No, not some G.I. Joe after school lesson of life, but a lesson in Sonar of which I have been a big fan of for the last 18 years.
I have studied the effects of sonar in seawater since March of 1988 and have prided myself in knowing a lot about the subject since I am a submarine sonarman. I taught junior sonarman-wannabees, guys just starting their naval careers, and I taught seasoned salts. I have taught officers and even sonarmen of foreign navies. Today I learned something that I had skimmed right over in the "Sonar bible" a hundred times and never thought to really think about it.
An essential part of sonar propagation theory is sound velocity, or how fast sound travels in the water. I know there are 3 things that directly affect sound velocity- temperature, salinity and pressure- and by how much each has an effect as their properties change.
We have a unit in sonar that is a repeater of a transducer face that measure the actual sound velocity in the water we are in; it is limited by ownship's depth but it a good place to start to understand how sound is traveling around our boat.
We can shoot a probe out of the boat that measures sound velocity (SSXVT) to a greater depth than the boat can go to, but they are expensive.
But all this knowledge of sound velocity, all the terminology we use to describe sound velocity, is wrong, and it has been wrong for quite sometime in the sonar world- even the sonarman's bible says it, but so quickly that you skim right over it like you are reading a credit card app.
Sound doesn't actually have velocity. To have velocity you must have mass and sound, in and of itself, is massless. Sound is created whenever something rotates, vibrates, or reciprocates. The molecules around the sound source excite the molecules around it which then expand outward in pressure waves. This is why there is no sound in outer space, no molecules in the empty vacuum of space to help the pressure wave expand. So, if a star exploded and nobody was around, would it still make a noise?
The correct terminology for the movement of the pressure wave is sound particle velocity.
But this would take too long to write down and we would have to change a lot of labels on sound velocity measuring equipment. We will just continue to use the wrong terminology and just know what is really happening.
If this is foreign to you do not fret. Most people could give 2-shits about sonar theory, including the hundreds of sonarmen in the U.S. Navy. to further complicate things I am including this daigram of a sound velocity profile, something near and dear to a sonarman's heart.
So there it is. Sonar Theory 101 completed.