Sonar

In the film The Hunt for Red October, Soviet submarine captain Marko Ramius pulls a “crazy Ivan” maneuver, suddenly turning his vessel away from its established course. Ramius made the turn because a blind spot was preventing him from seeing anything that might have been following his Red October submarine.

What do we mean by “see” in this context? Typhoon-class submarines like the Red October have an operational depth of 400 meters. Isn’t if kind of dark down there?

If you’ve seen the film (and it’s a great one), you know that the submarines were using sonar, a technology which relies on sound to “see” in the water. SONAR stands for SOund Navigation And Ranging. Navigation means finding your way. We met the word ranging a couple weeks ago, meaning an assessment of the distance between objects.

We learned last week that sound travels in air at about 340 meters/second. It turns out that sound travels faster in water than in air, at a speed of around 1500 meters/second. Just like the speed of sound can vary in air with atmospheric conditions, so can the speed of sound in the ocean vary with its “atmospheric conditions”, like water temperature, depth below the surface, and the salinity.

Why does sound travel more quickly in water than in air? Science has determined that the speed of sound is indirectly proportional to the compressibility K and density r of the medium (air or water) through which it travels:

v =  (Kr)^-1/2 

where 0 < K <1. Let’s do a quick back of the envelope calculation. Air has a ballpark density of about 1.0 grams/liter while water has a density of about 1 kilogram/liter.  Water’s compressibility is very small (K<<1) and air’s is about 20,000 times larger. So if we were to compare sounds speed in water to its speed in air, we’d find that:

v_w              (k₁ * 1000 grams/liter)^-1/2

—-  =         ————————–                     =   sqrt ( 20) = 4.5

v_a               (20,000 k₁ * 1 grams/liter)^-1/2

 

So sound works out to travel about more than four times faster in water than air!

Water’s compressibility is so small  that we usually consider it incompressible. You can read more about water’s compressibility at https://www.usgs.gov/special-topic/water-science-school/science/water-compressibility?qt-science_center_objects=0#qt-science_center_objects

Sonar comes in two flavors: active sonar and passive sonar. Active sonar is when the vessel  trying to sense things in its environment sends out a noise or a pulse – in the movie it’s called a “ping” – to see if it is echoed back after hitting another large object. In passive sonar, the sensor tries to make no noise but just listen for other things making noise, like the engines of another submarine. This is an advantage for military vessels that do not want to be found (heard), or for scientific missions that want to quietly listen to the ocean.

One significant difference between sonar and radar is that one requires a medium for transmission and the other does not. Like all sound, sonar needs a medium, like water or air, in order to propagate to the receiver. Like all electromagnetic waves, radar waves are transmitted electromagnetically, without any medium. This is why communications between earth and deep space probes are successful.

A second significant difference between sonar and radar is that sonar waves propagate as longitudinal waves, and radar waves as transverse waves. If you had a Slinky, a longitudinal wave would propagate by compressing and decompressing successive rings of the coil, whereas a transverse wave causes it to be perpendicularly displaced from the direction of propagation. It’s hard to describe with words, but you can find an animation showing the difference at this website: https://www.physicsclassroom.com/class/waves/Lesson-1/Categories-of-Waves#:~:text=Transverse%20waves%20are%20always%20characterized,direction%20that%20the%20wave%20moves.&text=Longitudinal%20waves%20are%20always%20characterized,being%20parallel%20to%20wave%20motion.

Who uses sonar? You’ve probably already guessed that ships and boats on the surface of the water also use sonar. Sonar is used by fishermen to find fish and by treasure hunters to locate sunken treasures. That’s how the wreck of the luxury passenger ship Titanic was found! Sonar is also used by scientists to study the environment and the geography of bodies of water like lakes and oceans.

Next week, before we leave the subject of underwater signals, we’ll learn how submarines – taking a lesson from dolphins and whales – communicate, even while deeply submerged in the ocean.