> ...they're tuned for the wrong frequencies. It's all booming jet engines, and I'm not sure that's going to work on the lower-midrange machine and fans-for-machines racket

As someone with a lifetime cumulative war-on-fan-noise tab running firmly into the four figures, I feel qualified to comment on this one. Noise-canceling headphones work exceptionally well on mid-range computer and lab equipment noise. The problem is that, given how they rely on destructive interference, they cannot cancel any sound which is out of their own speaker elements' range. The infra- and ultra- components of the noise still get through, and sum to an agonizing headache over time. Your mileage may vary.