Actually, IEMs not only block human speech, but they also do a better job of canceling low sounds than active circuity. This is from personal experience, but also from everything I've read about them. Take this example: http://www.earphonesolutions.com/coofsoiseaan.html - scroll down and you'll see that for low frequencies, Shure E2Cs provided a 28dB reduction vs 17dB with the Bose QuietComfort in their tests.