In this way, much of the sound does not dissipate out into the water in all directions, but instead is trapped within the channel, and can travel very long distances with little loss of energy (Figure 6.4.2).įigure 6.4.2 Sound propagation in the SOFAR channel. As the sound waves go deeper below the channel, they will be refracted upwards, back into the channel and the region of slower speed. As a result, sound waves moving from the SOFAR channel into shallower water will be refracted back towards the channel. As we saw with seismic waves, when these sound waves encounter a region of differing transmission speed, the waves tend to be refracted or bent back towards the region of lower speed. Waves that travel into shallower or deeper water outside of the sound channel are entering a region of faster sound transmission. Sound waves produced in the channel radiate out in all directions. The SOFAR channel is important because sounds produced in that region can be propagated over very long distances with little attenuation (loss of energy). At moderate depths lies the SOFAR channel, the region of slowest sound speed (PW). Sound speed is high at the surface due to the high temperatures, and is high at depth because of the high pressure. This zone of minimum speed is called the SOFAR channel ( Sound Fixing And Ranging) or the Deep Sound Channel (Figure 6.4.1).įigure 6.4.1 Profiles of temperature, pressure, and sound speed with depth. At moderate depths (between a few hundred and one thousand meters) there is a zone where both temperature and pressure are relatively low, so the speed of sound is at a minimum. Near the bottom, the extreme pressure dominates, and even though temperatures are low, the speed of sound increases with depth. As depth increases, the temperature and the speed of sound decline. The temperature effects dominate at the surface, so the speed of sound is fast in surface waters. At the surface, the pressure is low, but the temperature is at its highest point in the water column. To examine the way the speed of sound changes as a function of depth, we need to consider the vertical profiles for temperature and pressure. We have seen that these variables change with depth and location so to will the speed of sound differ in different regions of the ocean. However, as with sound in air, the speed of sound in the ocean is not constant it is influenced by a number of variables including temperature, salinity, and pressure, and an increase in any of these factors will lead to an increase in the speed of sound. In water, the sound is so much faster that the difference in arrival time between our ears becomes too small for us to interpret, and we lose the ability to localize the source.
Our brains can process that small difference in time of arrival to recognize the direction from which the sound came. A sound coming from our left will reach our left ear a fraction of a second before reaching our right ear. We localize sound sources when our brains detect the tiny differences in the time of arrival of sounds reaching our ears. This helps explain why we sometimes have difficulty localizing the source of a sound that we hear underwater. Since water is much denser than air, the speed of sound in water (about 1500 m/s) is approximately five times faster than the speed in air (around 330 m/s). In other words, sound travels faster through denser materials. Therefore, sound travels faster and more efficiently when the molecules are closer together and are better able to transfer their energy to neighboring particles. With ocean sounds, the energy is transmitted via water molecules vibrating back and forth parallel to the direction of the sound wave, and passing on the energy to adjacent molecules. Sound is a form of energy transmitted through pressure waves longitudinal or compressional waves similar to the seismic P-waves we discussed in section 3.3.
0 kommentar(er)
