Effective headphones

I was working at my office when I heard my wife laughing at her office down the hall.  She told me she had just realized she had been listening to some audio files for her research for about 30 minutes with her headphones on.  She had taken care to use the headphones because she didn't want to disturb me with the sound.  But she hadn't realized that the sound was not coming out of the headphones, but from the speakers.