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97 Rationally, if the combined speed v is moving towards the same direction as the signal propagation, effective propagation speed v’ will rise. However, if v moves in the opposite direction of propagation, v’ will decrease. As the result of water flow, a nar- row acoustic beam will bend slightly in the same direction. Nevertheless, the resulting effect is not significant. The propa- gation speed will change significantly if going through a differ- ent medium, for example from air into water or vice versa. This happen because of dielectric constant changes result in refrac- tion just as light bends from air through glass or water. Thus, the direction of propagating signal will be changed by refrac- tion. On a light note, in optometry the term ‘refraction’ refers to eye examination in the process of determining whether a pre- scription for spectacle improves a person’s vision. This applica- tion is known as refractometry. Figure 3.13 shows that reflection also occurs when the signal touches the boundary be- tween two media, then a part of the signal will be reflected back directly from the surface into water without going through the air. Reflection from the bottom of shallow water as with most swimming pools and beaches will also cause multipath effect Figure 3.13 Water surface causes refraction and reflection FONG et al 2011 In mathematics, the accepted signal rt can be written as: 98 in which the attenuation ceofficient α n indicates the decline in signal strength due to the absorption attenuation which can turn the signal energy into heat and loss because of reflection which depends on both distance and frequency while the original transmitted signal st is subject to a delay of τ n resulting in st + ࢚ ࢔ . N is the number of incident acoustic signal paths which is caused by multipath effect. In most cases, n=3 in short range shallow depth due to three signal paths that occur which are a reflection from the bottom reflection from the surface Direct LOS between the receiver and transmitter. Usually, there is an increase in n and t n due to the increasing of range and depth since more reflections will occur and the time taken for the signal to reach the receiver will increase. Reflec- tion loss because of water surface and bottom can be extremely different since the bottom may store deposits that make it une- ven. The molecular movement of the water surface that is caused by the propagating signal is very minimal as the carrier wave most probably does not carry enough energy to move the water substantially. Hence, just a very small fraction of the sig- nal will be sent into air from water. Almost all signals will be reflected back into the water. Acoustic pressure also does not match well with air just like an electrical current hitting a load with ‘impendence mismatch’. The same situation happens in the case from air into water. This explained why we have difficulty hearing anything from above when our heads are soaked in the swimming pool. Usually, this coupling problem not happen at 3.2 99 the bottom because deposited particles are ‘more friendly’ with the movement of water molecules. Better coupling will cause a certain portion to be reflected back into the water and others will be absorbed. This is good for communication because ab- sorption will have a negative impact on multipath, the bottom efficiently acts as cushions to shield some reflected signals and thereby declining N. The actual efficiency will rely on the com- position of the deposit. We have seen signal propagation in respect to time. Let us now see the signal propagation in respect to distance. Consider a sig- nal S d, whereby d is the distance travelled by the signal. The signal S will definitely weaken if d increases. Below is a basic mathematics equation that explained their relationship. Attenuation is usually indicated in dB, the signal loss is L is expressed as:- Note : L is not to be confused with the notation in Equation 2.2 that represents the number of levels available: It also can be simplified as: 3.3 3.4 3.5