Here's something a bit different from Sweetwater's Chief Soundware Engineer, Daniel Fisher:

"Due to a nasty cold, I have temporarily lost all the hearing in my right ear. My doctor explained that it is due to a vacuum caused by tubal blockage which is pulling my ear drum so far in one direction that it won't vibrate. (Wow! Extreme DC Bias!) Fortunately, it is a harmless and temporary condition. But I have discovered some very interesting things about on-axis and off-axis frequency response on a single ear. I never would have believed how many different variations there are based on the angle of the sound's source. If you want to try it yourself, put a high quality ear plug into one ear (please don't "invent" something from your toolbox!) and leave it in for an hour while doing your best to ignore it. Your brain will eventually stop paying attention to that ear and you will hear exactly what I'm talking about."