barometric level change ?

I seem to recall something in college that spoke about the human ear being able to "push back" against sound pressure levels that it found uncomfortable. It might be interesting to go a web site about such topics and see if this could be a factor (the situation you are describing). I am not certain how this could be, but I think it has to do with the tiny bones in the inner ear that provide the linkage to the eardrum. In any event, don't rely on your sound pressure meter, do as Garfish says (and perhaps as your ears are warning) turn it down a bit and save them for the long term.

I find that when I run my air conditioner it produces a positive pressure in the house, and it seems to affect the sound quality. I don't know if it's in my ear, or at the speaker.???

Mwalsdor, best let your family doctor look in your ears first. It might just be a case of too much wax distorting & dampening every sound that tries to reach your tympanum. Cadman, you have a very interesting problem. Have you ever measured the +ve pressure? It should not affect free standing diaphragms like electrostatics or planars. Guess only totally sealed woofer boxes would be affected, and to lesser extend the ported boxes be affected. Please post soon as you know the pressure at which your sound is audibly degraded. As a possible quick solution, suggest you leave a small vent open somewhere in your listening room when running the aircon to prevent pressure built up. Good luck.

I will try the venting process, but unfortunately I live in a newer apartment that is pretty sealed tight. What I have been doing is just turning the Air off when ever I want to play my system. By the way, I do have sealed speakers which could be affected, but like I said, I don't know if it's in my ear or not. Thanks for the suggetion! Maybe I'll close my fresh air intake for these occasions, and just re-cirrculate the air.

I don't know if this figures into your problem or not, but water is a more efficient carrier of sound than air. That means humid air will carry sound differently than dry air will. I've noticed that there is a good amount, but too little or too much humidity can make music sound worse.