Sonars run on the same principles as radar, but there are some key differences. If there weren't…they wouldn't have a different name. Both systems rely on waves to figure out where things are, but the waves radar uses (electromagnetic waves) don't need anything in order to move—they can travel even in a vacuum. Contrast that with sonar, and you've got a different story.

Sonar uses mechanical waves to detect and locate objects of interest. Whenever we talk about mechanical waves, we mean that they're physically moving things—like molecules—to get the wave (in this case, sound) from one place to another.

Say you're in a ball pit, except…without the smell of feet.

Gross.

In the ball pit, you decide to sink to the bottom of the "pool," but realize that it isn't really deep enough to get your dive on. Instead, your feet hit the bottom of the pit and spring you back up to the surface. Just in time, too. It was getting smelly down there.

Aside from your ability to smell things, you were acting just like one acoustic (sound) wave down in the ball pit. You displaced the plastic ball "molecules" by moving your body towards the bottom of the pit. Once you hit something you couldn't displace, though, there was nothing left to do but bounce back up to your original spot. As you came back up to the surface, those balls moved back to their original position.

That's exactly what acoustic waves do, except they don't have physical bodies. Instead, they push water molecules (or any other molecules, really), sending the energy down. The catch? The water molecules actually resist the motion; just like people, they don't like being squished together. When the water molecules get squeezed together, they're going to resist that pushing by pulling apart again.

That push and pull can also be called pressure. You'll get the maximum amount of pressure in a cycle when you reach the peak of the wave.

When we're talking about water, sonar is the top choice because acoustic waves are the only waves that can continue to compress and refract for long ranges under water. Even so, everything, including the

• seafloor depth
• ocean temperature
• water salinity
• wave currents

can hurt a sonar system's effectiveness.

Usually the computer and statistics can help figure all that out, but it's important to keep those things in mind.