Sonar: How Does it Work?
Sonar: How Does it Work?
Sonars might be state-of-the-art technology for surveying the ocean, but they aren't infallible. The ocean has a lot of unknowns that can degrade the quality of information received by a sonar, meaning that you'll need to add sound modeling and estimation into the system. For example, unlike the speed of light, the speed of sound isn't constant. In fact, it depends highly on temperature (T), salinity (S), and depth (D). The equation scientists use to handle all those factors is called Medwin's formula:
ν = 1449.2 + 4.6T – 0.055T2 + 0.000029T3 + (1.34 – 0.010T)(S – 35) + 0.16D
Example One
Say you've got the following reads on the parameters of an area for January through October. Based on Medwin's formula, ν, a.k.a. the speed of sound, is going to be:
| T (°C) | S (ppt) | D (m) | v (m/s) | |
| January | -5 | 32 | 100 | 1422.21875 |
| February | 0 | 32 | 101 | 1446.796 |
| March | 0 | 31 | 102 | 1445.472 |
| April | 2 | 35 | 103 | 1459.83032 |
| May | 14 | 39 | 104 | 1510.07976 |
| June | 15 | 33 | 105 | 1506.10375 |
| July | 25 | 33 | 100 | 1533.77625 |
| August | 25 | 31 | 98 | 1531.56425 |
| September | 20 | 32 | 99 | 1519.684 |
| October | 16 | 34 | 100 | 1510.32784 |
All that sound is based on the acoustic wave equation, which tells us how sound moves in a material. We use sound because it's one of the few types of waves that can move quickly through water, but we need a way to create the sound from pressure.
Crystals
To create some good, old-fashioned pressure, quartz and piezoelectricity come into play. Piezoelectricity is a property of crystals that lets them turn pressure into electricity. When you add pressure to two crystals (used as dipoles) you're going to generate some voltage, which can be changed to create a wave.
Once you've got the electricity, the transducer will be able to change that energy to a different output energy—in this case, a sound wave. Once the sound wave returns, it's going to be caught either by the transducer itself or a hydrophone. Hydrophones—like microphones—collect sounds. Although hydrophones can operate without transducers (like in passive sonar), and transducers can include hydrophones, a complete sonar system will have one of each.
The change in voltage is measured to determine the return scattering or reflections from the objects surrounding the voltage energy. In bi-static or multi-static sonar systems, there are a number of hydrophones or receivers that collect the sound waves when they echo back to the sonar.
Sonars: the wave of the future.