Understanding Basic Properties of Underwater Sound Propagation

Understanding Basic Properties of Underwater Sound Propagation

By: Mad Helmi Bin Ab. Majid (PhD Student)

Acoustic signal plays a very important role in underwater communication, localization and positioning system as other type of signals are poorly perform in the water. The sound (acoustic) travels at the speed of 384 m/s in the air while in the water sound travels at 1450 m/s i.e. about four times faster than in the air. However, this speed is varies according to few factors and understanding these factors are the key factor for successful underwater acoustic based application. Figure 1 illustrates how underwater sound speed varies as the depth of the ocean increases.

Figure 1 Speed change as a function of depth [1]

The underwater sound speed is affected by few factors which are temperature, salinity and pressure. The equation that represents the relationship between the sound speed and the three factors are given by

Sound speed (m/s) = 1449.2 + 4.6T - 0.055T2 + 0.00029T3 + (1.34 - 0.010T)(S - 35) + 0.016z

where

Where:
T is the temperature (掳C),
S is the salinity (parts per thousand), and
z is the depth (m).

Thus, observing from the above equation we can conclude that the sound speed is increases as T, S and z increases. Other important factors should be taken care in dealing with underwater sound is the variation of sound level as illustrated in example as shown in Figure 2.

Figure 2 Variation of sound level with respect to source location [2]

In conclusion, we could summarize that the studies of underwater sound properties are crucial for underwater application for accurate and reliable execution. The above factors become crucial when we are dealing with water depth which has significant changes of the temperature, salinity and pressure.

Reference(s):

1. How fast does sound travel? Retrieved June 3, 2015 from http://www.dosits.org/science/soundmovement/speedofsound/?CFID=3913853&CFTOKEN=47489084
2. An Introduction to Ocean Acoustic, Retrieved June 3, 2015 from http://www.meted.ucar.edu/oceans/acoustics/print.htm#page_4.1.0
3. What is Source Level and Received Level? Retrieved June 3, 2015 from http://www.surtass-lfa-eis.com/Terms/

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