Ouch. My ears.
If you travel a lot you may own noise canceling headphones. If you ever plugged them in to some of the airlines like JetBlue or British Airways, you might notice how loud they are and that if you don't ripped them off your head before the flight attendant starts speaking, your ears may bleed. On a recent transatlantic trip (recent = yesterday) I got the idea to plug in the "Dual Jack Adapter" to the single jack, thinking it might reduce the volume and what do you know, it actually worked. Apparently, if I had read the manual, I would have seen the following:
"The dual-plug adapter lowers the volume levels of strong audio output signals that are common on airline systems."