998; Standards and Curriculum Division, 1944; Davidson, 2005; Urick, 1983; Waite, 1998) 1.7 Typical Sound Speech Profiles Since the speed of sound in water can vary depending on the temperature, pressure and salinity, when these variables are taken into account in respect to water depth, sound speed profile can then be created to show how the sound will travel in that region of water. Sound profiles can also change based on the geography as well as other environmental influences such as the time of day or season of the year (Davidson, 2005). The primary difference in sound speed occurs in changes of depth, which typically includes changes in temperature. A sound velocity profile (SVP) refers to a group of propagation speeds and is the major instrument which anticipates range of sound travel. SVP could be gained from temperature and reveals water can be separated into three vertical areas. The surface layer is directly under the sea’s surface. Sound speed in this region is prone to daily and local changes of heating, cooling and wind. The surface layer may contain isothermal water. Thermocline is the layer below the surface layer. This layer is characterized as having water temperature that is greater than that of the lower water but cooler than that of the surface layer. Thereafter, the sound velocity decreases as it enters the deeper layer below the thermocline. The bottom of the thermocline layer varies upon latitude. In the deepest areas of the ocean where temperatures are stable, sound velocity can increase with the increased pressure. The surface layer is the closest level to the isothermal layer. This is a seasonal layer that is prone to a negative gradient in summer, while the water becomes warmer than the air in winter. There are positive gradients over major thermoclines (Federation of American Scientists, 1998). Development of sound speed profiles will also aid in further research and the development of sonar devices. The complexity of the estuarine tidal variables requires the use of sound speed formula comparisons based on formulae of Chen and Millero, MacKenzie and Medwin (Batton, 2004). Differences between their formulae have been as large as 0.28m/s and as small as 0m/s for calculations of sound speed in the water column, although other studies have found even higher levels of difference (Dinn et al, 1997). Batton (2004) also showed that sound speed formulae errors escalate as the beam angle increases. This demonstrates some of the challenges researchers face when trying to accurately create a profile. 1.8 Refraction Sound speed in the sea is connected to water density and influenced by temperature, pressure and salinity. The Ocean Thermal Structure is important tactically as researchers work to perfect sonar instruments. The ray theory is a process involving mapping up sound paths that are governed by Snells Law. Several paths could be recognized upon water sound velocity structure. Actual sound propagation is prone to attenuation. Snell’s Law of Refraction is a practical result of ray theory (a tracing out of sound paths) that states that for a ray incident on the interface of two media, the sine of the angle of incidence times the index of refraction of the first medium is equal to the sine of the angle of refraction times the index of refraction of the second medium. A sound wave, which enters another medium or a layer of the same medium that has different characteristics, will change direction and/or speed. “A sound ray always bends toward the region of slower sound speed” (Federation of American Scientists, 1998). It becomes a winding sound wave when it spreads in several layers; it may be negative (summer) or positive (winter) (Derencin, 2002). This temperature based variation is due to normal paths which are called refraction; the superior fraction of sound wave goes speedier than the lower one, which forces the sound wave to bend downward (Standards and Curriculum Division, 1944). Water temperature in the summer decreases in deep water, so the sound wave bends in the direction of the sea bottom. In this situation, sonar cant detect a submarine if it is at shallower depth. Water temperature in winter increases in deep water, so the sound wave bends in the direction of the sea surface. This means sonar cant detect a submarine if it is submerged near the sea bottom. Read More