When I have both my headsets earphones on, the music sounds normal, but when I take one off the one I still have on sounds extra loud. I have no idea why this is, can someone explain?
It’s a comparative effort. If the loud music is in both ears all you know is loud, and thereby it’s not as loud as when you only hear it through one ear. You’ve got near silence in one and loudness in the other, so it seems loud because you’re comparing it to nearly silence.
It should be related to Interaural Intensity Difference. Your brain is programmed to compare the sound perceived by both of ears. Usually your brain uses this data to determine the direction of sound. In case of single-ear IEM the pressure difference between both ears will be at max. One ear hears everything and the other ear is dead silent. So it comes down to perception.
Try single ear IEM in a crowded coffee shop. It wont sound that high then. The pressure difference will be lesser.