I found a pretty good way to nearly silence the roar from my TM's air conditioner and the roar from neighboring camper's generators...I bought these
Sony MDR-NC20 Traditional Closed Dynamic Noise Canceling Collapsible Headphones from Amazon.com. These things kill about 70% of low frequency steady noise (like AC or generator roar) without having much affect on normal conversational sound frequencies. Built into the headset is a microphone and amplifier that creates sound that's 180 degrees of out of phase and at the same volume as the noise...the net effect is the internally generated sound waves exactly cancel the external roar sound waves resulting in just a slight hiss.
Because the noise cancelling doesn't have much affect on conversational frequencies, if you
don't hook them up to music it's possible to have fairly normal conversation with other folks (no shouting needed). Since I normally camp solo these are great for me...but for those of you who travel as a couple, they would work though might look a little goofy. ;D
Anyway, I know they're not for everyone but figured there might be one or two folks here who don't care much about appearances if they can just make the sounds of air conditioners and generators go away. BTW, these headphones also are great for airline travel (jet noise...what jet noise??) and are pretty good at killing the thumpy-thump sounds from boom-boxes and boom-box cars.