One of the most frequent questions that I get as a chiropractor is what the popping sound is when we do an adjustment. No, it's not actually "popcorn" like we tell many kids. There is a physiological explanation to the "pop" or "crack" of a chiropractic adjustment.
First of all, when I explained what a subluxation is in "What is a Subluxation Anyway?", you may recall that, with a chiropractic adjustment, we are basically moving a misaligned bone back into place. When bones are misaligned like that and "stuck" out of position, they need a little coaxing to get back in place. And that takes years of training and continual learning to get down pat. That's called the chiropractic adjustment.
With a chiropractic adjustment, in order to move the bone back in place, the joint that joins the two bones needs to slightly gap apart. When that happens, little air/gas bubbles rush into the space that has just been opened a little bit and that is when you get the "pop" or "crack" sound. It's a lot less scary than most people think. The bones aren't crunching together or grinding against each other. There isn't any breaking or fracturing happening. It's simply air rushing into a newly opened space. When that happens, the bone is more easily able to get back into alignment.
As you can see, it's a pretty simple and gentle occurrence. The sound is usually what catches people off guard and gives that bit of a fear factor. And, if you were wondering, why a neck adjustment is so much louder than one in your midback or low back, it's simply an issue of proximity. Your neck is just simply much closer to your ears. So it's not that the "pop" in your neck is louder, it's just perceived as louder because your neck vertebrae are right by your ears.
So if you've ever been a little apprehensive about that sound, it's just air/gas bubbles moving around. Simple as that.