If you are into recording using microphones, you probably know how important mic distance and angle is for capturing a sound source. But even people who know nothing about recording, know from experience that you need to position yourself in front of a speaker to hear "better" audio. It applies when you listen to your home stereo, live concerts or any place where ears meet speakers. However, for some reason, many guitar players seem to ignore, or not pay enough attention to that fact.
Science - Know Your Enemy
The reason for the importance of microphone and ear positioning relative to a sound source is in the physics of sound waves. Sound waves are basically air pressure that's moving in a specific direction. The lower the frequency the more the waves spread in different directions, the higher they are the more focused and directional. Having said that, when you put a combo amp on the floor, horizontally, with the speaker facing your legs a few feet away, most of the high frequency content of your guitar tone will reach your ears only after bouncing off the walls, floor, ceiling and other surfaces in the room, once or a few times. Your brain will mix those reflection into what you perceive as the sound of your amp, but that's most of the time not doing justice with neither your amp not your guitar.
Problem #1
Reflections have less energy comparing to the direct source, so you hear less of those frequencies..
Problem #2
Reflections are always late, so your ear receives some frequencies (the lower ones) earlier than others and the "mix" that your brain creates is just different.
Problem #3
Reflections of the same frequencies can come from different directions, after hitting different surfaces and therefore they might be out of phase with each other, in which case, they will cancel each other out and won't be heard, or cause other frequencies to sound over exaggerated..
The Looper Test
Run the following test. Turn on your rig (while it's on the floor, where it normally is). Record a loop into your looper. Put the guitar down and turn down your amp volume to a reasonable level, so you won't hurt your ears! Now, stand where you normally would while playing and pay attention to the tone you're hearing. Now lie down on the floor (don't worry nobody's watching) and position your ear right in front of the center of the speaker. If you did that, you know exactly what I'm taking about and every extra word is just a waste of time. But for those who have never done such an experiment, the amount of high end frequencies that are going to hit your ear drum is enormous comparing to what you are used to. Not only that, they are going to be much more focused and detailed. But, since you normally set your amp to sound good pointing to your feet, it is probably going to sound way too bright and harsh.
What's The Point?
The point of this experiment is to raise your awareness to the directional attributes of sound waves. A thing that helps me getting better sound out of my amps, no to mention recording better sounds. You can dramatically change the way you experience your amp by simply tilting it a few degrees upwards or reposition it in the room. It can make it sound like a totally different amplifier - believe it or not.
If you are happy with your sound, just keep that knowledge for a rainy day and keep doing what you're doing. If you are missing top end or focus, do yourself a favour and experiment with different angles and positions before you waste money on unnecessary gear.
Cheers.